# GNU libgomp

# Introduction

This manual documents the usage of libgomp, the GNU Offloading and Multi Processing Runtime Library. This includes the GNU implementation of the OpenMP Application Programming Interface (API) for multi-platform shared-memory parallel programming in C/C++ and Fortran, and the GNU implementation of the OpenACC Application Programming Interface (API) for offloading of code to accelerator devices in C/C++ and Fortran.

Originally, libgomp implemented the GNU OpenMP Runtime Library. Based on this, support for OpenACC and offloading (both OpenACC and OpenMP 4’s target construct) has been added later on, and the library’s name changed to GNU Offloading and Multi Processing Runtime Library.

## Table of Contents

- 1 Enabling OpenMP
- 2 OpenMP Implementation Status
  - 2.1 OpenMP 4.5
  - 2.2 OpenMP 5.0
    - New features listed in Appendix B of the OpenMP specification
    - Other new OpenMP 5.0 features
  - 2.3 OpenMP 5.1
    - New features listed in Appendix B of the OpenMP specification
    - Other new OpenMP 5.1 features
- 3 OpenMP Runtime Library Routines
  - 3.1 `omp_get_active_level` – Number of parallel regions
  - 3.2 `omp_get_ancestor_thread_num` – Ancestor thread ID
  - 3.3 `omp_get_cancellation` – Whether cancellation support is enabled
  - 3.4 `omp_get_default_device` – Get the default device for target regions
  - 3.5 `omp_get_device_num` – Return device number of current device
  - 3.6 `omp_get_dynamic` – Dynamic teams setting
  - 3.7 `omp_get_initial_device` – Return device number of initial device
  - 3.8 `omp_get_level` – Obtain the current nesting level
  - 3.9 `omp_get_max_active_levels` – Current maximum number of active regions
  - 3.10 `omp_get_max_task_priority` – Maximum priority value
  - 3.11 `omp_get_max_teams` – Maximum number of teams of teams region
  - 3.12 `omp_get_max_threads` – Maximum number of threads of parallel region
  - 3.13 `omp_get_nested` – Nested parallel regions
  - 3.14 `omp_get_num_devices` – Number of target devices
  - 3.15 `omp_get_num_procs` – Number of processors online
  - 3.16 `omp_get_num_teams` – Number of teams
  - 3.17 `omp_get_num_threads` – Size of the active team
  - 3.18 `omp_get_proc_bind` – Whether threads may be moved between CPUs
  - 3.19 `omp_get_schedule` – Obtain the runtime scheduling method
  - 3.20 `omp_get_supported_active_levels` – Maximum number of active regions supported
  - 3.21 `omp_get_team_num` – Get team number
  - 3.22 `omp_get_team_size` – Number of threads in a team
  - 3.23 `omp_get_teams_thread_limit` – Maximum number of threads imposed by teams
  - 3.24 `omp_get_thread_limit` – Maximum number of threads
  - 3.25 `omp_get_thread_num` – Current thread ID
  - 3.26 `omp_in_parallel` – Whether a parallel region is active
  - 3.27 `omp_in_final` – Whether in final or included task region
  - 3.28 `omp_is_initial_device` – Whether executing on the host device
  - 3.29 `omp_set_default_device` – Set the default device for target regions
  - 3.30 `omp_set_dynamic` – Enable/disable dynamic teams
  - 3.31 `omp_set_max_active_levels` – Limits the number of active parallel regions
  - 3.32 `omp_set_nested` – Enable/disable nested parallel regions
  - 3.33 `omp_set_num_teams` – Set upper teams limit for teams construct
  - 3.34 `omp_set_num_threads` – Set upper team size limit
  - 3.35 `omp_set_schedule` – Set the runtime scheduling method
  - 3.36 `omp_set_teams_thread_limit` – Set upper thread limit for teams construct
  - 3.37 `omp_init_lock` – Initialize simple lock
  - 3.38 `omp_set_lock` – Wait for and set simple lock
  - 3.39 `omp_test_lock` – Test and set simple lock if available
  - 3.40 `omp_unset_lock` – Unset simple lock
  - 3.41 `omp_destroy_lock` – Destroy simple lock
  - 3.42 `omp_init_nest_lock` – Initialize nested lock
  - 3.43 `omp_set_nest_lock` – Wait for and set nested lock
  - 3.44 `omp_test_nest_lock` – Test and set nested lock if available
  - 3.45 `omp_unset_nest_lock` – Unset nested lock
  - 3.46 `omp_destroy_nest_lock` – Destroy nested lock
  - 3.47 `omp_get_wtick` – Get timer precision
  - 3.48 `omp_get_wtime` – Elapsed wall clock time
  - 3.49 `omp_fulfill_event` – Fulfill and destroy an OpenMP event
- 4 OpenMP Environment Variables
  - 4.1 `OMP_CANCELLATION` – Set whether cancellation is activated
  - 4.2 `OMP_DISPLAY_ENV` – Show OpenMP version and environment variables
  - 4.3 `OMP_DEFAULT_DEVICE` – Set the device used in target regions
  - 4.4 `OMP_DYNAMIC` – Dynamic adjustment of threads
  - 4.5 `OMP_MAX_ACTIVE_LEVELS` – Set the maximum number of nested parallel regions
  - 4.6 `OMP_MAX_TASK_PRIORITY` – Set the maximum priority
  - 4.7 `OMP_NESTED` – Nested parallel regions
  - 4.8 `OMP_NUM_TEAMS` – Specifies the number of teams to use by teams region
  - 4.9 `OMP_NUM_THREADS` – Specifies the number of threads to use
  - 4.10 `OMP_PROC_BIND` – Whether threads may be moved between CPUs
  - 4.11 `OMP_PLACES` – Specifies on which CPUs the threads should be placed
  - 4.12 `OMP_STACKSIZE` – Set default thread stack size
  - 4.13 `OMP_SCHEDULE` – How threads are scheduled
  - 4.14 `OMP_TARGET_OFFLOAD` – Controls offloading behaviour
  - 4.15 `OMP_TEAMS_THREAD_LIMIT` – Set the maximum number of threads imposed by teams
  - 4.16 `OMP_THREAD_LIMIT` – Set the maximum number of threads
  - 4.17 `OMP_WAIT_POLICY` – How waiting threads are handled
  - 4.18 `GOMP_CPU_AFFINITY` – Bind threads to specific CPUs
  - 4.19 `GOMP_DEBUG` – Enable debugging output
  - 4.20 `GOMP_STACKSIZE` – Set default thread stack size
  - 4.21 `GOMP_SPINCOUNT` – Set the busy-wait spin count
  - 4.22 `GOMP_RTEMS_THREAD_POOLS` – Set the RTEMS specific thread pools
- 5 Enabling OpenACC
- 6 OpenACC Runtime Library Routines
  - 6.1 `acc_get_num_devices` – Get number of devices for given device type
  - 6.2 `acc_set_device_type` – Set type of device accelerator to use.
  - 6.3 `acc_get_device_type` – Get type of device accelerator to be used.
  - 6.4 `acc_set_device_num` – Set device number to use.
  - 6.5 `acc_get_device_num` – Get device number to be used.
  - 6.6 `acc_get_property` – Get device property.
  - 6.7 `acc_async_test` – Test for completion of a specific asynchronous operation.
  - 6.8 `acc_async_test_all` – Tests for completion of all asynchronous operations.
  - 6.9 `acc_wait` – Wait for completion of a specific asynchronous operation.
  - 6.10 `acc_wait_all` – Waits for completion of all asynchronous operations.
  - 6.11 `acc_wait_all_async` – Wait for completion of all asynchronous operations.
  - 6.12 `acc_wait_async` – Wait for completion of asynchronous operations.
  - 6.13 `acc_init` – Initialize runtime for a specific device type.
  - 6.14 `acc_shutdown` – Shuts down the runtime for a specific device type.
  - 6.15 `acc_on_device` – Whether executing on a particular device
  - 6.16 `acc_malloc` – Allocate device memory.
  - 6.17 `acc_free` – Free device memory.
  - 6.18 `acc_copyin` – Allocate device memory and copy host memory to it.
  - 6.19 `acc_present_or_copyin` – If the data is not present on the device, allocate device memory and copy from host memory.
  - 6.20 `acc_create` – Allocate device memory and map it to host memory.
  - 6.21 `acc_present_or_create` – If the data is not present on the device, allocate device memory and map it to host memory.
  - 6.22 `acc_copyout` – Copy device memory to host memory.
  - 6.23 `acc_delete` – Free device memory.
  - 6.24 `acc_update_device` – Update device memory from mapped host memory.
  - 6.25 `acc_update_self` – Update host memory from mapped device memory.
  - 6.26 `acc_map_data` – Map previously allocated device memory to host memory.
  - 6.27 `acc_unmap_data` – Unmap device memory from host memory.
  - 6.28 `acc_deviceptr` – Get device pointer associated with specific host address.
  - 6.29 `acc_hostptr` – Get host pointer associated with specific device address.
  - 6.30 `acc_is_present` – Indicate whether host variable / array is present on device.
  - 6.31 `acc_memcpy_to_device` – Copy host memory to device memory.
  - 6.32 `acc_memcpy_from_device` – Copy device memory to host memory.
  - 6.33 `acc_attach` – Let device pointer point to device-pointer target.
  - 6.34 `acc_detach` – Let device pointer point to host-pointer target.
  - 6.35 `acc_get_current_cuda_device` – Get CUDA device handle.
  - 6.36 `acc_get_current_cuda_context` – Get CUDA context handle.
  - 6.37 `acc_get_cuda_stream` – Get CUDA stream handle.
  - 6.38 `acc_set_cuda_stream` – Set CUDA stream handle.
  - 6.39 `acc_prof_register` – Register callbacks.
  - 6.40 `acc_prof_unregister` – Unregister callbacks.
  - 6.41 `acc_prof_lookup` – Obtain inquiry functions.
  - 6.42 `acc_register_library` – Library registration.
- 7 OpenACC Environment Variables
  - 7.1 `ACC_DEVICE_TYPE`
  - 7.2 `ACC_DEVICE_NUM`
  - 7.3 `ACC_PROFLIB`
  - 7.4 `GCC_ACC_NOTIFY`
- 8 CUDA Streams Usage
- 9 OpenACC Library Interoperability
  - 9.1 Introduction
  - 9.2 First invocation: NVIDIA CUBLAS library API
  - 9.3 First invocation: OpenACC library API
  - 9.4 OpenACC library and environment variables
- 10 OpenACC Profiling Interface
  - 10.1 Implementation Status and Implementation-Defined Behavior
- 11 The libgomp ABI
  - 11.1 Implementing MASTER construct
  - 11.2 Implementing CRITICAL construct
  - 11.3 Implementing ATOMIC construct
  - 11.4 Implementing FLUSH construct
  - 11.5 Implementing BARRIER construct
  - 11.6 Implementing THREADPRIVATE construct
  - 11.7 Implementing PRIVATE clause
  - 11.8 Implementing FIRSTPRIVATE LASTPRIVATE COPYIN and COPYPRIVATE clauses
  - 11.9 Implementing REDUCTION clause
  - 11.10 Implementing PARALLEL construct
  - 11.11 Implementing FOR construct
  - 11.12 Implementing ORDERED construct
  - 11.13 Implementing SECTIONS construct
  - 11.14 Implementing SINGLE construct
  - 11.15 Implementing OpenACC’s PARALLEL construct
- 12 Reporting Bugs
- GNU General Public License
- GNU Free Documentation License
  - ADDENDUM: How to use this License for your documents
- Funding Free Software
- Library Index

## Short Table of Contents

- 1 Enabling OpenMP
- 2 OpenMP Implementation Status
- 3 OpenMP Runtime Library Routines
- 4 OpenMP Environment Variables
- 5 Enabling OpenACC
- 6 OpenACC Runtime Library Routines
- 7 OpenACC Environment Variables
- 8 CUDA Streams Usage
- 9 OpenACC Library Interoperability
- 10 OpenACC Profiling Interface
- 11 The libgomp ABI
- 12 Reporting Bugs
- GNU General Public License
- GNU Free Documentation License
- Funding Free Software
- Library Index

## 1 Enabling OpenMP

To activate the OpenMP extensions for C/C++ and Fortran, the compile-time flag `-fopenmp` must be specified. This enables the OpenMP directive `#pragma omp` in C/C++ and `!$omp` directives in free form, `c$omp`, `*$omp` and `!$omp` directives in fixed form, `!$` conditional compilation sentinels in free form and `c$`, `*$` and `!$` sentinels in fixed form, for Fortran. The flag also arranges for automatic linking of the OpenMP runtime library (OpenMP Runtime Library Routines).

A complete description of all OpenMP directives may be found in the OpenMP Application Program Interface manuals. See also OpenMP Implementation Status.

## 2 OpenMP Implementation Status

The `_OPENMP` preprocessor macro and Fortran’s `openmp_version` parameter, provided by `omp_lib.h` and the `omp_lib` module, have the value `201511` (i.e. OpenMP 4.5).

### 2.1 OpenMP 4.5

The OpenMP 4.5 specification is fully supported.

### 2.2 OpenMP 5.0

#### New features listed in Appendix B of the OpenMP specification

Description | Status | Comments
--- | --- | ---
Array shaping | N |
Array sections with non-unit strides in C and C++ | N |
Iterators | Y |
`metadirective` directive | N |
`declare variant` directive | P | simd traits not handled correctly
target-offload-var ICV and `OMP_TARGET_OFFLOAD` env variable | Y |
Nested-parallel changes to max-active-levels-var ICV | Y |
`requires` directive | P | Only fulfillable requirement are `atomic_default_mem_order` and `dynamic_allocators`
`teams` construct outside an enclosing target region | Y |
Non-rectangular loop nests | P | Only C/C++
`!=` as relational-op in canonical loop form for C/C++ | Y |
`nonmonotonic` as default loop schedule modifier for worksharing-loop constructs | Y |
Collapse of associated loops that are imperfectly nested loops | N |
Clauses `if`, `nontemporal` and `order(concurrent)` in `simd` construct | Y |
`atomic` constructs in `simd` | Y |
`loop` construct | Y |
`order(concurrent)` clause | Y |
`scan` directive and `in_scan` modifier for the `reduction` clause | Y |
`in_reduction` clause on `task` constructs | Y |
`in_reduction` clause on `target` constructs | P | `nowait` only stub
`task_reduction` clause with `taskgroup` | Y |
`task` modifier to `reduction` clause | Y |
`affinity` clause to `task` construct | Y | Stub only
`detach` clause to `task` construct | Y |
`omp_fulfill_event` runtime routine | Y |
`reduction` and `in_reduction` clauses on `taskloop` and `taskloop simd` constructs | Y |
`taskloop` construct cancelable by `cancel` construct | Y |
`mutexinoutset` dependence-type for `depend` clause | Y |
Predefined memory spaces, memory allocators, allocator traits | Y | Some are only stubs
Memory management routines | Y |
`allocate` directive | N |
`allocate` clause | P | initial support
`use_device_addr` clause on `target data` | Y |
`ancestor` modifier on `device` clause | P | Reverse offload unsupported
Implicit declare target directive | Y |
Discontiguous array section with `target update` construct | N |
C/C++’s lvalue expressions in `to`, `from` and `map` clauses | N |
C/C++’s lvalue expressions in `depend` clauses | Y |
Nested `declare target` directive | Y |
Combined `master` constructs | Y |
`depend` clause on `taskwait` | Y |
Weak memory ordering clauses on `atomic` and `flush` construct | Y |
`hint` clause on the `atomic` construct | Y | Stub only
`depobj` construct and depend objects | Y |
Lock hints were renamed to synchronization hints | Y |
`conditional` modifier to `lastprivate` clause | Y |
Map-order clarifications | P |
`close` map-type-modifier | Y |
Mapping C/C++ pointer variables and to assign the address of device memory mapped by an array section | P |
Mapping of Fortran pointer and allocatable variables, including pointer and allocatable components of variables | P | Mapping of vars with allocatable components unsupported
`defaultmap` extensions | Y |
`declare mapper` directive | N |
`omp_get_supported_active_levels` routine | Y |
Runtime routines and environment variables to display runtime thread affinity information | Y |
`omp_pause_resource` and `omp_pause_resource_all` runtime routines | Y |
`omp_get_device_num` runtime routine | Y |
OMPT interface | N |
OMPD interface | N |

#### Other new OpenMP 5.0 features

Description | Status | Comments
--- | --- | ---
Supporting C++’s range-based for loop | Y |

### 2.3 OpenMP 5.1

#### New features listed in Appendix B of the OpenMP specification

Description | Status | Comments
--- | --- | ---
OpenMP directive as C++ attribute specifiers | Y |
`omp_all_memory` reserved locator | N |
target_device trait in OpenMP Context | N |
`target_device` selector set in context selectors | N |
C/C++’s `declare variant` directive: elision support of preprocessed code | N |
`declare variant`: new clauses `adjust_args` and `append_args` | N |
`dispatch` construct | N |
device-specific ICV settings the environment variables | N |
assume directive | N |
`nothing` directive | Y |
`error` directive | Y |
`masked` construct | Y |
`scope` directive | Y |
Loop transformation constructs | N |
`strict` modifier in the `grainsize` and `num_tasks` clauses of the taskloop construct | Y |
`align` clause/modifier in `allocate` directive/clause and `allocator` directive | P | C/C++ on clause only
`thread_limit` clause to `target` construct | Y |
`has_device_addr` clause to `target` construct | Y |
iterators in `target update` motion clauses and `map` clauses | N |
indirect calls to the device version of a procedure or function in `target` regions | N |
`interop` directive | N |
`omp_interop_t` object support in runtime routines | N |
`nowait` clause in `taskwait` directive | N |
Extensions to the `atomic` directive | Y |
`seq_cst` clause on a `flush` construct | Y |
`inoutset` argument to the `depend` clause | N |
`private` and `firstprivate` argument to `default` clause in C and C++ | Y |
`present` argument to `defaultmap` clause | N |
`omp_set_num_teams`, `omp_set_teams_thread_limit`, `omp_get_max_teams`, `omp_get_teams_thread_limit` runtime routines | Y |
`omp_target_is_accessible` runtime routine | N |
`omp_target_memcpy_async` and `omp_target_memcpy_rect_async` runtime routines | N |
`omp_get_mapped_ptr` runtime routine | N |
`omp_calloc`, `omp_realloc`, `omp_aligned_alloc` and `omp_aligned_calloc` runtime routines | Y |
`omp_alloctrait_key_t` enum: `omp_atv_serialized` added, `omp_atv_default` changed | Y |
`omp_display_env` runtime routine | Y |
`ompt_scope_endpoint_t` enum: `ompt_scope_beginend` | N |
`ompt_sync_region_t` enum additions | N |
`ompt_state_t` enum: `ompt_state_wait_barrier_implementation` and `ompt_state_wait_barrier_teams` | N |
`ompt_callback_target_data_op_emi_t`, `ompt_callback_target_emi_t`, `ompt_callback_target_map_emi_t` and `ompt_callback_target_submit_emi_t` | N |
`ompt_callback_error_t` type | N |
`OMP_PLACES` syntax extensions | Y |
`OMP_NUM_TEAMS` and `OMP_TEAMS_THREAD_LIMIT` environment variables | Y |

#### Other new OpenMP 5.1 features

Description | Status | Comments
--- | --- | ---
Support of strictly structured blocks in Fortran | Y |
Support of structured block sequences in C/C++ | Y |
`unconstrained` and `reproducible` modifiers on `order` clause | Y |

## 3 OpenMP Runtime Library Routines

The runtime routines described here are defined by Section 3 of the OpenMP specification in version 4.5. The routines are structured in following three parts:

### 3.1 `omp_get_active_level` – Number of parallel regions

Description:

  This function returns the nesting level for the active parallel blocks, which enclose the calling call.

C/C++

  Prototype: | `int omp_get_active_level(void);`

Fortran:

  Interface: | `integer function omp_get_active_level()`

See also:

  `omp_get_level` – Obtain the current nesting level, `omp_get_max_active_levels` – Current maximum number of active regions, `omp_set_max_active_levels` – Limits the number of active parallel regions

Reference:

  OpenMP specification v4.5, Section 3.2.20.

### 3.2 `omp_get_ancestor_thread_num` – Ancestor thread ID

Description:

  This function returns the thread identification number for the given nesting level of the current thread. For values of level outside zero to `omp_get_level` -1 is returned; if level is `omp_get_level` the result is identical to `omp_get_thread_num`.

C/C++

  Prototype: | `int omp_get_ancestor_thread_num(int level);`

Fortran:

  Interface: | `integer function omp_get_ancestor_thread_num(level)`
  | `integer level`

See also:

  `omp_get_level` – Obtain the current nesting level, `omp_get_thread_num` – Current thread ID, `omp_get_team_size` – Number of threads in a team

Reference:

  OpenMP specification v4.5, Section 3.2.18.

### 3.3 `omp_get_cancellation` – Whether cancellation support is enabled

Description:

  This function returns `true` if cancellation is activated, `false` otherwise. Here, `true` and `false` represent their language-specific counterparts. Unless `OMP_CANCELLATION` is set true, cancellations are deactivated.

C/C++:

  Prototype: | `int omp_get_cancellation(void);`

Fortran:

  Interface: | `logical function omp_get_cancellation()`

See also:

  `OMP_CANCELLATION` – Set whether cancellation is activated

Reference:

  OpenMP specification v4.5, Section 3.2.9.

### 3.4 `omp_get_default_device` – Get the default device for target regions

Description:

  Get the default device for target regions without device clause.

C/C++:

  Prototype: | `int omp_get_default_device(void);`

Fortran:

  Interface: | `integer function omp_get_default_device()`

See also:

  `OMP_DEFAULT_DEVICE` – Set the device used in target regions, `omp_set_default_device` – Set the default device for target regions

Reference:

  OpenMP specification v4.5, Section 3.2.30.

### 3.5 `omp_get_device_num` – Return device number of current device

Description:

  This function returns a device number that represents the device that the current thread is executing on. For OpenMP 5.0, this must be equal to the value returned by the `omp_get_initial_device` function when called from the host.

C/C++

  Prototype: | `int omp_get_device_num(void);`

Fortran:

  Interface: | `integer function omp_get_device_num()`

See also:

  `omp_get_initial_device` – Return device number of initial device

Reference:

  OpenMP specification v5.0, Section 3.2.37.

### 3.6 `omp_get_dynamic` – Dynamic teams setting

Description:

  This function returns `true` if enabled, `false` otherwise. Here, `true` and `false` represent their language-specific counterparts.

  The dynamic team setting may be initialized at startup by the `OMP_DYNAMIC` environment variable or at runtime using `omp_set_dynamic`. If undefined, dynamic adjustment is disabled by default.

C/C++:

  Prototype: | `int omp_get_dynamic(void);`

Fortran:

  Interface: | `logical function omp_get_dynamic()`

See also:

  `omp_set_dynamic` – Enable/disable dynamic teams, `OMP_DYNAMIC` – Dynamic adjustment of threads

Reference:

  OpenMP specification v4.5, Section 3.2.8.

### 3.7 `omp_get_initial_device` – Return device number of initial device

Description:

  This function returns a device number that represents the host device. For OpenMP 5.1, this must be equal to the value returned by the `omp_get_num_devices` function.

C/C++

  Prototype: | `int omp_get_initial_device(void);`

Fortran:

  Interface: | `integer function omp_get_initial_device()`

See also:

  `omp_get_num_devices` – Number of target devices

Reference:

  OpenMP specification v4.5, Section 3.2.35.

### 3.8 `omp_get_level` – Obtain the current nesting level

Description:

  This function returns the nesting level for the parallel blocks, which enclose the calling call.

C/C++

  Prototype: | `int omp_get_level(void);`

Fortran:

  Interface: | `integer function omp_level()`

See also:

  `omp_get_active_level` – Number of parallel regions

Reference:

  OpenMP specification v4.5, Section 3.2.17.

### 3.9 `omp_get_max_active_levels` – Current maximum number of active regions

Description:

  This function obtains the maximum allowed number of nested, active parallel regions.

C/C++

  Prototype: | `int omp_get_max_active_levels(void);`

Fortran:

  Interface: | `integer function omp_get_max_active_levels()`

See also:

  `omp_set_max_active_levels` – Limits the number of active parallel regions, `omp_get_active_level` – Number of parallel regions

Reference:

  OpenMP specification v4.5, Section 3.2.16.

### 3.10 `omp_get_max_task_priority` – Maximum priority value

that can be set for tasks.

Description:

  This function obtains the maximum allowed priority number for tasks.

C/C++

  Prototype: | `int omp_get_max_task_priority(void);`

Fortran:

  Interface: | `integer function omp_get_max_task_priority()`

Reference:

  OpenMP specification v4.5, Section 3.2.29.

### 3.11 `omp_get_max_teams` – Maximum number of teams of teams region

Description:

  Return the maximum number of teams used for the teams region that does not use the clause `num_teams`.

C/C++:

  Prototype: | `int omp_get_max_teams(void);`

Fortran:

  Interface: | `integer function omp_get_max_teams()`

See also:

  `omp_set_num_teams` – Set upper teams limit for teams construct, `omp_get_num_teams` – Number of teams

Reference:

  OpenMP specification v5.1, Section 3.4.4.

### 3.12 `omp_get_max_threads` – Maximum number of threads of parallel region

Description:

  Return the maximum number of threads used for the current parallel region that does not use the clause `num_threads`.

C/C++:

  Prototype: | `int omp_get_max_threads(void);`

Fortran:

  Interface: | `integer function omp_get_max_threads()`

See also:

  `omp_set_num_threads` – Set upper team size limit, `omp_set_dynamic` – Enable/disable dynamic teams, `omp_get_thread_limit` – Maximum number of threads

Reference:

  OpenMP specification v4.5, Section 3.2.3.

### 3.13 `omp_get_nested` – Nested parallel regions

Description:

  This function returns `true` if nested parallel regions are enabled, `false` otherwise. Here, `true` and `false` represent their language-specific counterparts.

  The state of nested parallel regions at startup depends on several environment variables. If `OMP_MAX_ACTIVE_LEVELS` is defined and is set to greater than one, then nested parallel regions will be enabled. If not defined, then the value of the `OMP_NESTED` environment variable will be followed if defined. If neither are defined, then if either `OMP_NUM_THREADS` or `OMP_PROC_BIND` are defined with a list of more than one value, then nested parallel regions are enabled. If none of these are defined, then nested parallel regions are disabled by default.

  Nested parallel regions can be enabled or disabled at runtime using `omp_set_nested`, or by setting the maximum number of nested regions with `omp_set_max_active_levels` to one to disable, or above one to enable.

C/C++:

  Prototype: | `int omp_get_nested(void);`

Fortran:

  Interface: | `logical function omp_get_nested()`

See also:

  `omp_set_max_active_levels` – Limits the number of active parallel regions, `omp_set_nested` – Enable/disable nested parallel regions, `OMP_MAX_ACTIVE_LEVELS` – Set the maximum number of nested parallel regions, `OMP_NESTED` – Nested parallel regions

Reference:

  OpenMP specification v4.5, Section 3.2.11.

### 3.14 `omp_get_num_devices` – Number of target devices

Description:

  Returns the number of target devices.

C/C++:

  Prototype: | `int omp_get_num_devices(void);`

Fortran:

  Interface: | `integer function omp_get_num_devices()`

Reference:

  OpenMP specification v4.5, Section 3.2.31.

### 3.15 `omp_get_num_procs` – Number of processors online

Description:

  Returns the number of processors online on that device.

C/C++:

  Prototype: | `int omp_get_num_procs(void);`

Fortran:

  Interface: | `integer function omp_get_num_procs()`

Reference:

  OpenMP specification v4.5, Section 3.2.5.

### 3.16 `omp_get_num_teams` – Number of teams

Description:

  Returns the number of teams in the current team region.

C/C++:

  Prototype: | `int omp_get_num_teams(void);`

Fortran:

  Interface: | `integer function omp_get_num_teams()`

Reference:

  OpenMP specification v4.5, Section 3.2.32.

### 3.17 `omp_get_num_threads` – Size of the active team

Description:

  Returns the number of threads in the current team. In a sequential section of the program `omp_get_num_threads` returns 1.

  The default team size may be initialized at startup by the `OMP_NUM_THREADS` environment variable. At runtime, the size of the current team may be set either by the `NUM_THREADS` clause or by `omp_set_num_threads`. If none of the above were used to define a specific value and `OMP_DYNAMIC` is disabled, one thread per CPU online is used.

C/C++:

  Prototype: | `int omp_get_num_threads(void);`

Fortran:

  Interface: | `integer function omp_get_num_threads()`

See also:

  `omp_get_max_threads` – Maximum number of threads of parallel region, `omp_set_num_threads` – Set upper team size limit, `OMP_NUM_THREADS` – Specifies the number of threads to use

Reference:

  OpenMP specification v4.5, Section 3.2.2.

### 3.18 `omp_get_proc_bind` – Whether threads may be moved between CPUs

Description:

  This functions returns the currently active thread affinity policy, which is set via `OMP_PROC_BIND`. Possible values are `omp_proc_bind_false`, `omp_proc_bind_true`, `omp_proc_bind_primary`, `omp_proc_bind_master`, `omp_proc_bind_close` and `omp_proc_bind_spread`, where `omp_proc_bind_master` is an alias for `omp_proc_bind_primary`.

C/C++:

  Prototype: | `omp_proc_bind_t omp_get_proc_bind(void);`

Fortran:

  Interface: | `integer(kind=omp_proc_bind_kind) function omp_get_proc_bind()`

See also:

  `OMP_PROC_BIND` – Whether threads may be moved between CPUs, `OMP_PLACES` – Specifies on which CPUs the threads should be placed, `GOMP_CPU_AFFINITY` – Bind threads to specific CPUs,

Reference:

  OpenMP specification v4.5, Section 3.2.22.

### 3.19 `omp_get_schedule` – Obtain the runtime scheduling method

Description:

  Obtain the runtime scheduling method. The kind argument will be set to the value `omp_sched_static`, `omp_sched_dynamic`, `omp_sched_guided` or `omp_sched_auto`. The second argument, chunk_size, is set to the chunk size.

C/C++

  Prototype: | `void omp_get_schedule(omp_sched_t *kind, int *chunk_size);`

Fortran:

  Interface: | `subroutine omp_get_schedule(kind, chunk_size)`
  | `integer(kind=omp_sched_kind) kind`
  | `integer chunk_size`

See also:

  `omp_set_schedule` – Set the runtime scheduling method, `OMP_SCHEDULE` – How threads are scheduled

Reference:

  OpenMP specification v4.5, Section 3.2.13.

### 3.20 `omp_get_supported_active_levels` – Maximum number of active regions supported

Description:

  This function returns the maximum number of nested, active parallel regions supported by this implementation.

C/C++

  Prototype: | `int omp_get_supported_active_levels(void);`

Fortran:

  Interface: | `integer function omp_get_supported_active_levels()`

See also:

  `omp_get_max_active_levels` – Current maximum number of active regions, `omp_set_max_active_levels` – Limits the number of active parallel regions

Reference:

  OpenMP specification v5.0, Section 3.2.15.

### 3.21 `omp_get_team_num` – Get team number

Description:

  Returns the team number of the calling thread.

C/C++:

  Prototype: | `int omp_get_team_num(void);`

Fortran:

  Interface: | `integer function omp_get_team_num()`

Reference:

  OpenMP specification v4.5, Section 3.2.33.

### 3.22 `omp_get_team_size` – Number of threads in a team

Description:

  This function returns the number of threads in a thread team to which either the current thread or its ancestor belongs. For values of level outside zero to `omp_get_level`, -1 is returned; if level is zero, 1 is returned, and for `omp_get_level`, the result is identical to `omp_get_num_threads`.

C/C++:

  Prototype: | `int omp_get_team_size(int level);`

Fortran:

  Interface: | `integer function omp_get_team_size(level)`
  | `integer level`

See also:

  `omp_get_num_threads` – Size of the active team, `omp_get_level` – Obtain the current nesting level, `omp_get_ancestor_thread_num` – Ancestor thread ID

Reference:

  OpenMP specification v4.5, Section 3.2.19.

### 3.23 `omp_get_teams_thread_limit` – Maximum number of threads imposed by teams

Description:

  Return the maximum number of threads that will be able to participate in each team created by a teams construct.

C/C++:

  Prototype: | `int omp_get_teams_thread_limit(void);`

Fortran:

  Interface: | `integer function omp_get_teams_thread_limit()`

See also:

  `omp_set_teams_thread_limit` – Set upper thread limit for teams construct, `OMP_TEAMS_THREAD_LIMIT` – Set the maximum number of threads imposed by teams

Reference:

  OpenMP specification v5.1, Section 3.4.6.

### 3.24 `omp_get_thread_limit` – Maximum number of threads

Description:

  Return the maximum number of threads of the program.

C/C++:

  Prototype: | `int omp_get_thread_limit(void);`

Fortran:

  Interface: | `integer function omp_get_thread_limit()`

See also:

  `omp_get_max_threads` – Maximum number of threads of parallel region, `OMP_THREAD_LIMIT` – Set the maximum number of threads

Reference:

  OpenMP specification v4.5, Section 3.2.14.

### 3.25 `omp_get_thread_num` – Current thread ID

Description:

  Returns a unique thread identification number within the current team. In a sequential parts of the program, `omp_get_thread_num` always returns 0. In parallel regions the return value varies from 0 to `omp_get_num_threads`-1 inclusive. The return value of the primary thread of a team is always 0.

C/C++:

  Prototype: | `int omp_get_thread_num(void);`

Fortran:

  Interface: | `integer function omp_get_thread_num()`

See also:

  `omp_get_num_threads` – Size of the active team, `omp_get_ancestor_thread_num` – Ancestor thread ID

Reference:

  OpenMP specification v4.5, Section 3.2.4.

### 3.26 `omp_in_parallel` – Whether a parallel region is active

Description:

  This function returns `true` if currently running in parallel, `false` otherwise. Here, `true` and `false` represent their language-specific counterparts.

C/C++:

  Prototype: | `int omp_in_parallel(void);`

Fortran:

  Interface: | `logical function omp_in_parallel()`

Reference:

  OpenMP specification v4.5, Section 3.2.6.

### 3.27 `omp_in_final` – Whether in final or included task region

Description:

  This function returns `true` if currently running in a final or included task region, `false` otherwise. Here, `true` and `false` represent their language-specific counterparts.

C/C++:

  Prototype: | `int omp_in_final(void);`

Fortran:

  Interface: | `logical function omp_in_final()`

Reference:

  OpenMP specification v4.5, Section 3.2.21.

### 3.28 `omp_is_initial_device` – Whether executing on the host device

Description:

  This function returns `true` if currently running on the host device, `false` otherwise. Here, `true` and `false` represent their language-specific counterparts.

C/C++:

  Prototype: | `int omp_is_initial_device(void);`

Fortran:

  Interface: | `logical function omp_is_initial_device()`

Reference:

  OpenMP specification v4.5, Section 3.2.34.

### 3.29 `omp_set_default_device` – Set the default device for target regions

Description:

  Set the default device for target regions without device clause. The argument shall be a nonnegative device number.

C/C++:

  Prototype: | `void omp_set_default_device(int device_num);`

Fortran:

  Interface: | `subroutine omp_set_default_device(device_num)`
  | `integer device_num`

See also:

  `OMP_DEFAULT_DEVICE` – Set the device used in target regions, `omp_get_default_device` – Get the default device for target regions

Reference:

  OpenMP specification v4.5, Section 3.2.29.

### 3.30 `omp_set_dynamic` – Enable/disable dynamic teams

Description:

  Enable or disable the dynamic adjustment of the number of threads within a team. The function takes the language-specific equivalent of `true` and `false`, where `true` enables dynamic adjustment of team sizes and `false` disables it.

C/C++:

  Prototype: | `void omp_set_dynamic(int dynamic_threads);`

Fortran:

  Interface: | `subroutine omp_set_dynamic(dynamic_threads)`
  | `logical, intent(in) :: dynamic_threads`

See also:

  `OMP_DYNAMIC` – Dynamic adjustment of threads, `omp_get_dynamic` – Dynamic teams setting

Reference:

  OpenMP specification v4.5, Section 3.2.7.

### 3.31 `omp_set_max_active_levels` – Limits the number of active parallel regions

Description:

  This function limits the maximum allowed number of nested, active parallel regions. max_levels must be less or equal to the value returned by `omp_get_supported_active_levels`.

C/C++

  Prototype: | `void omp_set_max_active_levels(int max_levels);`

Fortran:

  Interface: | `subroutine omp_set_max_active_levels(max_levels)`
  | `integer max_levels`

See also:

  `omp_get_max_active_levels` – Current maximum number of active regions, `omp_get_active_level` – Number of parallel regions, `omp_get_supported_active_levels` – Maximum number of active regions supported

Reference:

  OpenMP specification v4.5, Section 3.2.15.

### 3.32 `omp_set_nested` – Enable/disable nested parallel regions

Description:

  Enable or disable nested parallel regions, i.e., whether team members are allowed to create new teams. The function takes the language-specific equivalent of `true` and `false`, where `true` enables dynamic adjustment of team sizes and `false` disables it.

  Enabling nested parallel regions will also set the maximum number of active nested regions to the maximum supported. Disabling nested parallel regions will set the maximum number of active nested regions to one.

C/C++:

  Prototype: | `void omp_set_nested(int nested);`

Fortran:

  Interface: | `subroutine omp_set_nested(nested)`
  | `logical, intent(in) :: nested`

See also:

  `omp_get_nested` – Nested parallel regions, `omp_set_max_active_levels` – Limits the number of active parallel regions, `OMP_MAX_ACTIVE_LEVELS` – Set the maximum number of nested parallel regions, `OMP_NESTED` – Nested parallel regions

Reference:

  OpenMP specification v4.5, Section 3.2.10.

### 3.33 `omp_set_num_teams` – Set upper teams limit for teams construct

Description:

  Specifies the upper bound for number of teams created by the teams construct which does not specify a `num_teams` clause. The argument of `omp_set_num_teams` shall be a positive integer.

C/C++:

  Prototype: | `void omp_set_num_teams(int num_teams);`

Fortran:

  Interface: | `subroutine omp_set_num_teams(num_teams)`
  | `integer, intent(in) :: num_teams`

See also:

  `OMP_NUM_TEAMS` – Specifies the number of teams to use by teams region, `omp_get_num_teams` – Number of teams, `omp_get_max_teams` – Maximum number of teams of teams region

Reference:

  OpenMP specification v5.1, Section 3.4.3.

### 3.34 `omp_set_num_threads` – Set upper team size limit

Description:

  Specifies the number of threads used by default in subsequent parallel sections, if those do not specify a `num_threads` clause. The argument of `omp_set_num_threads` shall be a positive integer.

C/C++:

  Prototype: | `void omp_set_num_threads(int num_threads);`

Fortran:

  Interface: | `subroutine omp_set_num_threads(num_threads)`
  | `integer, intent(in) :: num_threads`

See also:

  `OMP_NUM_THREADS` – Specifies the number of threads to use, `omp_get_num_threads` – Size of the active team, `omp_get_max_threads` – Maximum number of threads of parallel region

Reference:

  OpenMP specification v4.5, Section 3.2.1.

### 3.35 `omp_set_schedule` – Set the runtime scheduling method

Description:

  Sets the runtime scheduling method. The kind argument can have the value `omp_sched_static`, `omp_sched_dynamic`, `omp_sched_guided` or `omp_sched_auto`. Except for `omp_sched_auto`, the chunk size is set to the value of chunk_size if positive, or to the default value if zero or negative. For `omp_sched_auto` the chunk_size argument is ignored.

C/C++

  Prototype: | `void omp_set_schedule(omp_sched_t kind, int chunk_size);`

Fortran:

  Interface: | `subroutine omp_set_schedule(kind, chunk_size)`
  | `integer(kind=omp_sched_kind) kind`
  | `integer chunk_size`

See also:

  `omp_get_schedule` – Obtain the runtime scheduling method `OMP_SCHEDULE` – How threads are scheduled

Reference:

  OpenMP specification v4.5, Section 3.2.12.

### 3.36 `omp_set_teams_thread_limit` – Set upper thread limit for teams construct

Description:

  Specifies the upper bound for number of threads that will be available for each team created by the teams construct which does not specify a `thread_limit` clause. The argument of `omp_set_teams_thread_limit` shall be a positive integer.

C/C++:

  Prototype: | `void omp_set_teams_thread_limit(int thread_limit);`

Fortran:

  Interface: | `subroutine omp_set_teams_thread_limit(thread_limit)`
  | `integer, intent(in) :: thread_limit`

See also:

  `OMP_TEAMS_THREAD_LIMIT` – Set the maximum number of threads imposed by teams, `omp_get_teams_thread_limit` – Maximum number of threads imposed by teams, `omp_get_thread_limit` – Maximum number of threads

Reference:

  OpenMP specification v5.1, Section 3.4.5.

### 3.37 `omp_init_lock` – Initialize simple lock

Description:

  Initialize a simple lock. After initialization, the lock is in an unlocked state.

C/C++:

  Prototype: | `void omp_init_lock(omp_lock_t *lock);`

Fortran:

  Interface: | `subroutine omp_init_lock(svar)`
  | `integer(omp_lock_kind), intent(out) :: svar`

See also:

  `omp_destroy_lock` – Destroy simple lock

Reference:

  OpenMP specification v4.5, Section 3.3.1.

### 3.38 `omp_set_lock` – Wait for and set simple lock

Description:

  Before setting a simple lock, the lock variable must be initialized by `omp_init_lock`. The calling thread is blocked until the lock is available. If the lock is already held by the current thread, a deadlock occurs.

C/C++:

  Prototype: | `void omp_set_lock(omp_lock_t *lock);`

Fortran:

  Interface: | `subroutine omp_set_lock(svar)`
  | `integer(omp_lock_kind), intent(inout) :: svar`

See also:

  `omp_init_lock` – Initialize simple lock, `omp_test_lock` – Test and set simple lock if available, `omp_unset_lock` – Unset simple lock

Reference:

  OpenMP specification v4.5, Section 3.3.4.

### 3.39 `omp_test_lock` – Test and set simple lock if available

Description:

  Before setting a simple lock, the lock variable must be initialized by `omp_init_lock`. Contrary to `omp_set_lock`, `omp_test_lock` does not block if the lock is not available. This function returns `true` upon success, `false` otherwise. Here, `true` and `false` represent their language-specific counterparts.

C/C++:

  Prototype: | `int omp_test_lock(omp_lock_t *lock);`

Fortran:

  Interface: | `logical function omp_test_lock(svar)`
  | `integer(omp_lock_kind), intent(inout) :: svar`

See also:

  `omp_init_lock` – Initialize simple lock, `omp_set_lock` – Wait for and set simple lock, `omp_set_lock` – Wait for and set simple lock

Reference:

  OpenMP specification v4.5, Section 3.3.6.

### 3.40 `omp_unset_lock` – Unset simple lock

Description:

  A simple lock about to be unset must have been locked by `omp_set_lock` or `omp_test_lock` before. In addition, the lock must be held by the thread calling `omp_unset_lock`. Then, the lock becomes unlocked. If one or more threads attempted to set the lock before, one of them is chosen to, again, set the lock to itself.

C/C++:

  Prototype: | `void omp_unset_lock(omp_lock_t *lock);`

Fortran:

  Interface: | `subroutine omp_unset_lock(svar)`
  | `integer(omp_lock_kind), intent(inout) :: svar`

See also:

  `omp_set_lock` – Wait for and set simple lock, `omp_test_lock` – Test and set simple lock if available

Reference:

  OpenMP specification v4.5, Section 3.3.5.

### 3.41 `omp_destroy_lock` – Destroy simple lock

Description:

  Destroy a simple lock. In order to be destroyed, a simple lock must be in the unlocked state.

C/C++:

  Prototype: | `void omp_destroy_lock(omp_lock_t *lock);`

Fortran:

  Interface: | `subroutine omp_destroy_lock(svar)`
  | `integer(omp_lock_kind), intent(inout) :: svar`

See also:

  `omp_init_lock` – Initialize simple lock

Reference:

  OpenMP specification v4.5, Section 3.3.3.

### 3.42 `omp_init_nest_lock` – Initialize nested lock

Description:

  Initialize a nested lock. After initialization, the lock is in an unlocked state and the nesting count is set to zero.

C/C++:

  Prototype: | `void omp_init_nest_lock(omp_nest_lock_t *lock);`

Fortran:

  Interface: | `subroutine omp_init_nest_lock(nvar)`
  | `integer(omp_nest_lock_kind), intent(out) :: nvar`

See also:

  `omp_destroy_nest_lock` – Destroy nested lock

Reference:

  OpenMP specification v4.5, Section 3.3.1.

### 3.43 `omp_set_nest_lock` – Wait for and set nested lock

Description:

  Before setting a nested lock, the lock variable must be initialized by `omp_init_nest_lock`. The calling thread is blocked until the lock is available. If the lock is already held by the current thread, the nesting count for the lock is incremented.

C/C++:

  Prototype: | `void omp_set_nest_lock(omp_nest_lock_t *lock);`

Fortran:

  Interface: | `subroutine omp_set_nest_lock(nvar)`
  | `integer(omp_nest_lock_kind), intent(inout) :: nvar`

See also:

  `omp_init_nest_lock` – Initialize nested lock, `omp_unset_nest_lock` – Unset nested lock

Reference:

  OpenMP specification v4.5, Section 3.3.4.

### 3.44 `omp_test_nest_lock` – Test and set nested lock if available

Description:

  Before setting a nested lock, the lock variable must be initialized by `omp_init_nest_lock`. Contrary to `omp_set_nest_lock`, `omp_test_nest_lock` does not block if the lock is not available. If the lock is already held by the current thread, the new nesting count is returned. Otherwise, the return value equals zero.

C/C++:

  Prototype: | `int omp_test_nest_lock(omp_nest_lock_t *lock);`

Fortran:

  Interface: | `logical function omp_test_nest_lock(nvar)`
  | `integer(omp_nest_lock_kind), intent(inout) :: nvar`

See also:

  `omp_init_lock` – Initialize simple lock, `omp_set_lock` – Wait for and set simple lock, `omp_set_lock` – Wait for and set simple lock

Reference:

  OpenMP specification v4.5, Section 3.3.6.

### 3.45 `omp_unset_nest_lock` – Unset nested lock

Description:

  A nested lock about to be unset must have been locked by `omp_set_nested_lock` or `omp_test_nested_lock` before. In addition, the lock must be held by the thread calling `omp_unset_nested_lock`. If the nesting count drops to zero, the lock becomes unlocked. If one ore more threads attempted to set the lock before, one of them is chosen to, again, set the lock to itself.

C/C++:

  Prototype: | `void omp_unset_nest_lock(omp_nest_lock_t *lock);`

Fortran:

  Interface: | `subroutine omp_unset_nest_lock(nvar)`
  | `integer(omp_nest_lock_kind), intent(inout) :: nvar`

See also:

  `omp_set_nest_lock` – Wait for and set nested lock

Reference:

  OpenMP specification v4.5, Section 3.3.5.

### 3.46 `omp_destroy_nest_lock` – Destroy nested lock

Description:

  Destroy a nested lock. In order to be destroyed, a nested lock must be in the unlocked state and its nesting count must equal zero.

C/C++:

  Prototype: | `void omp_destroy_nest_lock(omp_nest_lock_t *);`

Fortran:

  Interface: | `subroutine omp_destroy_nest_lock(nvar)`
  | `integer(omp_nest_lock_kind), intent(inout) :: nvar`

See also:

  `omp_init_lock` – Initialize simple lock

Reference:

  OpenMP specification v4.5, Section 3.3.3.

### 3.47 `omp_get_wtick` – Get timer precision

Description:

  Gets the timer precision, i.e., the number of seconds between two successive clock ticks.

C/C++:

  Prototype: | `double omp_get_wtick(void);`

Fortran:

  Interface: | `double precision function omp_get_wtick()`

See also:

  `omp_get_wtime` – Elapsed wall clock time

Reference:

  OpenMP specification v4.5, Section 3.4.2.

### 3.48 `omp_get_wtime` – Elapsed wall clock time

Description:

  Elapsed wall clock time in seconds. The time is measured per thread, no guarantee can be made that two distinct threads measure the same time. Time is measured from some "time in the past", which is an arbitrary time guaranteed not to change during the execution of the program.

C/C++:

  Prototype: | `double omp_get_wtime(void);`

Fortran:

  Interface: | `double precision function omp_get_wtime()`

See also:

  `omp_get_wtick` – Get timer precision

Reference:

  OpenMP specification v4.5, Section 3.4.1.

### 3.49 `omp_fulfill_event` – Fulfill and destroy an OpenMP event

Description:

  Fulfill the event associated with the event handle argument. Currently, it is only used to fulfill events generated by detach clauses on task constructs - the effect of fulfilling the event is to allow the task to complete.

  The result of calling `omp_fulfill_event` with an event handle other than that generated by a detach clause is undefined. Calling it with an event handle that has already been fulfilled is also undefined.

C/C++:

  Prototype: | `void omp_fulfill_event(omp_event_handle_t event);`

Fortran:

  Interface: | `subroutine omp_fulfill_event(event)`
  | `integer (kind=omp_event_handle_kind) :: event`

Reference:

  OpenMP specification v5.0, Section 3.5.1.

## 4 OpenMP Environment Variables

The environment variables which beginning with `OMP_` are defined by section 4 of the OpenMP specification in version 4.5, while those beginning with `GOMP_` are GNU extensions.

### 4.1 `OMP_CANCELLATION` – Set whether cancellation is activated

Description:

  If set to `TRUE`, the cancellation is activated. If set to `FALSE` or if unset, cancellation is disabled and the `cancel` construct is ignored.

See also:

  `omp_get_cancellation` – Whether cancellation support is enabled

Reference:

  OpenMP specification v4.5, Section 4.11

### 4.2 `OMP_DISPLAY_ENV` – Show OpenMP version and environment variables

Description:

  If set to `TRUE`, the OpenMP version number and the values associated with the OpenMP environment variables are printed to `stderr`. If set to `VERBOSE`, it additionally shows the value of the environment variables which are GNU extensions. If undefined or set to `FALSE`, this information will not be shown.

Reference:

  OpenMP specification v4.5, Section 4.12

### 4.3 `OMP_DEFAULT_DEVICE` – Set the device used in target regions

Description:

  Set to choose the device which is used in a `target` region, unless the value is overridden by `omp_set_default_device` or by a `device` clause. The value shall be the nonnegative device number. If no device with the given device number exists, the code is executed on the host. If unset, device number 0 will be used.

See also:

  `omp_get_default_device` – Get the default device for target regions, `omp_set_default_device` – Set the default device for target regions,

Reference:

  OpenMP specification v4.5, Section 4.13

### 4.4 `OMP_DYNAMIC` – Dynamic adjustment of threads

Description:

  Enable or disable the dynamic adjustment of the number of threads within a team. The value of this environment variable shall be `TRUE` or `FALSE`. If undefined, dynamic adjustment is disabled by default.

See also:

  `omp_set_dynamic` – Enable/disable dynamic teams

Reference:

  OpenMP specification v4.5, Section 4.3

### 4.5 `OMP_MAX_ACTIVE_LEVELS` – Set the maximum number of nested parallel regions

Description:

  Specifies the initial value for the maximum number of nested parallel regions. The value of this variable shall be a positive integer. If undefined, then if `OMP_NESTED` is defined and set to true, or if `OMP_NUM_THREADS` or `OMP_PROC_BIND` are defined and set to a list with more than one item, the maximum number of nested parallel regions will be initialized to the largest number supported, otherwise it will be set to one.

See also:

  `omp_set_max_active_levels` – Limits the number of active parallel regions, `OMP_NESTED` – Nested parallel regions

Reference:

  OpenMP specification v4.5, Section 4.9

### 4.6 `OMP_MAX_TASK_PRIORITY` – Set the maximum priority

number that can be set for a task.

Description:

  Specifies the initial value for the maximum priority value that can be set for a task. The value of this variable shall be a non-negative integer, and zero is allowed. If undefined, the default priority is 0.

See also:

  `omp_get_max_task_priority` – Maximum priority value

Reference:

  OpenMP specification v4.5, Section 4.14

### 4.7 `OMP_NESTED` – Nested parallel regions

Description:

  Enable or disable nested parallel regions, i.e., whether team members are allowed to create new teams. The value of this environment variable shall be `TRUE` or `FALSE`. If set to `TRUE`, the number of maximum active nested regions supported will by default be set to the maximum supported, otherwise it will be set to one. If `OMP_MAX_ACTIVE_LEVELS` is defined, its setting will override this setting. If both are undefined, nested parallel regions are enabled if `OMP_NUM_THREADS` or `OMP_PROC_BINDS` are defined to a list with more than one item, otherwise they are disabled by default.

See also:

  `omp_set_max_active_levels` – Limits the number of active parallel regions, `omp_set_nested` – Enable/disable nested parallel regions

Reference:

  OpenMP specification v4.5, Section 4.6

### 4.8 `OMP_NUM_TEAMS` – Specifies the number of teams to use by teams region

Description:

  Specifies the upper bound for number of teams to use in teams regions without explicit `num_teams` clause. The value of this variable shall be a positive integer. If undefined it defaults to 0 which means implementation defined upper bound.

See also:

  `omp_set_num_teams` – Set upper teams limit for teams construct

Reference:

  OpenMP specification v5.1, Section 6.23

### 4.9 `OMP_NUM_THREADS` – Specifies the number of threads to use

Description:

  Specifies the default number of threads to use in parallel regions. The value of this variable shall be a comma-separated list of positive integers; the value specifies the number of threads to use for the corresponding nested level. Specifying more than one item in the list will automatically enable nesting by default. If undefined one thread per CPU is used.

See also:

  `omp_set_num_threads` – Set upper team size limit, `OMP_NESTED` – Nested parallel regions

Reference:

  OpenMP specification v4.5, Section 4.2

### 4.10 `OMP_PROC_BIND` – Whether threads may be moved between CPUs

Description:

  Specifies whether threads may be moved between processors. If set to `TRUE`, OpenMP threads should not be moved; if set to `FALSE` they may be moved. Alternatively, a comma separated list with the values `PRIMARY`, `MASTER`, `CLOSE` and `SPREAD` can be used to specify the thread affinity policy for the corresponding nesting level. With `PRIMARY` and `MASTER` the worker threads are in the same place partition as the primary thread. With `CLOSE` those are kept close to the primary thread in contiguous place partitions. And with `SPREAD` a sparse distribution across the place partitions is used. Specifying more than one item in the list will automatically enable nesting by default.

  When undefined, `OMP_PROC_BIND` defaults to `TRUE` when `OMP_PLACES` or `GOMP_CPU_AFFINITY` is set and `FALSE` otherwise.

See also:

  `omp_get_proc_bind` – Whether threads may be moved between CPUs, `GOMP_CPU_AFFINITY` – Bind threads to specific CPUs, `OMP_NESTED` – Nested parallel regions, `OMP_PLACES` – Specifies on which CPUs the threads should be placed

Reference:

  OpenMP specification v4.5, Section 4.4

### 4.11 `OMP_PLACES` – Specifies on which CPUs the threads should be placed

Description:

  The thread placement can be either specified using an abstract name or by an explicit list of the places. The abstract names `threads`, `cores`, `sockets`, `ll_caches` and `numa_domains` can be optionally followed by a positive number in parentheses, which denotes the how many places shall be created. With `threads` each place corresponds to a single hardware thread; `cores` to a single core with the corresponding number of hardware threads; with `sockets` the place corresponds to a single socket; with `ll_caches` to a set of cores that shares the last level cache on the device; and `numa_domains` to a set of cores for which their closest memory on the device is the same memory and at a similar distance from the cores. The resulting placement can be shown by setting the `OMP_DISPLAY_ENV` environment variable.

  Alternatively, the placement can be specified explicitly as comma-separated list of places. A place is specified by set of nonnegative numbers in curly braces, denoting the hardware threads. The curly braces can be omitted when only a single number has been specified. The hardware threads belonging to a place can either be specified as comma-separated list of nonnegative thread numbers or using an interval. Multiple places can also be either specified by a comma-separated list of places or by an interval. To specify an interval, a colon followed by the count is placed after the hardware thread number or the place. Optionally, the length can be followed by a colon and the stride number – otherwise a unit stride is assumed. Placing an exclamation mark (`!`) directly before a curly brace or numbers inside the curly braces (excluding intervals) will exclude those hardware threads.

  For instance, the following specifies the same places list: `"{0,1,2}, {3,4,6}, {7,8,9}, {10,11,12}"`; `"{0:3}, {3:3}, {7:3}, {10:3}"`; and `"{0:2}:4:3"`.

  If `OMP_PLACES` and `GOMP_CPU_AFFINITY` are unset and `OMP_PROC_BIND` is either unset or `false`, threads may be moved between CPUs following no placement policy.

See also:

  `OMP_PROC_BIND` – Whether threads may be moved between CPUs, `GOMP_CPU_AFFINITY` – Bind threads to specific CPUs, `omp_get_proc_bind` – Whether threads may be moved between CPUs, `OMP_DISPLAY_ENV` – Show OpenMP version and environment variables

Reference:

  OpenMP specification v4.5, Section 4.5

### 4.12 `OMP_STACKSIZE` – Set default thread stack size

Description:

  Set the default thread stack size in kilobytes, unless the number is suffixed by `B`, `K`, `M` or `G`, in which case the size is, respectively, in bytes, kilobytes, megabytes or gigabytes. This is different from `pthread_attr_setstacksize` which gets the number of bytes as an argument. If the stack size cannot be set due to system constraints, an error is reported and the initial stack size is left unchanged. If undefined, the stack size is system dependent.

Reference:

  OpenMP specification v4.5, Section 4.7

### 4.13 `OMP_SCHEDULE` – How threads are scheduled

Description:

  Allows to specify `schedule type` and `chunk size`. The value of the variable shall have the form: `type[,chunk]` where `type` is one of `static`, `dynamic`, `guided` or `auto` The optional `chunk` size shall be a positive integer. If undefined, dynamic scheduling and a chunk size of 1 is used.

See also:

  `omp_set_schedule` – Set the runtime scheduling method

Reference:

  OpenMP specification v4.5, Sections 2.7.1.1 and 4.1

### 4.14 `OMP_TARGET_OFFLOAD` – Controls offloading behaviour

Description:

  Specifies the behaviour with regard to offloading code to a device. This variable can be set to one of three values - `MANDATORY`, `DISABLED` or `DEFAULT`.

  If set to `MANDATORY`, the program will terminate with an error if the offload device is not present or is not supported. If set to `DISABLED`, then offloading is disabled and all code will run on the host. If set to `DEFAULT`, the program will try offloading to the device first, then fall back to running code on the host if it cannot.

  If undefined, then the program will behave as if `DEFAULT` was set.

Reference:

  OpenMP specification v5.0, Section 6.17

### 4.15 `OMP_TEAMS_THREAD_LIMIT` – Set the maximum number of threads imposed by teams

Description:

  Specifies an upper bound for the number of threads to use by each contention group created by a teams construct without explicit `thread_limit` clause. The value of this variable shall be a positive integer. If undefined, the value of 0 is used which stands for an implementation defined upper limit.

See also:

  `OMP_THREAD_LIMIT` – Set the maximum number of threads, `omp_set_teams_thread_limit` – Set upper thread limit for teams construct

Reference:

  OpenMP specification v5.1, Section 6.24

### 4.16 `OMP_THREAD_LIMIT` – Set the maximum number of threads

Description:

  Specifies the number of threads to use for the whole program. The value of this variable shall be a positive integer. If undefined, the number of threads is not limited.

See also:

  `OMP_NUM_THREADS` – Specifies the number of threads to use, `omp_get_thread_limit` – Maximum number of threads

Reference:

  OpenMP specification v4.5, Section 4.10

### 4.17 `OMP_WAIT_POLICY` – How waiting threads are handled

Description:

  Specifies whether waiting threads should be active or passive. If the value is `PASSIVE`, waiting threads should not consume CPU power while waiting; while the value is `ACTIVE` specifies that they should. If undefined, threads wait actively for a short time before waiting passively.

See also:

  `GOMP_SPINCOUNT` – Set the busy-wait spin count

Reference:

  OpenMP specification v4.5, Section 4.8

### 4.18 `GOMP_CPU_AFFINITY` – Bind threads to specific CPUs

Description:

  Binds threads to specific CPUs. The variable should contain a space-separated or comma-separated list of CPUs. This list may contain different kinds of entries: either single CPU numbers in any order, a range of CPUs (M-N) or a range with some stride (M-N:S). CPU numbers are zero based. For example, `GOMP_CPU_AFFINITY="0 3 1-2 4-15:2"` will bind the initial thread to CPU 0, the second to CPU 3, the third to CPU 1, the fourth to CPU 2, the fifth to CPU 4, the sixth through tenth to CPUs 6, 8, 10, 12, and 14 respectively and then start assigning back from the beginning of the list. `GOMP_CPU_AFFINITY=0` binds all threads to CPU 0.

  There is no libgomp library routine to determine whether a CPU affinity specification is in effect. As a workaround, language-specific library functions, e.g., `getenv` in C or `GET_ENVIRONMENT_VARIABLE` in Fortran, may be used to query the setting of the `GOMP_CPU_AFFINITY` environment variable. A defined CPU affinity on startup cannot be changed or disabled during the runtime of the application.

  If both `GOMP_CPU_AFFINITY` and `OMP_PROC_BIND` are set, `OMP_PROC_BIND` has a higher precedence. If neither has been set and `OMP_PROC_BIND` is unset, or when `OMP_PROC_BIND` is set to `FALSE`, the host system will handle the assignment of threads to CPUs.

See also:

  `OMP_PLACES` – Specifies on which CPUs the threads should be placed, `OMP_PROC_BIND` – Whether threads may be moved between CPUs

### 4.19 `GOMP_DEBUG` – Enable debugging output

Description:

  Enable debugging output. The variable should be set to `0` (disabled, also the default if not set), or `1` (enabled).

  If enabled, some debugging output will be printed during execution. This is currently not specified in more detail, and subject to change.

### 4.20 `GOMP_STACKSIZE` – Set default thread stack size

Description:

  Set the default thread stack size in kilobytes. This is different from `pthread_attr_setstacksize` which gets the number of bytes as an argument. If the stack size cannot be set due to system constraints, an error is reported and the initial stack size is left unchanged. If undefined, the stack size is system dependent.

See also:

  `OMP_STACKSIZE` – Set default thread stack size

Reference:

  GCC Patches Mailinglist, GCC Patches Mailinglist

### 4.21 `GOMP_SPINCOUNT` – Set the busy-wait spin count

Description:

  Determines how long a threads waits actively with consuming CPU power before waiting passively without consuming CPU power. The value may be either `INFINITE`, `INFINITY` to always wait actively or an integer which gives the number of spins of the busy-wait loop. The integer may optionally be followed by the following suffixes acting as multiplication factors: `k` (kilo, thousand), `M` (mega, million), `G` (giga, billion), or `T` (tera, trillion). If undefined, 0 is used when `OMP_WAIT_POLICY` is `PASSIVE`, 300,000 is used when `OMP_WAIT_POLICY` is undefined and 30 billion is used when `OMP_WAIT_POLICY` is `ACTIVE`. If there are more OpenMP threads than available CPUs, 1000 and 100 spins are used for `OMP_WAIT_POLICY` being `ACTIVE` or undefined, respectively; unless the `GOMP_SPINCOUNT` is lower or `OMP_WAIT_POLICY` is `PASSIVE`.

See also:

  `OMP_WAIT_POLICY` – How waiting threads are handled

### 4.22 `GOMP_RTEMS_THREAD_POOLS` – Set the RTEMS specific thread pools

Description:

  This environment variable is only used on the RTEMS real-time operating system. It determines the scheduler instance specific thread pools. The format for `GOMP_RTEMS_THREAD_POOLS` is a list of optional `<thread-pool-count>[$<priority>]@<scheduler-name>` configurations separated by `:` where:

  - `<thread-pool-count>` is the thread pool count for this scheduler instance.
  - `$<priority>` is an optional priority for the worker threads of a thread pool according to `pthread_setschedparam`. In case a priority value is omitted, then a worker thread will inherit the priority of the OpenMP primary thread that created it. The priority of the worker thread is not changed after creation, even if a new OpenMP primary thread using the worker has a different priority.
  - `@<scheduler-name>` is the scheduler instance name according to the RTEMS application configuration.

  In case no thread pool configuration is specified for a scheduler instance, then each OpenMP primary thread of this scheduler instance will use its own dynamically allocated thread pool. To limit the worker thread count of the thread pools, each OpenMP primary thread must call `omp_set_num_threads`.

Example:

  Lets suppose we have three scheduler instances `IO`, `WRK0`, and `WRK1` with `GOMP_RTEMS_THREAD_POOLS` set to `"1@WRK0:3$4@WRK1"`. Then there are no thread pool restrictions for scheduler instance `IO`. In the scheduler instance `WRK0` there is one thread pool available. Since no priority is specified for this scheduler instance, the worker thread inherits the priority of the OpenMP primary thread that created it. In the scheduler instance `WRK1` there are three thread pools available and their worker threads run at priority four.

## 5 Enabling OpenACC

To activate the OpenACC extensions for C/C++ and Fortran, the compile-time flag `-fopenacc` must be specified. This enables the OpenACC directive `#pragma acc` in C/C++ and `!$acc` directives in free form, `c$acc`, `*$acc` and `!$acc` directives in fixed form, `!$` conditional compilation sentinels in free form and `c$`, `*$` and `!$` sentinels in fixed form, for Fortran. The flag also arranges for automatic linking of the OpenACC runtime library (OpenACC Runtime Library Routines).

See https://gcc.gnu.org/wiki/OpenACC for more information.

A complete description of all OpenACC directives accepted may be found in the OpenACC Application Programming Interface manual, version 2.6.

## 6 OpenACC Runtime Library Routines

The runtime routines described here are defined by section 3 of the OpenACC specifications in version 2.6. They have C linkage, and do not throw exceptions. Generally, they are available only for the host, with the exception of `acc_on_device`, which is available for both the host and the acceleration device.

### 6.1 `acc_get_num_devices` – Get number of devices for given device type

Description

  This function returns a value indicating the number of devices available for the device type specified in devicetype.

C/C++:

  Prototype: | `int acc_get_num_devices(acc_device_t devicetype);`

Fortran:

  Interface: | `integer function acc_get_num_devices(devicetype)`
  | `integer(kind=acc_device_kind) devicetype`

Reference:

  OpenACC specification v2.6, section 3.2.1.

### 6.2 `acc_set_device_type` – Set type of device accelerator to use.

Description

  This function indicates to the runtime library which device type, specified in devicetype, to use when executing a parallel or kernels region.

C/C++:

  Prototype: | `acc_set_device_type(acc_device_t devicetype);`

Fortran:

  Interface: | `subroutine acc_set_device_type(devicetype)`
  | `integer(kind=acc_device_kind) devicetype`

Reference:

  OpenACC specification v2.6, section 3.2.2.

### 6.3 `acc_get_device_type` – Get type of device accelerator to be used.

Description

  This function returns what device type will be used when executing a parallel or kernels region.

  This function returns `acc_device_none` if `acc_get_device_type` is called from `acc_ev_device_init_start`, `acc_ev_device_init_end` callbacks of the OpenACC Profiling Interface (OpenACC Profiling Interface), that is, if the device is currently being initialized.

C/C++:

  Prototype: | `acc_device_t acc_get_device_type(void);`

Fortran:

  Interface: | `function acc_get_device_type(void)`
  | `integer(kind=acc_device_kind) acc_get_device_type`

Reference:

  OpenACC specification v2.6, section 3.2.3.

### 6.4 `acc_set_device_num` – Set device number to use.

Description

  This function will indicate to the runtime which device number, specified by devicenum, associated with the specified device type devicetype.

C/C++:

  Prototype: | `acc_set_device_num(int devicenum, acc_device_t devicetype);`

Fortran:

  Interface: | `subroutine acc_set_device_num(devicenum, devicetype)`
  | `integer devicenum`
  | `integer(kind=acc_device_kind) devicetype`

Reference:

  OpenACC specification v2.6, section 3.2.4.

### 6.5 `acc_get_device_num` – Get device number to be used.

Description

  This function returns which device number associated with the specified device type devicetype, will be used when executing a parallel or kernels region.

C/C++:

  Prototype: | `int acc_get_device_num(acc_device_t devicetype);`

Fortran:

  Interface: | `function acc_get_device_num(devicetype)`
  | `integer(kind=acc_device_kind) devicetype`
  | `integer acc_get_device_num`

Reference:

  OpenACC specification v2.6, section 3.2.5.

### 6.6 `acc_get_property` – Get device property.

Description

  These routines return the value of the specified property for the device being queried according to devicenum and devicetype. Integer-valued and string-valued properties are returned by `acc_get_property` and `acc_get_property_string` respectively. The Fortran `acc_get_property_string` subroutine returns the string retrieved in its fourth argument while the remaining entry points are functions, which pass the return value as their result.

  Note for Fortran, only: the OpenACC technical committee corrected and, hence, modified the interface introduced in OpenACC 2.6. The kind-value parameter `acc_device_property` has been renamed to `acc_device_property_kind` for consistency and the return type of the `acc_get_property` function is now a `c_size_t` integer instead of a `acc_device_property` integer. The parameter `acc_device_property` will continue to be provided, but might be removed in a future version of GCC.

C/C++:

  Prototype: | `size_t acc_get_property(int devicenum, acc_device_t devicetype, acc_device_property_t property);`
  Prototype: | `const char *acc_get_property_string(int devicenum, acc_device_t devicetype, acc_device_property_t property);`

Fortran:

  Interface: | `function acc_get_property(devicenum, devicetype, property)`
  Interface: | `subroutine acc_get_property_string(devicenum, devicetype, property, string)`
  | `use ISO_C_Binding, only: c_size_t`
  | `integer devicenum`
  | `integer(kind=acc_device_kind) devicetype`
  | `integer(kind=acc_device_property_kind) property`
  | `integer(kind=c_size_t) acc_get_property`
  | `character(*) string`

Reference:

  OpenACC specification v2.6, section 3.2.6.

### 6.7 `acc_async_test` – Test for completion of a specific asynchronous operation.

Description

  This function tests for completion of the asynchronous operation specified in arg. In C/C++, a non-zero value will be returned to indicate the specified asynchronous operation has completed. While Fortran will return a `true`. If the asynchronous operation has not completed, C/C++ returns a zero and Fortran returns a `false`.

C/C++:

  Prototype: | `int acc_async_test(int arg);`

Fortran:

  Interface: | `function acc_async_test(arg)`
  | `integer(kind=acc_handle_kind) arg`
  | `logical acc_async_test`

Reference:

  OpenACC specification v2.6, section 3.2.9.

### 6.8 `acc_async_test_all` – Tests for completion of all asynchronous operations.

Description

  This function tests for completion of all asynchronous operations. In C/C++, a non-zero value will be returned to indicate all asynchronous operations have completed. While Fortran will return a `true`. If any asynchronous operation has not completed, C/C++ returns a zero and Fortran returns a `false`.

C/C++:

  Prototype: | `int acc_async_test_all(void);`

Fortran:

  Interface: | `function acc_async_test()`
  | `logical acc_get_device_num`

Reference:

  OpenACC specification v2.6, section 3.2.10.

### 6.9 `acc_wait` – Wait for completion of a specific asynchronous operation.

Description

  This function waits for completion of the asynchronous operation specified in arg.

C/C++:

  Prototype: | `acc_wait(arg);`
  Prototype (OpenACC 1.0 compatibility): | `acc_async_wait(arg);`

Fortran:

  Interface: | `subroutine acc_wait(arg)`
  | `integer(acc_handle_kind) arg`
  Interface (OpenACC 1.0 compatibility): | `subroutine acc_async_wait(arg)`
  | `integer(acc_handle_kind) arg`

Reference:

  OpenACC specification v2.6, section 3.2.11.

### 6.10 `acc_wait_all` – Waits for completion of all asynchronous operations.

Description

  This function waits for the completion of all asynchronous operations.

C/C++:

  Prototype: | `acc_wait_all(void);`
  Prototype (OpenACC 1.0 compatibility): | `acc_async_wait_all(void);`

Fortran:

  Interface: | `subroutine acc_wait_all()`
  Interface (OpenACC 1.0 compatibility): | `subroutine acc_async_wait_all()`

Reference:

  OpenACC specification v2.6, section 3.2.13.

### 6.11 `acc_wait_all_async` – Wait for completion of all asynchronous operations.

Description

  This function enqueues a wait operation on the queue async for any and all asynchronous operations that have been previously enqueued on any queue.

C/C++:

  Prototype: | `acc_wait_all_async(int async);`

Fortran:

  Interface: | `subroutine acc_wait_all_async(async)`
  | `integer(acc_handle_kind) async`

Reference:

  OpenACC specification v2.6, section 3.2.14.

### 6.12 `acc_wait_async` – Wait for completion of asynchronous operations.

Description

  This function enqueues a wait operation on queue async for any and all asynchronous operations enqueued on queue arg.

C/C++:

  Prototype: | `acc_wait_async(int arg, int async);`

Fortran:

  Interface: | `subroutine acc_wait_async(arg, async)`
  | `integer(acc_handle_kind) arg, async`

Reference:

  OpenACC specification v2.6, section 3.2.12.

### 6.13 `acc_init` – Initialize runtime for a specific device type.

Description

  This function initializes the runtime for the device type specified in devicetype.

C/C++:

  Prototype: | `acc_init(acc_device_t devicetype);`

Fortran:

  Interface: | `subroutine acc_init(devicetype)`
  | `integer(acc_device_kind) devicetype`

Reference:

  OpenACC specification v2.6, section 3.2.7.

### 6.14 `acc_shutdown` – Shuts down the runtime for a specific device type.

Description

  This function shuts down the runtime for the device type specified in devicetype.

C/C++:

  Prototype: | `acc_shutdown(acc_device_t devicetype);`

Fortran:

  Interface: | `subroutine acc_shutdown(devicetype)`
  | `integer(acc_device_kind) devicetype`

Reference:

  OpenACC specification v2.6, section 3.2.8.

### 6.15 `acc_on_device` – Whether executing on a particular device

Description:

  This function returns whether the program is executing on a particular device specified in devicetype. In C/C++ a non-zero value is returned to indicate the device is executing on the specified device type. In Fortran, `true` will be returned. If the program is not executing on the specified device type C/C++ will return a zero, while Fortran will return `false`.

C/C++:

  Prototype: | `acc_on_device(acc_device_t devicetype);`

Fortran:

  Interface: | `function acc_on_device(devicetype)`
  | `integer(acc_device_kind) devicetype`
  | `logical acc_on_device`

Reference:

  OpenACC specification v2.6, section 3.2.17.

### 6.16 `acc_malloc` – Allocate device memory.

Description

  This function allocates len bytes of device memory. It returns the device address of the allocated memory.

C/C++:

  Prototype: | `d_void* acc_malloc(size_t len);`

Reference:

  OpenACC specification v2.6, section 3.2.18.

### 6.17 `acc_free` – Free device memory.

Description

  Free previously allocated device memory at the device address `a`.

C/C++:

  Prototype: | `acc_free(d_void *a);`

Reference:

  OpenACC specification v2.6, section 3.2.19.

### 6.18 `acc_copyin` – Allocate device memory and copy host memory to it.

Description

  In C/C++, this function allocates len bytes of device memory and maps it to the specified host address in a. The device address of the newly allocated device memory is returned.

  In Fortran, two (2) forms are supported. In the first form, a specifies a contiguous array section. The second form a specifies a variable or array element and len specifies the length in bytes.

C/C++:

  Prototype: | `void *acc_copyin(h_void *a, size_t len);`
  Prototype: | `void *acc_copyin_async(h_void *a, size_t len, int async);`

Fortran:

  Interface: | `subroutine acc_copyin(a)`
  | `type, dimension(:[,:]...) :: a`
  Interface: | `subroutine acc_copyin(a, len)`
  | `type, dimension(:[,:]...) :: a`
  | `integer len`
  Interface: | `subroutine acc_copyin_async(a, async)`
  | `type, dimension(:[,:]...) :: a`
  | `integer(acc_handle_kind) :: async`
  Interface: | `subroutine acc_copyin_async(a, len, async)`
  | `type, dimension(:[,:]...) :: a`
  | `integer len`
  | `integer(acc_handle_kind) :: async`

Reference:

  OpenACC specification v2.6, section 3.2.20.

### 6.19 `acc_present_or_copyin` – If the data is not present on the device, allocate device memory and copy from host memory.

Description

  This function tests if the host data specified by a and of length len is present or not. If it is not present, then device memory will be allocated and the host memory copied. The device address of the newly allocated device memory is returned.

  In Fortran, two (2) forms are supported. In the first form, a specifies a contiguous array section. The second form a specifies a variable or array element and len specifies the length in bytes.

  Note that `acc_present_or_copyin` and `acc_pcopyin` exist for backward compatibility with OpenACC 2.0; use `acc_copyin` – Allocate device memory and copy host memory to it. instead.

C/C++:

  Prototype: | `void *acc_present_or_copyin(h_void *a, size_t len);`
  Prototype: | `void *acc_pcopyin(h_void *a, size_t len);`

Fortran:

  Interface: | `subroutine acc_present_or_copyin(a)`
  | `type, dimension(:[,:]...) :: a`
  Interface: | `subroutine acc_present_or_copyin(a, len)`
  | `type, dimension(:[,:]...) :: a`
  | `integer len`
  Interface: | `subroutine acc_pcopyin(a)`
  | `type, dimension(:[,:]...) :: a`
  Interface: | `subroutine acc_pcopyin(a, len)`
  | `type, dimension(:[,:]...) :: a`
  | `integer len`

Reference:

  OpenACC specification v2.6, section 3.2.20.

### 6.20 `acc_create` – Allocate device memory and map it to host memory.

Description

  This function allocates device memory and maps it to host memory specified by the host address a with a length of len bytes. In C/C++, the function returns the device address of the allocated device memory.

  In Fortran, two (2) forms are supported. In the first form, a specifies a contiguous array section. The second form a specifies a variable or array element and len specifies the length in bytes.

C/C++:

  Prototype: | `void *acc_create(h_void *a, size_t len);`
  Prototype: | `void *acc_create_async(h_void *a, size_t len, int async);`

Fortran:

  Interface: | `subroutine acc_create(a)`
  | `type, dimension(:[,:]...) :: a`
  Interface: | `subroutine acc_create(a, len)`
  | `type, dimension(:[,:]...) :: a`
  | `integer len`
  Interface: | `subroutine acc_create_async(a, async)`
  | `type, dimension(:[,:]...) :: a`
  | `integer(acc_handle_kind) :: async`
  Interface: | `subroutine acc_create_async(a, len, async)`
  | `type, dimension(:[,:]...) :: a`
  | `integer len`
  | `integer(acc_handle_kind) :: async`

Reference:

  OpenACC specification v2.6, section 3.2.21.

### 6.21 `acc_present_or_create` – If the data is not present on the device, allocate device memory and map it to host memory.

Description

  This function tests if the host data specified by a and of length len is present or not. If it is not present, then device memory will be allocated and mapped to host memory. In C/C++, the device address of the newly allocated device memory is returned.

  In Fortran, two (2) forms are supported. In the first form, a specifies a contiguous array section. The second form a specifies a variable or array element and len specifies the length in bytes.

  Note that `acc_present_or_create` and `acc_pcreate` exist for backward compatibility with OpenACC 2.0; use `acc_create` – Allocate device memory and map it to host memory. instead.

C/C++:

  Prototype: | `void *acc_present_or_create(h_void *a, size_t len)`
  Prototype: | `void *acc_pcreate(h_void *a, size_t len)`

Fortran:

  Interface: | `subroutine acc_present_or_create(a)`
  | `type, dimension(:[,:]...) :: a`
  Interface: | `subroutine acc_present_or_create(a, len)`
  | `type, dimension(:[,:]...) :: a`
  | `integer len`
  Interface: | `subroutine acc_pcreate(a)`
  | `type, dimension(:[,:]...) :: a`
  Interface: | `subroutine acc_pcreate(a, len)`
  | `type, dimension(:[,:]...) :: a`
  | `integer len`

Reference:

  OpenACC specification v2.6, section 3.2.21.

### 6.22 `acc_copyout` – Copy device memory to host memory.

Description

  This function copies mapped device memory to host memory which is specified by host address a for a length len bytes in C/C++.

  In Fortran, two (2) forms are supported. In the first form, a specifies a contiguous array section. The second form a specifies a variable or array element and len specifies the length in bytes.

C/C++:

  Prototype: | `acc_copyout(h_void *a, size_t len);`
  Prototype: | `acc_copyout_async(h_void *a, size_t len, int async);`
  Prototype: | `acc_copyout_finalize(h_void *a, size_t len);`
  Prototype: | `acc_copyout_finalize_async(h_void *a, size_t len, int async);`

Fortran:

  Interface: | `subroutine acc_copyout(a)`
  | `type, dimension(:[,:]...) :: a`
  Interface: | `subroutine acc_copyout(a, len)`
  | `type, dimension(:[,:]...) :: a`
  | `integer len`
  Interface: | `subroutine acc_copyout_async(a, async)`
  | `type, dimension(:[,:]...) :: a`
  | `integer(acc_handle_kind) :: async`
  Interface: | `subroutine acc_copyout_async(a, len, async)`
  | `type, dimension(:[,:]...) :: a`
  | `integer len`
  | `integer(acc_handle_kind) :: async`
  Interface: | `subroutine acc_copyout_finalize(a)`
  | `type, dimension(:[,:]...) :: a`
  Interface: | `subroutine acc_copyout_finalize(a, len)`
  | `type, dimension(:[,:]...) :: a`
  | `integer len`
  Interface: | `subroutine acc_copyout_finalize_async(a, async)`
  | `type, dimension(:[,:]...) :: a`
  | `integer(acc_handle_kind) :: async`
  Interface: | `subroutine acc_copyout_finalize_async(a, len, async)`
  | `type, dimension(:[,:]...) :: a`
  | `integer len`
  | `integer(acc_handle_kind) :: async`

Reference:

  OpenACC specification v2.6, section 3.2.22.

### 6.23 `acc_delete` – Free device memory.

Description

  This function frees previously allocated device memory specified by the device address a and the length of len bytes.

  In Fortran, two (2) forms are supported. In the first form, a specifies a contiguous array section. The second form a specifies a variable or array element and len specifies the length in bytes.

C/C++:

  Prototype: | `acc_delete(h_void *a, size_t len);`
  Prototype: | `acc_delete_async(h_void *a, size_t len, int async);`
  Prototype: | `acc_delete_finalize(h_void *a, size_t len);`
  Prototype: | `acc_delete_finalize_async(h_void *a, size_t len, int async);`

Fortran:

  Interface: | `subroutine acc_delete(a)`
  | `type, dimension(:[,:]...) :: a`
  Interface: | `subroutine acc_delete(a, len)`
  | `type, dimension(:[,:]...) :: a`
  | `integer len`
  Interface: | `subroutine acc_delete_async(a, async)`
  | `type, dimension(:[,:]...) :: a`
  | `integer(acc_handle_kind) :: async`
  Interface: | `subroutine acc_delete_async(a, len, async)`
  | `type, dimension(:[,:]...) :: a`
  | `integer len`
  | `integer(acc_handle_kind) :: async`
  Interface: | `subroutine acc_delete_finalize(a)`
  | `type, dimension(:[,:]...) :: a`
  Interface: | `subroutine acc_delete_finalize(a, len)`
  | `type, dimension(:[,:]...) :: a`
  | `integer len`
  Interface: | `subroutine acc_delete_async_finalize(a, async)`
  | `type, dimension(:[,:]...) :: a`
  | `integer(acc_handle_kind) :: async`
  Interface: | `subroutine acc_delete_async_finalize(a, len, async)`
  | `type, dimension(:[,:]...) :: a`
  | `integer len`
  | `integer(acc_handle_kind) :: async`

Reference:

  OpenACC specification v2.6, section 3.2.23.

### 6.24 `acc_update_device` – Update device memory from mapped host memory.

Description

  This function updates the device copy from the previously mapped host memory. The host memory is specified with the host address a and a length of len bytes.

  In Fortran, two (2) forms are supported. In the first form, a specifies a contiguous array section. The second form a specifies a variable or array element and len specifies the length in bytes.

C/C++:

  Prototype: | `acc_update_device(h_void *a, size_t len);`
  Prototype: | `acc_update_device(h_void *a, size_t len, async);`

Fortran:

  Interface: | `subroutine acc_update_device(a)`
  | `type, dimension(:[,:]...) :: a`
  Interface: | `subroutine acc_update_device(a, len)`
  | `type, dimension(:[,:]...) :: a`
  | `integer len`
  Interface: | `subroutine acc_update_device_async(a, async)`
  | `type, dimension(:[,:]...) :: a`
  | `integer(acc_handle_kind) :: async`
  Interface: | `subroutine acc_update_device_async(a, len, async)`
  | `type, dimension(:[,:]...) :: a`
  | `integer len`
  | `integer(acc_handle_kind) :: async`

Reference:

  OpenACC specification v2.6, section 3.2.24.

### 6.25 `acc_update_self` – Update host memory from mapped device memory.

Description

  This function updates the host copy from the previously mapped device memory. The host memory is specified with the host address a and a length of len bytes.

  In Fortran, two (2) forms are supported. In the first form, a specifies a contiguous array section. The second form a specifies a variable or array element and len specifies the length in bytes.

C/C++:

  Prototype: | `acc_update_self(h_void *a, size_t len);`
  Prototype: | `acc_update_self_async(h_void *a, size_t len, int async);`

Fortran:

  Interface: | `subroutine acc_update_self(a)`
  | `type, dimension(:[,:]...) :: a`
  Interface: | `subroutine acc_update_self(a, len)`
  | `type, dimension(:[,:]...) :: a`
  | `integer len`
  Interface: | `subroutine acc_update_self_async(a, async)`
  | `type, dimension(:[,:]...) :: a`
  | `integer(acc_handle_kind) :: async`
  Interface: | `subroutine acc_update_self_async(a, len, async)`
  | `type, dimension(:[,:]...) :: a`
  | `integer len`
  | `integer(acc_handle_kind) :: async`

Reference:

  OpenACC specification v2.6, section 3.2.25.

### 6.26 `acc_map_data` – Map previously allocated device memory to host memory.

Description

  This function maps previously allocated device and host memory. The device memory is specified with the device address d. The host memory is specified with the host address h and a length of len.

C/C++:

  Prototype: | `acc_map_data(h_void *h, d_void *d, size_t len);`

Reference:

  OpenACC specification v2.6, section 3.2.26.

### 6.27 `acc_unmap_data` – Unmap device memory from host memory.

Description

  This function unmaps previously mapped device and host memory. The latter specified by h.

C/C++:

  Prototype: | `acc_unmap_data(h_void *h);`

Reference:

  OpenACC specification v2.6, section 3.2.27.

### 6.28 `acc_deviceptr` – Get device pointer associated with specific host address.

Description

  This function returns the device address that has been mapped to the host address specified by h.

C/C++:

  Prototype: | `void *acc_deviceptr(h_void *h);`

Reference:

  OpenACC specification v2.6, section 3.2.28.

### 6.29 `acc_hostptr` – Get host pointer associated with specific device address.

Description

  This function returns the host address that has been mapped to the device address specified by d.

C/C++:

  Prototype: | `void *acc_hostptr(d_void *d);`

Reference:

  OpenACC specification v2.6, section 3.2.29.

### 6.30 `acc_is_present` – Indicate whether host variable / array is present on device.

Description

  This function indicates whether the specified host address in a and a length of len bytes is present on the device. In C/C++, a non-zero value is returned to indicate the presence of the mapped memory on the device. A zero is returned to indicate the memory is not mapped on the device.

  In Fortran, two (2) forms are supported. In the first form, a specifies a contiguous array section. The second form a specifies a variable or array element and len specifies the length in bytes. If the host memory is mapped to device memory, then a `true` is returned. Otherwise, a `false` is return to indicate the mapped memory is not present.

C/C++:

  Prototype: | `int acc_is_present(h_void *a, size_t len);`

Fortran:

  Interface: | `function acc_is_present(a)`
  | `type, dimension(:[,:]...) :: a`
  | `logical acc_is_present`
  Interface: | `function acc_is_present(a, len)`
  | `type, dimension(:[,:]...) :: a`
  | `integer len`
  | `logical acc_is_present`

Reference:

  OpenACC specification v2.6, section 3.2.30.

### 6.31 `acc_memcpy_to_device` – Copy host memory to device memory.

Description

  This function copies host memory specified by host address of src to device memory specified by the device address dest for a length of bytes bytes.

C/C++:

  Prototype: | `acc_memcpy_to_device(d_void *dest, h_void *src, size_t bytes);`

Reference:

  OpenACC specification v2.6, section 3.2.31.

### 6.32 `acc_memcpy_from_device` – Copy device memory to host memory.

Description

  This function copies host memory specified by host address of src from device memory specified by the device address dest for a length of bytes bytes.

C/C++:

  Prototype: | `acc_memcpy_from_device(d_void *dest, h_void *src, size_t bytes);`

Reference:

  OpenACC specification v2.6, section 3.2.32.

### 6.33 `acc_attach` – Let device pointer point to device-pointer target.

Description

  This function updates a pointer on the device from pointing to a host-pointer address to pointing to the corresponding device data.

C/C++:

  Prototype: | `acc_attach(h_void **ptr);`
  Prototype: | `acc_attach_async(h_void **ptr, int async);`

Reference:

  OpenACC specification v2.6, section 3.2.34.

### 6.34 `acc_detach` – Let device pointer point to host-pointer target.

Description

  This function updates a pointer on the device from pointing to a device-pointer address to pointing to the corresponding host data.

C/C++:

  Prototype: | `acc_detach(h_void **ptr);`
  Prototype: | `acc_detach_async(h_void **ptr, int async);`
  Prototype: | `acc_detach_finalize(h_void **ptr);`
  Prototype: | `acc_detach_finalize_async(h_void **ptr, int async);`

Reference:

  OpenACC specification v2.6, section 3.2.35.

### 6.35 `acc_get_current_cuda_device` – Get CUDA device handle.

Description

  This function returns the CUDA device handle. This handle is the same as used by the CUDA Runtime or Driver API’s.

C/C++:

  Prototype: | `void *acc_get_current_cuda_device(void);`

Reference:

  OpenACC specification v2.6, section A.2.1.1.

### 6.36 `acc_get_current_cuda_context` – Get CUDA context handle.

Description

  This function returns the CUDA context handle. This handle is the same as used by the CUDA Runtime or Driver API’s.

C/C++:

  Prototype: | `void *acc_get_current_cuda_context(void);`

Reference:

  OpenACC specification v2.6, section A.2.1.2.

### 6.37 `acc_get_cuda_stream` – Get CUDA stream handle.

Description

  This function returns the CUDA stream handle for the queue async. This handle is the same as used by the CUDA Runtime or Driver API’s.

C/C++:

  Prototype: | `void *acc_get_cuda_stream(int async);`

Reference:

  OpenACC specification v2.6, section A.2.1.3.

### 6.38 `acc_set_cuda_stream` – Set CUDA stream handle.

Description

  This function associates the stream handle specified by stream with the queue async.

  This cannot be used to change the stream handle associated with `acc_async_sync`.

  The return value is not specified.

C/C++:

  Prototype: | `int acc_set_cuda_stream(int async, void *stream);`

Reference:

  OpenACC specification v2.6, section A.2.1.4.

### 6.39 `acc_prof_register` – Register callbacks.

Description:

  This function registers callbacks.

C/C++:

  Prototype: | `void acc_prof_register (acc_event_t, acc_prof_callback, acc_register_t);`

See also:

  OpenACC Profiling Interface

Reference:

  OpenACC specification v2.6, section 5.3.

### 6.40 `acc_prof_unregister` – Unregister callbacks.

Description:

  This function unregisters callbacks.

C/C++:

  Prototype: | `void acc_prof_unregister (acc_event_t, acc_prof_callback, acc_register_t);`

See also:

  OpenACC Profiling Interface

Reference:

  OpenACC specification v2.6, section 5.3.

### 6.41 `acc_prof_lookup` – Obtain inquiry functions.

Description:

  Function to obtain inquiry functions.

C/C++:

  Prototype: | `acc_query_fn acc_prof_lookup (const char *);`

See also:

  OpenACC Profiling Interface

Reference:

  OpenACC specification v2.6, section 5.3.

### 6.42 `acc_register_library` – Library registration.

Description:

  Function for library registration.

C/C++:

  Prototype: | `void acc_register_library (acc_prof_reg, acc_prof_reg, acc_prof_lookup_func);`

See also:

  OpenACC Profiling Interface, `ACC_PROFLIB`

Reference:

  OpenACC specification v2.6, section 5.3.

## 7 OpenACC Environment Variables

The variables `ACC_DEVICE_TYPE` and `ACC_DEVICE_NUM` are defined by section 4 of the OpenACC specification in version 2.0. The variable `ACC_PROFLIB` is defined by section 4 of the OpenACC specification in version 2.6. The variable `GCC_ACC_NOTIFY` is used for diagnostic purposes.

### 7.1 `ACC_DEVICE_TYPE`

Reference:

  OpenACC specification v2.6, section 4.1.

### 7.2 `ACC_DEVICE_NUM`

Reference:

  OpenACC specification v2.6, section 4.2.

### 7.3 `ACC_PROFLIB`

See also:

  `acc_register_library` – Library registration., OpenACC Profiling Interface

Reference:

  OpenACC specification v2.6, section 4.3.

### 7.4 `GCC_ACC_NOTIFY`

Description:

  Print debug information pertaining to the accelerator.

## 8 CUDA Streams Usage

This applies to the `nvptx` plugin only.

The library provides elements that perform asynchronous movement of data and asynchronous operation of computing constructs. This asynchronous functionality is implemented by making use of CUDA streams1.

The primary means by that the asynchronous functionality is accessed is through the use of those OpenACC directives which make use of the `async` and `wait` clauses. When the `async` clause is first used with a directive, it creates a CUDA stream. If an `async-argument` is used with the `async` clause, then the stream is associated with the specified `async-argument`.

Following the creation of an association between a CUDA stream and the `async-argument` of an `async` clause, both the `wait` clause and the `wait` directive can be used. When either the clause or directive is used after stream creation, it creates a rendezvous point whereby execution waits until all operations associated with the `async-argument`, that is, stream, have completed.

Normally, the management of the streams that are created as a result of using the `async` clause, is done without any intervention by the caller. This implies the association between the `async-argument` and the CUDA stream will be maintained for the lifetime of the program. However, this association can be changed through the use of the library function `acc_set_cuda_stream`. When the function `acc_set_cuda_stream` is called, the CUDA stream that was originally associated with the `async` clause will be destroyed. Caution should be taken when changing the association as subsequent references to the `async-argument` refer to a different CUDA stream.

## 9 OpenACC Library Interoperability

### 9.1 Introduction

The OpenACC library uses the CUDA Driver API, and may interact with programs that use the Runtime library directly, or another library based on the Runtime library, e.g., CUBLAS2. This chapter describes the use cases and what changes are required in order to use both the OpenACC library and the CUBLAS and Runtime libraries within a program.

### 9.2 First invocation: NVIDIA CUBLAS library API

In this first use case (see below), a function in the CUBLAS library is called prior to any of the functions in the OpenACC library. More specifically, the function `cublasCreate()`.

When invoked, the function initializes the library and allocates the hardware resources on the host and the device on behalf of the caller. Once the initialization and allocation has completed, a handle is returned to the caller. The OpenACC library also requires initialization and allocation of hardware resources. Since the CUBLAS library has already allocated the hardware resources for the device, all that is left to do is to initialize the OpenACC library and acquire the hardware resources on the host.

Prior to calling the OpenACC function that initializes the library and allocate the host hardware resources, you need to acquire the device number that was allocated during the call to `cublasCreate()`. The invoking of the runtime library function `cudaGetDevice()` accomplishes this. Once acquired, the device number is passed along with the device type as parameters to the OpenACC library function `acc_set_device_num()`.

Once the call to `acc_set_device_num()` has completed, the OpenACC library uses the context that was created during the call to `cublasCreate()`. In other words, both libraries will be sharing the same context.

        /* Create the handle */
        s = cublasCreate(&h);
        if (s != CUBLAS_STATUS_SUCCESS)
        {
            fprintf(stderr, "cublasCreate failed %d\n", s);
            exit(EXIT_FAILURE);
        }

        /* Get the device number */
        e = cudaGetDevice(&dev);
        if (e != cudaSuccess)
        {
            fprintf(stderr, "cudaGetDevice failed %d\n", e);
            exit(EXIT_FAILURE);
        }

        /* Initialize OpenACC library and use device 'dev' */
        acc_set_device_num(dev, acc_device_nvidia);

Use Case 1

### 9.3 First invocation: OpenACC library API

In this second use case (see below), a function in the OpenACC library is called prior to any of the functions in the CUBLAS library. More specificially, the function `acc_set_device_num()`.

In the use case presented here, the function `acc_set_device_num()` is used to both initialize the OpenACC library and allocate the hardware resources on the host and the device. In the call to the function, the call parameters specify which device to use and what device type to use, i.e., `acc_device_nvidia`. It should be noted that this is but one method to initialize the OpenACC library and allocate the appropriate hardware resources. Other methods are available through the use of environment variables and these will be discussed in the next section.

Once the call to `acc_set_device_num()` has completed, other OpenACC functions can be called as seen with multiple calls being made to `acc_copyin()`. In addition, calls can be made to functions in the CUBLAS library. In the use case a call to `cublasCreate()` is made subsequent to the calls to `acc_copyin()`. As seen in the previous use case, a call to `cublasCreate()` initializes the CUBLAS library and allocates the hardware resources on the host and the device. However, since the device has already been allocated, `cublasCreate()` will only initialize the CUBLAS library and allocate the appropriate hardware resources on the host. The context that was created as part of the OpenACC initialization is shared with the CUBLAS library, similarly to the first use case.

        dev = 0;

        acc_set_device_num(dev, acc_device_nvidia);

        /* Copy the first set to the device */
        d_X = acc_copyin(&h_X[0], N * sizeof (float));
        if (d_X == NULL)
        {
            fprintf(stderr, "copyin error h_X\n");
            exit(EXIT_FAILURE);
        }

        /* Copy the second set to the device */
        d_Y = acc_copyin(&h_Y1[0], N * sizeof (float));
        if (d_Y == NULL)
        {
            fprintf(stderr, "copyin error h_Y1\n");
            exit(EXIT_FAILURE);
        }

        /* Create the handle */
        s = cublasCreate(&h);
        if (s != CUBLAS_STATUS_SUCCESS)
        {
            fprintf(stderr, "cublasCreate failed %d\n", s);
            exit(EXIT_FAILURE);
        }

        /* Perform saxpy using CUBLAS library function */
        s = cublasSaxpy(h, N, &alpha, d_X, 1, d_Y, 1);
        if (s != CUBLAS_STATUS_SUCCESS)
        {
            fprintf(stderr, "cublasSaxpy failed %d\n", s);
            exit(EXIT_FAILURE);
        }

        /* Copy the results from the device */
        acc_memcpy_from_device(&h_Y1[0], d_Y, N * sizeof (float));

Use Case 2

### 9.4 OpenACC library and environment variables

There are two environment variables associated with the OpenACC library that may be used to control the device type and device number: `ACC_DEVICE_TYPE` and `ACC_DEVICE_NUM`, respectively. These two environment variables can be used as an alternative to calling `acc_set_device_num()`. As seen in the second use case, the device type and device number were specified using `acc_set_device_num()`. If however, the aforementioned environment variables were set, then the call to `acc_set_device_num()` would not be required.

The use of the environment variables is only relevant when an OpenACC function is called prior to a call to `cudaCreate()`. If `cudaCreate()` is called prior to a call to an OpenACC function, then you must call `acc_set_device_num()`3

## 10 OpenACC Profiling Interface

### 10.1 Implementation Status and Implementation-Defined Behavior

We’re implementing the OpenACC Profiling Interface as defined by the OpenACC 2.6 specification. We’re clarifying some aspects here as implementation-defined behavior, while they’re still under discussion within the OpenACC Technical Committee.

This implementation is tuned to keep the performance impact as low as possible for the (very common) case that the Profiling Interface is not enabled. This is relevant, as the Profiling Interface affects all the hot code paths (in the target code, not in the offloaded code). Users of the OpenACC Profiling Interface can be expected to understand that performance will be impacted to some degree once the Profiling Interface has gotten enabled: for example, because of the runtime (libgomp) calling into a third-party library for every event that has been registered.

We’re not yet accounting for the fact that OpenACC events may occur during event processing. We just handle one case specially, as required by CUDA 9.0 `nvprof`, that `acc_get_device_type` (`acc_get_device_type` – Get type of device accelerator to be used.)) may be called from `acc_ev_device_init_start`, `acc_ev_device_init_end` callbacks.

We’re not yet implementing initialization via a `acc_register_library` function that is either statically linked in, or dynamically via `LD_PRELOAD`. Initialization via `acc_register_library` functions dynamically loaded via the `ACC_PROFLIB` environment variable does work, as does directly calling `acc_prof_register`, `acc_prof_unregister`, `acc_prof_lookup`.

As currently there are no inquiry functions defined, calls to `acc_prof_lookup` will always return `NULL`.

There aren’t separate start, stop events defined for the event types `acc_ev_create`, `acc_ev_delete`, `acc_ev_alloc`, `acc_ev_free`. It’s not clear if these should be triggered before or after the actual device-specific call is made. We trigger them after.

Remarks about data provided to callbacks:

`acc_prof_info.event_type`

  It’s not clear if for nested event callbacks (for example, `acc_ev_enqueue_launch_start` as part of a parent compute construct), this should be set for the nested event (`acc_ev_enqueue_launch_start`), or if the value of the parent construct should remain (`acc_ev_compute_construct_start`). In this implementation, the value will generally correspond to the innermost nested event type.

`acc_prof_info.device_type`
  - For `acc_ev_compute_construct_start`, and in presence of an `if` clause with false argument, this will still refer to the offloading device type. It’s not clear if that’s the expected behavior.
  - Complementary to the item before, for `acc_ev_compute_construct_end`, this is set to `acc_device_host` in presence of an `if` clause with false argument. It’s not clear if that’s the expected behavior.
`acc_prof_info.thread_id`

  Always `-1`; not yet implemented.

`acc_prof_info.async`
  - Not yet implemented correctly for `acc_ev_compute_construct_start`.
  - In a compute construct, for host-fallback execution/`acc_device_host` it will always be `acc_async_sync`. It’s not clear if that’s the expected behavior.
  - For `acc_ev_device_init_start` and `acc_ev_device_init_end`, it will always be `acc_async_sync`. It’s not clear if that’s the expected behavior.
`acc_prof_info.async_queue`

  There is no limited number of asynchronous queues in libgomp. This will always have the same value as `acc_prof_info.async`.

`acc_prof_info.src_file`

  Always `NULL`; not yet implemented.

`acc_prof_info.func_name`

  Always `NULL`; not yet implemented.

`acc_prof_info.line_no`

  Always `-1`; not yet implemented.

`acc_prof_info.end_line_no`

  Always `-1`; not yet implemented.

`acc_prof_info.func_line_no`

  Always `-1`; not yet implemented.

`acc_prof_info.func_end_line_no`

  Always `-1`; not yet implemented.

`acc_event_info.event_type`, `acc_event_info.*.event_type`

  Relating to `acc_prof_info.event_type` discussed above, in this implementation, this will always be the same value as `acc_prof_info.event_type`.

`acc_event_info.*.parent_construct`
  - Will be `acc_construct_parallel` for all OpenACC compute constructs as well as many OpenACC Runtime API calls; should be the one matching the actual construct, or `acc_construct_runtime_api`, respectively.
  - Will be `acc_construct_enter_data` or `acc_construct_exit_data` when processing variable mappings specified in OpenACC declare directives; should be `acc_construct_declare`.
  - For implicit `acc_ev_device_init_start`, `acc_ev_device_init_end`, and explicit as well as implicit `acc_ev_alloc`, `acc_ev_free`, `acc_ev_enqueue_upload_start`, `acc_ev_enqueue_upload_end`, `acc_ev_enqueue_download_start`, and `acc_ev_enqueue_download_end`, will be `acc_construct_parallel`; should reflect the real parent construct.
`acc_event_info.*.implicit`

  For `acc_ev_alloc`, `acc_ev_free`, `acc_ev_enqueue_upload_start`, `acc_ev_enqueue_upload_end`, `acc_ev_enqueue_download_start`, and `acc_ev_enqueue_download_end`, this currently will be `1` also for explicit usage.

`acc_event_info.data_event.var_name`

  Always `NULL`; not yet implemented.

`acc_event_info.data_event.host_ptr`

  For `acc_ev_alloc`, and `acc_ev_free`, this is always `NULL`.

`typedef union acc_api_info`

  … as printed in 5.2.3. Third Argument: API-Specific Information. This should obviously be `typedef struct acc_api_info`.

`acc_api_info.device_api`

  Possibly not yet implemented correctly for `acc_ev_compute_construct_start`, `acc_ev_device_init_start`, `acc_ev_device_init_end`: will always be `acc_device_api_none` for these event types. For `acc_ev_enter_data_start`, it will be `acc_device_api_none` in some cases.

`acc_api_info.device_type`

  Always the same as `acc_prof_info.device_type`.

`acc_api_info.vendor`

  Always `-1`; not yet implemented.

`acc_api_info.device_handle`

  Always `NULL`; not yet implemented.

`acc_api_info.context_handle`

  Always `NULL`; not yet implemented.

`acc_api_info.async_handle`

  Always `NULL`; not yet implemented.

Remarks about certain event types:

`acc_ev_device_init_start`, `acc_ev_device_init_end`
  - When a compute construct triggers implicit `acc_ev_device_init_start` and `acc_ev_device_init_end` events, they currently aren’t nested within the corresponding `acc_ev_compute_construct_start` and `acc_ev_compute_construct_end`, but they’re currently observed before `acc_ev_compute_construct_start`. It’s not clear what to do: the standard asks us provide a lot of details to the `acc_ev_compute_construct_start` callback, without (implicitly) initializing a device before?
  - Callbacks for these event types will not be invoked for calls to the `acc_set_device_type` and `acc_set_device_num` functions. It’s not clear if they should be.
`acc_ev_enter_data_start`, `acc_ev_enter_data_end`, `acc_ev_exit_data_start`, `acc_ev_exit_data_end`
  - Callbacks for these event types will also be invoked for OpenACC host_data constructs. It’s not clear if they should be.
  - Callbacks for these event types will also be invoked when processing variable mappings specified in OpenACC declare directives. It’s not clear if they should be.

Callbacks for the following event types will be invoked, but dispatch and information provided therein has not yet been thoroughly reviewed:

- `acc_ev_alloc`
- `acc_ev_free`
- `acc_ev_update_start`, `acc_ev_update_end`
- `acc_ev_enqueue_upload_start`, `acc_ev_enqueue_upload_end`
- `acc_ev_enqueue_download_start`, `acc_ev_enqueue_download_end`

During device initialization, and finalization, respectively, callbacks for the following event types will not yet be invoked:

- `acc_ev_alloc`
- `acc_ev_free`

Callbacks for the following event types have not yet been implemented, so currently won’t be invoked:

- `acc_ev_device_shutdown_start`, `acc_ev_device_shutdown_end`
- `acc_ev_runtime_shutdown`
- `acc_ev_create`, `acc_ev_delete`
- `acc_ev_wait_start`, `acc_ev_wait_end`

For the following runtime library functions, not all expected callbacks will be invoked (mostly concerning implicit device initialization):

- `acc_get_num_devices`
- `acc_set_device_type`
- `acc_get_device_type`
- `acc_set_device_num`
- `acc_get_device_num`
- `acc_init`
- `acc_shutdown`

Aside from implicit device initialization, for the following runtime library functions, no callbacks will be invoked for shared-memory offloading devices (it’s not clear if they should be):

- `acc_malloc`
- `acc_free`
- `acc_copyin`, `acc_present_or_copyin`, `acc_copyin_async`
- `acc_create`, `acc_present_or_create`, `acc_create_async`
- `acc_copyout`, `acc_copyout_async`, `acc_copyout_finalize`, `acc_copyout_finalize_async`
- `acc_delete`, `acc_delete_async`, `acc_delete_finalize`, `acc_delete_finalize_async`
- `acc_update_device`, `acc_update_device_async`
- `acc_update_self`, `acc_update_self_async`
- `acc_map_data`, `acc_unmap_data`
- `acc_memcpy_to_device`, `acc_memcpy_to_device_async`
- `acc_memcpy_from_device`, `acc_memcpy_from_device_async`

## 11 The libgomp ABI

The following sections present notes on the external ABI as presented by libgomp. Only maintainers should need them.

### 11.1 Implementing MASTER construct

    if (omp_get_thread_num () == 0)
      block

Alternately, we generate two copies of the parallel subfunction and only include this in the version run by the primary thread. Surely this is not worthwhile though...

### 11.2 Implementing CRITICAL construct

Without a specified name,

      void GOMP_critical_start (void);
      void GOMP_critical_end (void);

so that we don’t get COPY relocations from libgomp to the main application.

With a specified name, use omp_set_lock and omp_unset_lock with name being transformed into a variable declared like

      omp_lock_t gomp_critical_user_<name> __attribute__((common))

Ideally the ABI would specify that all zero is a valid unlocked state, and so we wouldn’t need to initialize this at startup.

### 11.3 Implementing ATOMIC construct

The target should implement the `__sync` builtins.

Failing that we could add

      void GOMP_atomic_enter (void)
      void GOMP_atomic_exit (void)

which reuses the regular lock code, but with yet another lock object private to the library.

### 11.4 Implementing FLUSH construct

Expands to the `__sync_synchronize` builtin.

### 11.5 Implementing BARRIER construct

      void GOMP_barrier (void)

### 11.6 Implementing THREADPRIVATE construct

In _most_ cases we can map this directly to `__thread`. Except that OMP allows constructors for C++ objects. We can either refuse to support this (how often is it used?) or we can implement something akin to .ctors.

Even more ideally, this ctor feature is handled by extensions to the main pthreads library. Failing that, we can have a set of entry points to register ctor functions to be called.

### 11.7 Implementing PRIVATE clause

In association with a PARALLEL, or within the lexical extent of a PARALLEL block, the variable becomes a local variable in the parallel subfunction.

In association with FOR or SECTIONS blocks, create a new automatic variable within the current function. This preserves the semantic of new variable creation.

### 11.8 Implementing FIRSTPRIVATE LASTPRIVATE COPYIN and COPYPRIVATE clauses

This seems simple enough for PARALLEL blocks. Create a private struct for communicating between the parent and subfunction. In the parent, copy in values for scalar and "small" structs; copy in addresses for others TREE_ADDRESSABLE types. In the subfunction, copy the value into the local variable.

It is not clear what to do with bare FOR or SECTION blocks. The only thing I can figure is that we do something like:

    #pragma omp for firstprivate(x) lastprivate(y)
    for (int i = 0; i < n; ++i)
      body;

which becomes

    {
      int x = x, y;

      // for stuff

      if (i == n)
        y = y;
    }

where the "x=x" and "y=y" assignments actually have different uids for the two variables, i.e. not something you could write directly in C. Presumably this only makes sense if the "outer" x and y are global variables.

COPYPRIVATE would work the same way, except the structure broadcast would have to happen via SINGLE machinery instead.

### 11.9 Implementing REDUCTION clause

The private struct mentioned in the previous section should have a pointer to an array of the type of the variable, indexed by the thread’s team_id. The thread stores its final value into the array, and after the barrier, the primary thread iterates over the array to collect the values.

### 11.10 Implementing PARALLEL construct

      #pragma omp parallel
      {
        body;
      }

becomes

      void subfunction (void *data)
      {
        use data;
        body;
      }

      setup data;
      GOMP_parallel_start (subfunction, &data, num_threads);
      subfunction (&data);
      GOMP_parallel_end ();

      void GOMP_parallel_start (void (*fn)(void *), void *data, unsigned num_threads)

The FN argument is the subfunction to be run in parallel.

The DATA argument is a pointer to a structure used to communicate data in and out of the subfunction, as discussed above with respect to FIRSTPRIVATE et al.

The NUM_THREADS argument is 1 if an IF clause is present and false, or the value of the NUM_THREADS clause, if present, or 0.

The function needs to create the appropriate number of threads and/or launch them from the dock. It needs to create the team structure and assign team ids.

      void GOMP_parallel_end (void)

Tears down the team and returns us to the previous `omp_in_parallel()` state.

### 11.11 Implementing FOR construct

      #pragma omp parallel for
      for (i = lb; i <= ub; i++)
        body;

becomes

      void subfunction (void *data)
      {
        long _s0, _e0;
        while (GOMP_loop_static_next (&_s0, &_e0))
        {
          long _e1 = _e0, i;
          for (i = _s0; i < _e1; i++)
            body;
        }
        GOMP_loop_end_nowait ();
      }

      GOMP_parallel_loop_static (subfunction, NULL, 0, lb, ub+1, 1, 0);
      subfunction (NULL);
      GOMP_parallel_end ();

      #pragma omp for schedule(runtime)
      for (i = 0; i < n; i++)
        body;

becomes

      {
        long i, _s0, _e0;
        if (GOMP_loop_runtime_start (0, n, 1, &_s0, &_e0))
          do {
            long _e1 = _e0;
            for (i = _s0, i < _e0; i++)
              body;
          } while (GOMP_loop_runtime_next (&_s0, _&e0));
        GOMP_loop_end ();
      }

Note that while it looks like there is trickiness to propagating a non-constant STEP, there isn’t really. We’re explicitly allowed to evaluate it as many times as we want, and any variables involved should automatically be handled as PRIVATE or SHARED like any other variables. So the expression should remain evaluable in the subfunction. We can also pull it into a local variable if we like, but since its supposed to remain unchanged, we can also not if we like.

If we have SCHEDULE(STATIC), and no ORDERED, then we ought to be able to get away with no work-sharing context at all, since we can simply perform the arithmetic directly in each thread to divide up the iterations. Which would mean that we wouldn’t need to call any of these routines.

There are separate routines for handling loops with an ORDERED clause. Bookkeeping for that is non-trivial...

### 11.12 Implementing ORDERED construct

      void GOMP_ordered_start (void)
      void GOMP_ordered_end (void)

### 11.13 Implementing SECTIONS construct

A block as

      #pragma omp sections
      {
        #pragma omp section
        stmt1;
        #pragma omp section
        stmt2;
        #pragma omp section
        stmt3;
      }

becomes

      for (i = GOMP_sections_start (3); i != 0; i = GOMP_sections_next ())
        switch (i)
          {
          case 1:
            stmt1;
            break;
          case 2:
            stmt2;
            break;
          case 3:
            stmt3;
            break;
          }
      GOMP_barrier ();

### 11.14 Implementing SINGLE construct

A block like

      #pragma omp single
      {
        body;
      }

becomes

      if (GOMP_single_start ())
        body;
      GOMP_barrier ();

while

      #pragma omp single copyprivate(x)
        body;

becomes

      datap = GOMP_single_copy_start ();
      if (datap == NULL)
        {
          body;
          data.x = x;
          GOMP_single_copy_end (&data);
        }
      else
        x = datap->x;
      GOMP_barrier ();

### 11.15 Implementing OpenACC’s PARALLEL construct

      void GOACC_parallel ()

## 12 Reporting Bugs

Bugs in the GNU Offloading and Multi Processing Runtime Library should be reported via Bugzilla. Please add "openacc", or "openmp", or both to the keywords field in the bug report, as appropriate.

## GNU General Public License

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   6. Requiring indemnification of licensors and authors of that material by anyone who conveys the material (or modified versions of it) with contractual assumptions of liability to the recipient, for any liability that these contractual assumptions directly impose on those licensors and authors.

   All other non-permissive additional terms are considered “further restrictions” within the meaning of section 10. If the Program as you received it, or any part of it, contains a notice stating that it is governed by this License along with a term that is a further restriction, you may remove that term. If a license document contains a further restriction but permits relicensing or conveying under this License, you may add to a covered work material governed by the terms of that license document, provided that the further restriction does not survive such relicensing or conveying.

   If you add terms to a covered work in accord with this section, you must place, in the relevant source files, a statement of the additional terms that apply to those files, or a notice indicating where to find the applicable terms.

   Additional terms, permissive or non-permissive, may be stated in the form of a separately written license, or stated as exceptions; the above requirements apply either way.

8. Termination.

   You may not propagate or modify a covered work except as expressly provided under this License. Any attempt otherwise to propagate or modify it is void, and will automatically terminate your rights under this License (including any patent licenses granted under the third paragraph of section 11).

   However, if you cease all violation of this License, then your license from a particular copyright holder is reinstated (a) provisionally, unless and until the copyright holder explicitly and finally terminates your license, and (b) permanently, if the copyright holder fails to notify you of the violation by some reasonable means prior to 60 days after the cessation.

   Moreover, your license from a particular copyright holder is reinstated permanently if the copyright holder notifies you of the violation by some reasonable means, this is the first time you have received notice of violation of this License (for any work) from that copyright holder, and you cure the violation prior to 30 days after your receipt of the notice.

   Termination of your rights under this section does not terminate the licenses of parties who have received copies or rights from you under this License. If your rights have been terminated and not permanently reinstated, you do not qualify to receive new licenses for the same material under section 10.

9. Acceptance Not Required for Having Copies.

   You are not required to accept this License in order to receive or run a copy of the Program. Ancillary propagation of a covered work occurring solely as a consequence of using peer-to-peer transmission to receive a copy likewise does not require acceptance. However, nothing other than this License grants you permission to propagate or modify any covered work. These actions infringe copyright if you do not accept this License. Therefore, by modifying or propagating a covered work, you indicate your acceptance of this License to do so.

10. Automatic Licensing of Downstream Recipients.

    Each time you convey a covered work, the recipient automatically receives a license from the original licensors, to run, modify and propagate that work, subject to this License. You are not responsible for enforcing compliance by third parties with this License.

    An “entity transaction” is a transaction transferring control of an organization, or substantially all assets of one, or subdividing an organization, or merging organizations. If propagation of a covered work results from an entity transaction, each party to that transaction who receives a copy of the work also receives whatever licenses to the work the party’s predecessor in interest had or could give under the previous paragraph, plus a right to possession of the Corresponding Source of the work from the predecessor in interest, if the predecessor has it or can get it with reasonable efforts.

    You may not impose any further restrictions on the exercise of the rights granted or affirmed under this License. For example, you may not impose a license fee, royalty, or other charge for exercise of rights granted under this License, and you may not initiate litigation (including a cross-claim or counterclaim in a lawsuit) alleging that any patent claim is infringed by making, using, selling, offering for sale, or importing the Program or any portion of it.

11. Patents.

    A “contributor” is a copyright holder who authorizes use under this License of the Program or a work on which the Program is based. The work thus licensed is called the contributor’s “contributor version”.

    A contributor’s “essential patent claims” are all patent claims owned or controlled by the contributor, whether already acquired or hereafter acquired, that would be infringed by some manner, permitted by this License, of making, using, or selling its contributor version, but do not include claims that would be infringed only as a consequence of further modification of the contributor version. For purposes of this definition, “control” includes the right to grant patent sublicenses in a manner consistent with the requirements of this License.

    Each contributor grants you a non-exclusive, worldwide, royalty-free patent license under the contributor’s essential patent claims, to make, use, sell, offer for sale, import and otherwise run, modify and propagate the contents of its contributor version.

    In the following three paragraphs, a “patent license” is any express agreement or commitment, however denominated, not to enforce a patent (such as an express permission to practice a patent or covenant not to sue for patent infringement). To “grant” such a patent license to a party means to make such an agreement or commitment not to enforce a patent against the party.

    If you convey a covered work, knowingly relying on a patent license, and the Corresponding Source of the work is not available for anyone to copy, free of charge and under the terms of this License, through a publicly available network server or other readily accessible means, then you must either (1) cause the Corresponding Source to be so available, or (2) arrange to deprive yourself of the benefit of the patent license for this particular work, or (3) arrange, in a manner consistent with the requirements of this License, to extend the patent license to downstream recipients. “Knowingly relying” means you have actual knowledge that, but for the patent license, your conveying the covered work in a country, or your recipient’s use of the covered work in a country, would infringe one or more identifiable patents in that country that you have reason to believe are valid.

    If, pursuant to or in connection with a single transaction or arrangement, you convey, or propagate by procuring conveyance of, a covered work, and grant a patent license to some of the parties receiving the covered work authorizing them to use, propagate, modify or convey a specific copy of the covered work, then the patent license you grant is automatically extended to all recipients of the covered work and works based on it.

    A patent license is “discriminatory” if it does not include within the scope of its coverage, prohibits the exercise of, or is conditioned on the non-exercise of one or more of the rights that are specifically granted under this License. You may not convey a covered work if you are a party to an arrangement with a third party that is in the business of distributing software, under which you make payment to the third party based on the extent of your activity of conveying the work, and under which the third party grants, to any of the parties who would receive the covered work from you, a discriminatory patent license (a) in connection with copies of the covered work conveyed by you (or copies made from those copies), or (b) primarily for and in connection with specific products or compilations that contain the covered work, unless you entered into that arrangement, or that patent license was granted, prior to 28 March 2007.

    Nothing in this License shall be construed as excluding or limiting any implied license or other defenses to infringement that may otherwise be available to you under applicable patent law.

12. No Surrender of Others’ Freedom.

    If conditions are imposed on you (whether by court order, agreement or otherwise) that contradict the conditions of this License, they do not excuse you from the conditions of this License. If you cannot convey a covered work so as to satisfy simultaneously your obligations under this License and any other pertinent obligations, then as a consequence you may not convey it at all. For example, if you agree to terms that obligate you to collect a royalty for further conveying from those to whom you convey the Program, the only way you could satisfy both those terms and this License would be to refrain entirely from conveying the Program.

13. Use with the GNU Affero General Public License.

    Notwithstanding any other provision of this License, you have permission to link or combine any covered work with a work licensed under version 3 of the GNU Affero General Public License into a single combined work, and to convey the resulting work. The terms of this License will continue to apply to the part which is the covered work, but the special requirements of the GNU Affero General Public License, section 13, concerning interaction through a network will apply to the combination as such.

14. Revised Versions of this License.

    The Free Software Foundation may publish revised and/or new versions of the GNU General Public License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns.

    Each version is given a distinguishing version number. If the Program specifies that a certain numbered version of the GNU General Public License “or any later version” applies to it, you have the option of following the terms and conditions either of that numbered version or of any later version published by the Free Software Foundation. If the Program does not specify a version number of the GNU General Public License, you may choose any version ever published by the Free Software Foundation.

    If the Program specifies that a proxy can decide which future versions of the GNU General Public License can be used, that proxy’s public statement of acceptance of a version permanently authorizes you to choose that version for the Program.

    Later license versions may give you additional or different permissions. However, no additional obligations are imposed on any author or copyright holder as a result of your choosing to follow a later version.

15. Disclaimer of Warranty.

    THERE IS NO WARRANTY FOR THE PROGRAM, TO THE EXTENT PERMITTED BY APPLICABLE LAW. EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES PROVIDE THE PROGRAM “AS IS” WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE PROGRAM IS WITH YOU. SHOULD THE PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING, REPAIR OR CORRECTION.

16. Limitation of Liability.

    IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MODIFIES AND/OR CONVEYS THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES, INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED TO LOSS OF DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER PROGRAMS), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.

17. Interpretation of Sections 15 and 16.

    If the disclaimer of warranty and limitation of liability provided above cannot be given local legal effect according to their terms, reviewing courts shall apply local law that most closely approximates an absolute waiver of all civil liability in connection with the Program, unless a warranty or assumption of liability accompanies a copy of the Program in return for a fee.

### END OF TERMS AND CONDITIONS

### How to Apply These Terms to Your New Programs

If you develop a new program, and you want it to be of the greatest possible use to the public, the best way to achieve this is to make it free software which everyone can redistribute and change under these terms.

To do so, attach the following notices to the program. It is safest to attach them to the start of each source file to most effectively state the exclusion of warranty; and each file should have at least the “copyright” line and a pointer to where the full notice is found.

    one line to give the program's name and a brief idea of what it does.
    Copyright (C) year name of author

    This program is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation, either version 3 of the License, or (at
    your option) any later version.

    This program is distributed in the hope that it will be useful, but
    WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
    General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program.  If not, see https://www.gnu.org/licenses/.

Also add information on how to contact you by electronic and paper mail.

If the program does terminal interaction, make it output a short notice like this when it starts in an interactive mode:

    program Copyright (C) year name of author
    This program comes with ABSOLUTELY NO WARRANTY; for details type ‘show w’.
    This is free software, and you are welcome to redistribute it
    under certain conditions; type ‘show c’ for details.

The hypothetical commands ‘`show w`’ and ‘`show c`’ should show the appropriate parts of the General Public License. Of course, your program’s commands might be different; for a GUI interface, you would use an “about box”.

You should also get your employer (if you work as a programmer) or school, if any, to sign a “copyright disclaimer” for the program, if necessary. For more information on this, and how to apply and follow the GNU GPL, see https://www.gnu.org/licenses/.

The GNU General Public License does not permit incorporating your program into proprietary programs. If your program is a subroutine library, you may consider it more useful to permit linking proprietary applications with the library. If this is what you want to do, use the GNU Lesser General Public License instead of this License. But first, please read https://www.gnu.org/licenses/why-not-lgpl.html.

## GNU Free Documentation License

Version 1.3, 3 November 2008

    Copyright © 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc.
    https://fsf.org/

    Everyone is permitted to copy and distribute verbatim copies
    of this license document, but changing it is not allowed.

0. PREAMBLE

   The purpose of this License is to make a manual, textbook, or other functional and useful document free in the sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying it, either commercially or noncommercially. Secondarily, this License preserves for the author and publisher a way to get credit for their work, while not being considered responsible for modifications made by others.

   This License is a kind of “copyleft”, which means that derivative works of the document must themselves be free in the same sense. It complements the GNU General Public License, which is a copyleft license designed for free software.

   We have designed this License in order to use it for manuals for free software, because free software needs free documentation: a free program should come with manuals providing the same freedoms that the software does. But this License is not limited to software manuals; it can be used for any textual work, regardless of subject matter or whether it is published as a printed book. We recommend this License principally for works whose purpose is instruction or reference.

1. APPLICABILITY AND DEFINITIONS

   This License applies to any manual or other work, in any medium, that contains a notice placed by the copyright holder saying it can be distributed under the terms of this License. Such a notice grants a world-wide, royalty-free license, unlimited in duration, to use that work under the conditions stated herein. The “Document”, below, refers to any such manual or work. Any member of the public is a licensee, and is addressed as “you”. You accept the license if you copy, modify or distribute the work in a way requiring permission under copyright law.

   A “Modified Version” of the Document means any work containing the Document or a portion of it, either copied verbatim, or with modifications and/or translated into another language.

   A “Secondary Section” is a named appendix or a front-matter section of the Document that deals exclusively with the relationship of the publishers or authors of the Document to the Document’s overall subject (or to related matters) and contains nothing that could fall directly within that overall subject. (Thus, if the Document is in part a textbook of mathematics, a Secondary Section may not explain any mathematics.) The relationship could be a matter of historical connection with the subject or with related matters, or of legal, commercial, philosophical, ethical or political position regarding them.

   The “Invariant Sections” are certain Secondary Sections whose titles are designated, as being those of Invariant Sections, in the notice that says that the Document is released under this License. If a section does not fit the above definition of Secondary then it is not allowed to be designated as Invariant. The Document may contain zero Invariant Sections. If the Document does not identify any Invariant Sections then there are none.

   The “Cover Texts” are certain short passages of text that are listed, as Front-Cover Texts or Back-Cover Texts, in the notice that says that the Document is released under this License. A Front-Cover Text may be at most 5 words, and a Back-Cover Text may be at most 25 words.

   A “Transparent” copy of the Document means a machine-readable copy, represented in a format whose specification is available to the general public, that is suitable for revising the document straightforwardly with generic text editors or (for images composed of pixels) generic paint programs or (for drawings) some widely available drawing editor, and that is suitable for input to text formatters or for automatic translation to a variety of formats suitable for input to text formatters. A copy made in an otherwise Transparent file format whose markup, or absence of markup, has been arranged to thwart or discourage subsequent modification by readers is not Transparent. An image format is not Transparent if used for any substantial amount of text. A copy that is not “Transparent” is called “Opaque”.

   Examples of suitable formats for Transparent copies include plain ASCII without markup, Texinfo input format, LaTeX input format, SGML or XML using a publicly available DTD, and standard-conforming simple HTML, PostScript or PDF designed for human modification. Examples of transparent image formats include PNG, XCF and JPG. Opaque formats include proprietary formats that can be read and edited only by proprietary word processors, SGML or XML for which the DTD and/or processing tools are not generally available, and the machine-generated HTML, PostScript or PDF produced by some word processors for output purposes only.

   The “Title Page” means, for a printed book, the title page itself, plus such following pages as are needed to hold, legibly, the material this License requires to appear in the title page. For works in formats which do not have any title page as such, “Title Page” means the text near the most prominent appearance of the work’s title, preceding the beginning of the body of the text.

   The “publisher” means any person or entity that distributes copies of the Document to the public.

   A section “Entitled XYZ” means a named subunit of the Document whose title either is precisely XYZ or contains XYZ in parentheses following text that translates XYZ in another language. (Here XYZ stands for a specific section name mentioned below, such as “Acknowledgements”, “Dedications”, “Endorsements”, or “History”.) To “Preserve the Title” of such a section when you modify the Document means that it remains a section “Entitled XYZ” according to this definition.

   The Document may include Warranty Disclaimers next to the notice which states that this License applies to the Document. These Warranty Disclaimers are considered to be included by reference in this License, but only as regards disclaiming warranties: any other implication that these Warranty Disclaimers may have is void and has no effect on the meaning of this License.

2. VERBATIM COPYING

   You may copy and distribute the Document in any medium, either commercially or noncommercially, provided that this License, the copyright notices, and the license notice saying this License applies to the Document are reproduced in all copies, and that you add no other conditions whatsoever to those of this License. You may not use technical measures to obstruct or control the reading or further copying of the copies you make or distribute. However, you may accept compensation in exchange for copies. If you distribute a large enough number of copies you must also follow the conditions in section 3.

   You may also lend copies, under the same conditions stated above, and you may publicly display copies.

3. COPYING IN QUANTITY

   If you publish printed copies (or copies in media that commonly have printed covers) of the Document, numbering more than 100, and the Document’s license notice requires Cover Texts, you must enclose the copies in covers that carry, clearly and legibly, all these Cover Texts: Front-Cover Texts on the front cover, and Back-Cover Texts on the back cover. Both covers must also clearly and legibly identify you as the publisher of these copies. The front cover must present the full title with all words of the title equally prominent and visible. You may add other material on the covers in addition. Copying with changes limited to the covers, as long as they preserve the title of the Document and satisfy these conditions, can be treated as verbatim copying in other respects.

   If the required texts for either cover are too voluminous to fit legibly, you should put the first ones listed (as many as fit reasonably) on the actual cover, and continue the rest onto adjacent pages.

   If you publish or distribute Opaque copies of the Document numbering more than 100, you must either include a machine-readable Transparent copy along with each Opaque copy, or state in or with each Opaque copy a computer-network location from which the general network-using public has access to download using public-standard network protocols a complete Transparent copy of the Document, free of added material. If you use the latter option, you must take reasonably prudent steps, when you begin distribution of Opaque copies in quantity, to ensure that this Transparent copy will remain thus accessible at the stated location until at least one year after the last time you distribute an Opaque copy (directly or through your agents or retailers) of that edition to the public.

   It is requested, but not required, that you contact the authors of the Document well before redistributing any large number of copies, to give them a chance to provide you with an updated version of the Document.

4. MODIFICATIONS

   You may copy and distribute a Modified Version of the Document under the conditions of sections 2 and 3 above, provided that you release the Modified Version under precisely this License, with the Modified Version filling the role of the Document, thus licensing distribution and modification of the Modified Version to whoever possesses a copy of it. In addition, you must do these things in the Modified Version:

   1. Use in the Title Page (and on the covers, if any) a title distinct from that of the Document, and from those of previous versions (which should, if there were any, be listed in the History section of the Document). You may use the same title as a previous version if the original publisher of that version gives permission.
   2. List on the Title Page, as authors, one or more persons or entities responsible for authorship of the modifications in the Modified Version, together with at least five of the principal authors of the Document (all of its principal authors, if it has fewer than five), unless they release you from this requirement.
   3. State on the Title page the name of the publisher of the Modified Version, as the publisher.
   4. Preserve all the copyright notices of the Document.
   5. Add an appropriate copyright notice for your modifications adjacent to the other copyright notices.
   6. Include, immediately after the copyright notices, a license notice giving the public permission to use the Modified Version under the terms of this License, in the form shown in the Addendum below.
   7. Preserve in that license notice the full lists of Invariant Sections and required Cover Texts given in the Document’s license notice.
   8. Include an unaltered copy of this License.
   9. Preserve the section Entitled “History”, Preserve its Title, and add to it an item stating at least the title, year, new authors, and publisher of the Modified Version as given on the Title Page. If there is no section Entitled “History” in the Document, create one stating the title, year, authors, and publisher of the Document as given on its Title Page, then add an item describing the Modified Version as stated in the previous sentence.
   10. Preserve the network location, if any, given in the Document for public access to a Transparent copy of the Document, and likewise the network locations given in the Document for previous versions it was based on. These may be placed in the “History” section. You may omit a network location for a work that was published at least four years before the Document itself, or if the original publisher of the version it refers to gives permission.
   11. For any section Entitled “Acknowledgements” or “Dedications”, Preserve the Title of the section, and preserve in the section all the substance and tone of each of the contributor acknowledgements and/or dedications given therein.
   12. Preserve all the Invariant Sections of the Document, unaltered in their text and in their titles. Section numbers or the equivalent are not considered part of the section titles.
   13. Delete any section Entitled “Endorsements”. Such a section may not be included in the Modified Version.
   14. Do not retitle any existing section to be Entitled “Endorsements” or to conflict in title with any Invariant Section.
   15. Preserve any Warranty Disclaimers.

   If the Modified Version includes new front-matter sections or appendices that qualify as Secondary Sections and contain no material copied from the Document, you may at your option designate some or all of these sections as invariant. To do this, add their titles to the list of Invariant Sections in the Modified Version’s license notice. These titles must be distinct from any other section titles.

   You may add a section Entitled “Endorsements”, provided it contains nothing but endorsements of your Modified Version by various parties—for example, statements of peer review or that the text has been approved by an organization as the authoritative definition of a standard.

   You may add a passage of up to five words as a Front-Cover Text, and a passage of up to 25 words as a Back-Cover Text, to the end of the list of Cover Texts in the Modified Version. Only one passage of Front-Cover Text and one of Back-Cover Text may be added by (or through arrangements made by) any one entity. If the Document already includes a cover text for the same cover, previously added by you or by arrangement made by the same entity you are acting on behalf of, you may not add another; but you may replace the old one, on explicit permission from the previous publisher that added the old one.

   The author(s) and publisher(s) of the Document do not by this License give permission to use their names for publicity for or to assert or imply endorsement of any Modified Version.

5. COMBINING DOCUMENTS

   You may combine the Document with other documents released under this License, under the terms defined in section 4 above for modified versions, provided that you include in the combination all of the Invariant Sections of all of the original documents, unmodified, and list them all as Invariant Sections of your combined work in its license notice, and that you preserve all their Warranty Disclaimers.

   The combined work need only contain one copy of this License, and multiple identical Invariant Sections may be replaced with a single copy. If there are multiple Invariant Sections with the same name but different contents, make the title of each such section unique by adding at the end of it, in parentheses, the name of the original author or publisher of that section if known, or else a unique number. Make the same adjustment to the section titles in the list of Invariant Sections in the license notice of the combined work.

   In the combination, you must combine any sections Entitled “History” in the various original documents, forming one section Entitled “History”; likewise combine any sections Entitled “Acknowledgements”, and any sections Entitled “Dedications”. You must delete all sections Entitled “Endorsements.”

6. COLLECTIONS OF DOCUMENTS

   You may make a collection consisting of the Document and other documents released under this License, and replace the individual copies of this License in the various documents with a single copy that is included in the collection, provided that you follow the rules of this License for verbatim copying of each of the documents in all other respects.

   You may extract a single document from such a collection, and distribute it individually under this License, provided you insert a copy of this License into the extracted document, and follow this License in all other respects regarding verbatim copying of that document.

7. AGGREGATION WITH INDEPENDENT WORKS

   A compilation of the Document or its derivatives with other separate and independent documents or works, in or on a volume of a storage or distribution medium, is called an “aggregate” if the copyright resulting from the compilation is not used to limit the legal rights of the compilation’s users beyond what the individual works permit. When the Document is included in an aggregate, this License does not apply to the other works in the aggregate which are not themselves derivative works of the Document.

   If the Cover Text requirement of section 3 is applicable to these copies of the Document, then if the Document is less than one half of the entire aggregate, the Document’s Cover Texts may be placed on covers that bracket the Document within the aggregate, or the electronic equivalent of covers if the Document is in electronic form. Otherwise they must appear on printed covers that bracket the whole aggregate.

8. TRANSLATION

   Translation is considered a kind of modification, so you may distribute translations of the Document under the terms of section 4. Replacing Invariant Sections with translations requires special permission from their copyright holders, but you may include translations of some or all Invariant Sections in addition to the original versions of these Invariant Sections. You may include a translation of this License, and all the license notices in the Document, and any Warranty Disclaimers, provided that you also include the original English version of this License and the original versions of those notices and disclaimers. In case of a disagreement between the translation and the original version of this License or a notice or disclaimer, the original version will prevail.

   If a section in the Document is Entitled “Acknowledgements”, “Dedications”, or “History”, the requirement (section 4) to Preserve its Title (section 1) will typically require changing the actual title.

9. TERMINATION

   You may not copy, modify, sublicense, or distribute the Document except as expressly provided under this License. Any attempt otherwise to copy, modify, sublicense, or distribute it is void, and will automatically terminate your rights under this License.

   However, if you cease all violation of this License, then your license from a particular copyright holder is reinstated (a) provisionally, unless and until the copyright holder explicitly and finally terminates your license, and (b) permanently, if the copyright holder fails to notify you of the violation by some reasonable means prior to 60 days after the cessation.

   Moreover, your license from a particular copyright holder is reinstated permanently if the copyright holder notifies you of the violation by some reasonable means, this is the first time you have received notice of violation of this License (for any work) from that copyright holder, and you cure the violation prior to 30 days after your receipt of the notice.

   Termination of your rights under this section does not terminate the licenses of parties who have received copies or rights from you under this License. If your rights have been terminated and not permanently reinstated, receipt of a copy of some or all of the same material does not give you any rights to use it.

10. FUTURE REVISIONS OF THIS LICENSE

    The Free Software Foundation may publish new, revised versions of the GNU Free Documentation License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns. See https://www.gnu.org/copyleft/.

    Each version of the License is given a distinguishing version number. If the Document specifies that a particular numbered version of this License “or any later version” applies to it, you have the option of following the terms and conditions either of that specified version or of any later version that has been published (not as a draft) by the Free Software Foundation. If the Document does not specify a version number of this License, you may choose any version ever published (not as a draft) by the Free Software Foundation. If the Document specifies that a proxy can decide which future versions of this License can be used, that proxy’s public statement of acceptance of a version permanently authorizes you to choose that version for the Document.

11. RELICENSING

    “Massive Multiauthor Collaboration Site” (or “MMC Site”) means any World Wide Web server that publishes copyrightable works and also provides prominent facilities for anybody to edit those works. A public wiki that anybody can edit is an example of such a server. A “Massive Multiauthor Collaboration” (or “MMC”) contained in the site means any set of copyrightable works thus published on the MMC site.

    “CC-BY-SA” means the Creative Commons Attribution-Share Alike 3.0 license published by Creative Commons Corporation, a not-for-profit corporation with a principal place of business in San Francisco, California, as well as future copyleft versions of that license published by that same organization.

    “Incorporate” means to publish or republish a Document, in whole or in part, as part of another Document.

    An MMC is “eligible for relicensing” if it is licensed under this License, and if all works that were first published under this License somewhere other than this MMC, and subsequently incorporated in whole or in part into the MMC, (1) had no cover texts or invariant sections, and (2) were thus incorporated prior to November 1, 2008.

    The operator of an MMC Site may republish an MMC contained in the site under CC-BY-SA on the same site at any time before August 1, 2009, provided the MMC is eligible for relicensing.

### ADDENDUM: How to use this License for your documents

To use this License in a document you have written, include a copy of the License in the document and put the following copyright and license notices just after the title page:

      Copyright (C)  year  your name.
      Permission is granted to copy, distribute and/or modify this document
      under the terms of the GNU Free Documentation License, Version 1.3
      or any later version published by the Free Software Foundation;
      with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
      Texts.  A copy of the license is included in the section entitled ``GNU
      Free Documentation License''.

If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, replace the “with...Texts.” line with this:

        with the Invariant Sections being list their titles, with
        the Front-Cover Texts being list, and with the Back-Cover Texts
        being list.

If you have Invariant Sections without Cover Texts, or some other combination of the three, merge those two alternatives to suit the situation.

If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software.

## Funding Free Software

If you want to have more free software a few years from now, it makes sense for you to help encourage people to contribute funds for its development. The most effective approach known is to encourage commercial redistributors to donate.

Users of free software systems can boost the pace of development by encouraging for-a-fee distributors to donate part of their selling price to free software developers—the Free Software Foundation, and others.

The way to convince distributors to do this is to demand it and expect it from them. So when you compare distributors, judge them partly by how much they give to free software development. Show distributors they must compete to be the one who gives the most.

To make this approach work, you must insist on numbers that you can compare, such as, “We will donate ten dollars to the Frobnitz project for each disk sold.” Don’t be satisfied with a vague promise, such as “A portion of the profits are donated,” since it doesn’t give a basis for comparison.

Even a precise fraction “of the profits from this disk” is not very meaningful, since creative accounting and unrelated business decisions can greatly alter what fraction of the sales price counts as profit. If the price you pay is $50, ten percent of the profit is probably less than a dollar; it might be a few cents, or nothing at all.

Some redistributors do development work themselves. This is useful too; but to keep everyone honest, you need to inquire how much they do, and what kind. Some kinds of development make much more long-term difference than others. For example, maintaining a separate version of a program contributes very little; maintaining the standard version of a program for the whole community contributes much. Easy new ports contribute little, since someone else would surely do them; difficult ports such as adding a new CPU to the GNU Compiler Collection contribute more; major new features or packages contribute the most.

By establishing the idea that supporting further development is “the proper thing to do” when distributing free software for a fee, we can assure a steady flow of resources into making more free software.

    Copyright © 1994 Free Software Foundation, Inc.
    Verbatim copying and redistribution of this section is permitted
    without royalty; alteration is not permitted.

## Library Index

Jump to: | A E F I

| Index Entry |  | Section

A |  |
| acc_get_property: |  | acc_get_property
| acc_get_property_string: |  | acc_get_property

E |  |
| Environment Variable: |  | OMP_CANCELLATION
| Environment Variable: |  | OMP_DISPLAY_ENV
| Environment Variable: |  | OMP_DEFAULT_DEVICE
| Environment Variable: |  | OMP_DYNAMIC
| Environment Variable: |  | OMP_MAX_ACTIVE_LEVELS
| Environment Variable: |  | OMP_MAX_TASK_PRIORITY
| Environment Variable: |  | OMP_NESTED
| Environment Variable: |  | OMP_NUM_TEAMS
| Environment Variable: |  | OMP_NUM_THREADS
| Environment Variable: |  | OMP_PROC_BIND
| Environment Variable: |  | OMP_PLACES
| Environment Variable: |  | OMP_STACKSIZE
| Environment Variable: |  | OMP_SCHEDULE
| Environment Variable: |  | OMP_TARGET_OFFLOAD
| Environment Variable: |  | OMP_TEAMS_THREAD_LIMIT
| Environment Variable: |  | OMP_THREAD_LIMIT
| Environment Variable: |  | OMP_WAIT_POLICY
| Environment Variable: |  | GOMP_CPU_AFFINITY
| Environment Variable: |  | GOMP_DEBUG
| Environment Variable: |  | GOMP_STACKSIZE
| Environment Variable: |  | GOMP_SPINCOUNT
| Environment Variable: |  | GOMP_RTEMS_THREAD_POOLS

F |  |
| FDL, GNU Free Documentation License: |  | GNU Free Documentation License

I |  |
| Implementation specific setting: |  | OMP_NESTED
| Implementation specific setting: |  | OMP_NUM_THREADS
| Implementation specific setting: |  | OMP_SCHEDULE
| Implementation specific setting: |  | OMP_TARGET_OFFLOAD
| Implementation specific setting: |  | GOMP_STACKSIZE
| Implementation specific setting: |  | GOMP_SPINCOUNT
| Implementation specific setting: |  | GOMP_RTEMS_THREAD_POOLS
| Introduction: |  | Top

Jump to: | A E F I

#### Footnotes

##### (1)

See "Stream Management" in "CUDA Driver API", TRM-06703-001, Version 5.5, for additional information

##### (2)

See section 2.26, "Interactions with the CUDA Driver API" in "CUDA Runtime API", Version 5.5, and section 2.27, "VDPAU Interoperability", in "CUDA Driver API", TRM-06703-001, Version 5.5, for additional information on library interoperability.

##### (3)

More complete information about `ACC_DEVICE_TYPE` and `ACC_DEVICE_NUM` can be found in sections 4.1 and 4.2 of the OpenACC Application Programming Interface”, Version 2.6.