`type stat = Stdlib.Gc.stat = {`

1. `minor_words : float;`
   (*

   Number of words allocated in the minor heap since the program was started. This number is accurate in byte-code programs, but only an approximation in programs compiled to native code.

   *)
2. `promoted_words : float;`
   (*

   Number of words allocated in the minor heap that survived a minor collection and were moved to the major heap since the program was started.

   *)
3. `major_words : float;`
   (*

   Number of words allocated in the major heap, including the promoted words, since the program was started.

   *)
4. `minor_collections : int;`
   (*

   Number of minor collections since the program was started.

   *)
5. `major_collections : int;`
   (*

   Number of major collection cycles completed since the program was started.

   *)
6. `heap_words : int;`
   (*

   Total size of the major heap, in words.

   *)
7. `heap_chunks : int;`
   (*

   Number of contiguous pieces of memory that make up the major heap.

   *)
8. `live_words : int;`
   (*

   Number of words of live data in the major heap, including the header words.

   *)
9. `live_blocks : int;`
   (*

   Number of live blocks in the major heap.

   *)
10. `free_words : int;`
    (*

    Number of words in the free list.

    *)
11. `free_blocks : int;`
    (*

    Number of blocks in the free list.

    *)
12. `largest_free : int;`
    (*

    Size (in words) of the largest block in the free list.

    *)
13. `fragments : int;`
    (*

    Number of wasted words due to fragmentation. These are 1-words free blocks placed between two live blocks. They are not available for allocation.

    *)
14. `compactions : int;`
    (*

    Number of heap compactions since the program was started.

    *)
15. `top_heap_words : int;`
    (*

    Maximum size reached by the major heap, in words.

    *)
16. `stack_size : int;`
    (*

    Current size of the stack, in words.

    - since 3.12.0
    *)
17. `forced_major_collections : int;`
    (*

    Number of forced full major collections completed since the program was started.

    - since 4.12.0
    *)

`}`

The memory management counters are returned in a `stat` record.

The total amount of memory allocated by the program since it was started is (in words) `minor_words + major_words - promoted_words`. Multiply by the word size (4 on a 32-bit machine, 8 on a 64-bit machine) to get the number of bytes.

`type control = Stdlib.Gc.control = {`

1. `minor_heap_size : int;`
   (*

   The size (in words) of the minor heap. Changing this parameter will trigger a minor collection. Default: 32k.

   *)
2. `major_heap_increment : int;`
   (*

   The minimum number of words to add to the major heap when increasing it. Default: 124k.

   *)
3. `space_overhead : int;`
   (*

   The major GC speed is computed from this parameter. This is the memory that will be "wasted" because the GC does not immediately collect unreachable blocks. It is expressed as a percentage of the memory used for live data. The GC will work more (use more CPU time and collect blocks more eagerly) if `space_overhead` is smaller. Default: 80.

   *)
4. `verbose : int;`
   (*

   This value controls the GC messages on standard error output. It is a sum of some of the following flags, to print messages on the corresponding events:

   - `0x001` Start of major GC cycle.
   - `0x002` Minor collection and major GC slice.
   - `0x004` Growing and shrinking of the heap.
   - `0x008` Resizing of stacks and memory manager tables.
   - `0x010` Heap compaction.
   - `0x020` Change of GC parameters.
   - `0x040` Computation of major GC slice size.
   - `0x080` Calling of finalisation functions.
   - `0x100` Bytecode executable search at start-up.
   - `0x200` Computation of compaction triggering condition. Default: 0.
   *)
5. `max_overhead : int;`
   (*

   Heap compaction is triggered when the estimated amount of "wasted" memory is more than `max_overhead` percent of the amount of live data. If `max_overhead` is set to 0, heap compaction is triggered at the end of each major GC cycle (this setting is intended for testing purposes only). If `max_overhead >= 1000000`, compaction is never triggered. Default: 500.

   *)
6. `stack_limit : int;`
   (*

   The maximum size of the stack (in words). This is only relevant to the byte-code runtime, as the native code runtime uses the operating system's stack. Default: 256k.

   *)
7. `allocation_policy : int;`
   (*

   The policy used for allocating in the heap. Possible values are 0 and 1. 0 is the next-fit policy, which is quite fast but can result in fragmentation. 1 is the first-fit policy, which can be slower in some cases but can be better for programs with fragmentation problems. Default: 0.

   - since 3.11.0
   *)
8. `window_size : int;`
   (*

   The size of the window used by the major GC for smoothing out variations in its workload. This is an integer between 1 and 50. Default: 1.

   - since 2.5.0 and OCaml 4.03.0
   *)
9. `custom_major_ratio : int;`
   (*

   Target ratio of floating garbage to major heap size for out-of-heap memory held by custom values located in the major heap. The GC speed is adjusted to try to use this much memory for dead values that are not yet collected. Expressed as a percentage of major heap size. The default value keeps the out-of-heap floating garbage about the same size as the in-heap overhead. Note: this only applies to values allocated with `caml_alloc_custom_mem` (e.g. bigarrays). Default: 44.

   - since 2.10.0 and OCaml 4.08.0
   *)
10. `custom_minor_ratio : int;`
    (*

    Bound on floating garbage for out-of-heap memory held by custom values in the minor heap. A minor GC is triggered when this much memory is held by custom values located in the minor heap. Expressed as a percentage of minor heap size. Note: this only applies to values allocated with `caml_alloc_custom_mem` (e.g. bigarrays). Default: 100.

    - since 2.10.0 and OCaml 4.08.0
    *)
11. `custom_minor_max_size : int;`
    (*

    Maximum amount of out-of-heap memory for each custom value allocated in the minor heap. When a custom value is allocated on the minor heap and holds more than this many bytes, only this value is counted against `custom_minor_ratio` and the rest is directly counted against `custom_major_ratio`. Note: this only applies to values allocated with `caml_alloc_custom_mem` (e.g. bigarrays). Default: 8192 bytes.

    - since 2.10.0 and OCaml 4.08.0
    *)

`}`

The GC parameters are given as a `control` record. Note that these parameters can also be initialised by setting the OCAMLRUNPARAM environment variable. See the documentation of ocamlrun.

`val stat : unit -> stat`

Return the current values of the memory management counters in a `stat` record. This function examines every heap block to get the statistics.

`val quick_stat : unit -> stat`

Same as `stat` except that `live_words`, `live_blocks`, `free_words`, `free_blocks`, `largest_free`, and `fragments` are set to 0. This function is much faster than `stat` because it does not need to go through the heap.

`val counters : unit -> float * float * float`

Return `(minor_words, promoted_words, major_words)`. This function is as fast at `quick_stat`.

`val minor_words : unit -> float`

Number of words allocated in the minor heap since the program was started. This number is accurate in byte-code programs, but only an approximation in programs compiled to native code.

In native code this function does not allocate.

- since 2.5.3 and OCaml 4.04

`val get : unit -> control`

Return the current values of the GC parameters in a `control` record.

`val set : control -> unit`

`set r` changes the GC parameters according to the `control` record `r`. The normal usage is: `Gc.set { (Gc.get()) with Gc.verbose = 0x00d }`

`val minor : unit -> unit`

Trigger a minor collection.

`val major_slice : int -> int`

Do a minor collection and a slice of major collection. The argument is the size of the slice, 0 to use the automatically-computed slice size. In all cases, the result is the computed slice size.

`val major : unit -> unit`

Do a minor collection and finish the current major collection cycle.

`val full_major : unit -> unit`

Do a minor collection, finish the current major collection cycle, and perform a complete new cycle. This will collect all currently unreachable blocks.

`val compact : unit -> unit`

Perform a full major collection and compact the heap. Note that heap compaction is a lengthy operation.

`val print_stat : _ BatInnerIO.output -> unit`

Print the current values of the memory management counters (in human-readable form) into the channel argument.

`val allocated_bytes : unit -> float`

Return the total number of bytes allocated since the program was started. It is returned as a `float` to avoid overflow problems with `int` on 32-bit machines.

`val get_minor_free : unit -> int`

Return the current size of the free space inside the minor heap.

- since 2.5.0 and OCaml 4.03.0

`val finalise : ('a -> unit) -> 'a -> unit`

`finalise f v` registers `f` as a finalisation function for `v`. `v` must be heap-allocated. `f` will be called with `v` as argument at some point between the first time `v` becomes unreachable and the time `v` is collected by the GC. Several functions can be registered for the same value, or even several instances of the same function. Each instance will be called once (or never, if the program terminates before `v` becomes unreachable).

The GC will call the finalisation functions in the order of deallocation. When several values become unreachable at the same time (i.e. during the same GC cycle), the finalisation functions will be called in the reverse order of the corresponding calls to `finalise`. If `finalise` is called in the same order as the values are allocated, that means each value is finalised before the values it depends upon. Of course, this becomes false if additional dependencies are introduced by assignments.

Anything reachable from the closure of finalisation functions is considered reachable, so the following code will not work as expected:

- `let v = ... in Gc.finalise (fun x -> ...) v`

Instead you should write:

- `let f = fun x -> ... ;; let v = ... in Gc.finalise f v`

The `f` function can use all features of OCaml, including assignments that make the value reachable again. It can also loop forever (in this case, the other finalisation functions will not be called during the execution of f, unless it calls `finalise_release`). It can call `finalise` on `v` or other values to register other functions or even itself. It can raise an exception; in this case the exception will interrupt whatever the program was doing when the function was called.

`finalise` will raise `Invalid_argument` if `v` is not heap-allocated. Some examples of values that are not heap-allocated are integers, constant constructors, booleans, the empty array, the empty list, the unit value. The exact list of what is heap-allocated or not is implementation-dependent. Some constant values can be heap-allocated but never deallocated during the lifetime of the program, for example a list of integer constants; this is also implementation-dependent. You should also be aware that compiler optimisations may duplicate some immutable values, for example floating-point numbers when stored into arrays, so they can be finalised and collected while another copy is still in use by the program.

The results of calling `String.make`, `String.create`, `Array.make`, and `Pervasives.ref` are guaranteed to be heap-allocated and non-constant except when the length argument is `0`.

`val finalise_last : (unit -> unit) -> 'a -> unit`

same as `finalise` except the value is not given as argument. So you can't use the given value for the computation of the finalisation function. The benefit is that the function is called after the value is unreachable for the last time instead of the first time. So contrary to `finalise` the value will never be reachable again or used again. In particular every weak pointers and ephemerons that contained this value as key or data is unset before running the finalisation function. Moreover the finalisation function attached with `GC.finalise` are always called before the finalisation function attached with `GC.finalise_last`.

- since 2.11.0 and OCaml 4.04

`val finalise_release : unit -> unit`

A finalisation function may call `finalise_release` to tell the GC that it can launch the next finalisation function without waiting for the current one to return.

`type alarm = Stdlib.Gc.alarm`

An alarm is a piece of data that calls a user function at the end of each major GC cycle. The following functions are provided to create and delete alarms.

`val create_alarm : (unit -> unit) -> alarm`

`create_alarm f` will arrange for `f` to be called at the end of each major GC cycle, starting with the current cycle or the next one. A value of type `alarm` is returned that you can use to call `delete_alarm`.

`val delete_alarm : alarm -> unit`

`delete_alarm a` will stop the calls to the function associated to `a`. Calling `delete_alarm a` again has no effect.

`val eventlog_pause : unit -> unit`

`eventlog_pause ()` will pause the collection of traces in the runtime. Traces are collected if the program is linked to the instrumented runtime and started with the environment variable OCAML_EVENTLOG_ENABLED. Events are flushed to disk after pausing, and no new events will be recorded until `eventlog_resume` is called.

`val eventlog_resume : unit -> unit`

`eventlog_resume ()` will resume the collection of traces in the runtime. Traces are collected if the program is linked to the instrumented runtime and started with the environment variable OCAML_EVENTLOG_ENABLED. This call can be used after calling `eventlog_pause`, or if the program was started with OCAML_EVENTLOG_ENABLED=p. (which pauses the collection of traces before the first event.)

`module Memprof : sig ... end`

`Memprof` is a sampling engine for allocated memory words. Every allocated word has a probability of being sampled equal to a configurable sampling rate. Once a block is sampled, it becomes tracked. A tracked block triggers a user-defined callback as soon as it is allocated, promoted or deallocated.

`type suspended_collection_work = Stdlib.Gc.suspended_collection_work`
`val ramp_up : (unit -> 'a) -> 'a * suspended_collection_work`
`val ramp_down : suspended_collection_work -> unit`