# Chapter 4. Foreign Interface

Chez Scheme provides two ways to interact with "foreign" code, i.e., code written in other languages. The first is via subprocess creation and communication, which is discussed in Section 4.1. The second is via static or dynamic loading and invocation from Scheme of procedures written in C and invocation from C of procedures written in Scheme. These mechanisms are discussed in Sections 4.2 through 4.4.

The method for static loading of C object code is dependent upon which machine you are running; see the installation instructions distributed with Chez Scheme.

### Section 4.1. Subprocess Communication

Two procedures, `system` and `process`, are used to create subprocesses. Both procedures accept a single string argument and create a subprocess to execute the shell command contained in the string. The `system` procedure waits for the process to exit before returning, however, while the `process` procedure returns immediately without waiting for the process to exit. The standard input and output files of a subprocess created by `system` may be used to communicate with the user's console. The standard input and output files of a subprocess created by `process` may be used to communicate with the Scheme process.

procedure: `(system command)`
returns: see below
libraries: `(chezscheme)`

`command` must be a string.

The `system` procedure creates a subprocess to perform the operation specified by `command`. The subprocess may communicate with the user through the same console input and console output files used by the Scheme process. After creating the subprocess, `system` waits for the process to exit before returning.

When the subprocess exits, `system` returns the exit code for the subprocess, unless (on Unix-based systems) a signal caused the subprocess to terminate, in which case `system` returns the negation of the signal that caused the termination, e.g., -1 for `SIGHUP`.

procedure: `(open-process-ports command)`
procedure: `(open-process-ports command b-mode)`
procedure: `(open-process-ports command b-mode ?transcoder)`
returns: see below
libraries: `(chezscheme)`

`command` must be a string. If `?transcoder` is present and not `#f`, it must be a transcoder, and this procedure creates textual ports, each of whose transcoder is `?transcoder`. Otherwise, this procedure returns binary ports. `b-mode` specifies the buffer mode used by each of the ports returned by this procedure and defaults to `block`. Buffer modes are described in Section 7.2 of The Scheme Programming Language, 4th Edition.

`open-process-ports` creates a subprocess to perform the operation specified by `command`. Unlike `system`, `process` returns immediately after creating the subprocess, i.e., without waiting for the subprocess to terminate. It returns four values:

1. `to-stdin` is an output port to which Scheme can send output to the subprocess through the subprocess's standard input file.

2. `from-stdout` is an input port from which Scheme can read input from the subprocess through the subprocess's standard output file.

3. `from-stderr` is an input port from which Scheme can read input from the subprocess through the subprocess's standard error file.

4. `process-id` is an integer identifying the created subprocess provided by the host operating system.

If the process exits or closes its standard output file descriptor, any procedure that reads input from `from-stdout` will return an end-of-file object. Similarly, if the process exits or closes its standard error file descriptor, any procedure that reads input from `from-stderr` will return an end-of-file object.

The predicate `input-port-ready?` may be used to detect whether input has been sent by the subprocess to Scheme.

It is sometimes necessary to force output to be sent immediately to the subprocess by invoking `flush-output-port` on `to-stdin`, since Chez Scheme buffers the output for efficiency.

On UNIX systems, the `process-id` is the process identifier for the shell created to execute `command`. If `command` is used to invoke an executable file rather than a shell command, it may be useful to prepend `command` with the string `"exec "`, which causes the shell to load and execute the named executable directly, without forking a new process---the shell equivalent of a tail call. This will reduce by one the number of subprocesses created and cause `process-id` to reflect the process identifier for the executable once the shell has transferred control.

procedure: `(process command)`
returns: see explanation
libraries: `(chezscheme)`

`command` must be a string.

`process` is similar to `open-process-ports`, but less general. It does not return a port from which the subprocess's standard error output can be read, and it always creates textual ports. It returns a list of three values rather than the four separate values of `open-process-ports`. The returned list contains, in order: `from-stdout`, `to-stdin`, and `process-id`, which correspond to the second, first, and fourth return values of `open-process-ports`.

### Section 4.2. Calling out of Scheme

Chez Scheme's foreign-procedure interface allows a Scheme program to invoke procedures written in C or in languages that obey the same calling conventions as C. Two steps are necessary before foreign procedures can be invoked from Scheme. First, the foreign procedure must be compiled and loaded, either statically or dynamically, as described in Section 4.7. Then, access to the foreign procedure must be established in Scheme, as described in this section. Once access to a foreign procedure has been established it may be called as an ordinary Scheme procedure.

Since foreign procedures operate independently of the Scheme memory management and exception handling system, great care must be taken when using them. Although the foreign-procedure interface provides type checking (at optimize levels less than 3) and type conversion, the programmer must ensure that the sharing of data between Scheme and foreign procedures is done safely by specifying proper argument and result types.

Scheme-callable wrappers for foreign procedures can also be created via `ftype-ref` and function ftypes (Section 4.5).

syntax: `(foreign-procedure conv ... entry-exp (param-type ...) res-type)`
returns: a procedure
libraries: `(chezscheme)`

`entry-exp` must evaluate to a string representing a valid foreign procedure entry point or an integer representing the address of the foreign procedure. The `param-types` and `res-type` must be symbols or structured forms as described below. When a `foreign-procedure` expression is evaluated, a Scheme procedure is created that will invoke the foreign procedure specified by `entry-exp`. When the procedure is called each argument is checked and converted according to the specified `param-type` before it is passed to the foreign procedure. The result of the foreign procedure call is converted as specified by the `res-type`. Multiple procedures may be created for the same foreign entry.

Each `conv` adjusts the calling convention to be used. A `#f` is allowed as `conv` to indicate the default calling convention on the target machine (so the `#f` has no effect). Three other conventions are currently supported under Windows: `__stdcall`, `__cdecl`, and `__com` (32-bit only). Since `__cdecl` is the default, specifying `__cdecl` is equivalent to specifying `#f` or no convention. Additionally, `conv` can be `__collect_safe` to indicate that garbage collection is allowed concurrently with a call of the foreign procedure, `__atomic` to indicate that the foreign procedure never calls back to Scheme or its C library routines, `__alloc` to modify a preceding `__atomic` to allow allocation via Scheme's C library routines, `__disable_interrupts` to disable Scheme interrupts during the call, `__errno` or `__get_last_error` (Windows only) to get an error value as an additional result when the foreign procedure is called, or `__varargs` or `(__varargs_after n)` to indicate that the procedure uses a convention that works with a variable number of arguments after the first `n` (which may differ from the convention used for the fixed-argument variant on some platforms and conventions). `__varargs` is a shorthand for `(__varargs_after 1)`.

Use `__stdcall` to access most Windows API procedures. Use `__cdecl` for Windows API varargs procedures, for C library procedures, and for most other procedures. Use `__com` to invoke COM interface methods; COM uses the `__stdcall` convention but additionally performs the indirections necessary to obtain the correct method from a COM instance. The address of the COM instance must be passed as the first argument, which should normally be declared as `iptr`. For the `__com` interface only, `entry-exp` must evaluate to the byte offset of the method in the COM vtable. For example,

`(foreign-procedure __com 12 (iptr double-float) integer-32)`

creates an interface to a COM method at offset 12 in the vtable encapsulated within the COM instance passed as the first argument, with the second argument being a double float and the return value being an integer.

Use `__collect_safe` to declare that garbage collection is allowed concurrently with the foreign procedure. The `__collect_safe` declaration allows concurrent collection by deactivating the current thread (see `fork-thread`) when the foreign procedure is called, and the thread is activated again when the foreign procedure returns. The `__collect_safe` declaration is useful, for example, when calling a blocking I/O call to allow other Scheme threads to run normally. Refrain from passing collectable memory to a `__collect_safe` foreign procedure, or use `lock-object` to lock the memory in place; see also `Sdeactivate_thread`. The `__collect_safe` declaration has no effect on a non-threaded version of the system.

For example, calling the C `sleep` function with the default convention will block other Scheme threads from performing a garbage collection, but adding the `__collect_safe` declaration avoids that problem:

`(define c-sleep
(foreign-procedure __collect_safe "sleep" (unsigned) unsigned))
(c-sleep 10) ; sleeps for 10 seconds without blocking other threads`

If a foreign procedure with `__collect_safe` can invoke callables, then each callable should also be declared with `__collect_safe` so that the callable reactivates the thread.

Use `__atomic` to declare that the foreign procedure never calls back to Scheme, such as through a foreign callable, and never calls Scheme's C library routines as described in Section 4.10. If `__alloc` is used after `__atomic`, then the foreign procedure is allowed to call Scheme C library routines that merely allocate, such as allocating a string or pair. Using `__atomic` may allow a more efficient implementation of calls to the foreign function. The `__atomic` and `__collect_safe` declarations cannot be used at the same time.

Use `__disable_interrupts` to add a `(disable-interrupts)` call just before a call to the foreign procedure and a `(enable-interrupts)` call just afterward. The `__disable_interrupts` convention is intended more for use with foreign callables, where it is less readily implemented by using `with-interrupts-disabled` or by calling `disable-interrupts` and `enable-interrupts`.

Use `__errno` to make calling a foreign procedure return two values instead of one: the foreign procedure's direct result and the value of the C library's `errno` immediately after the foreign procedure returns (and before that value can change for the current thread). On Windows, see `current-errno-source` for more information about `errno`, or use `__get_last_error` instead of `__errno` to get a result from `GetLastError` instead of `errno`.

Complete type checking and conversion is performed on the parameters to a foreign procedure. The types `scheme-object`, `string`, `wstring`, `u8*`, `u16*`, `u32*`, `utf-8`, `utf-16`, `utf-16le`, `utf-16be`, `utf-32`, `utf-32le`, and `utf-32be`, must be used with caution, however, since they allow allocated Scheme objects to be used in places the Scheme memory management system cannot control. No problems will arise as long as such objects are not retained in foreign variables or data structures while Scheme code is running; as long as they are not passed as arguments to a `__collect_safe` procedure, since garbage collection can occur only while Scheme code is running or when concurrent garbage collection is enabled; and as long as the foreign procedure does not reenter Scheme via a callable before returning. Other parameter types are converted to equivalent foreign representations and consequently they can be retained indefinitely in foreign variables and data structures.

For argument types `string`, `wstring`, `utf-8`, `utf-16`, `utf-16le`, `utf-16be`, `utf-32`, `utf-32le`, and `utf-32be`, an argument is converted to a fresh object that is passed to the foreign procedure. Since the fresh object is not accessible for locking before the call, it can never be treated correctly for a `__collect_safe` foreign procedure, so those types are disallowed as argument types for a `__collect_safe` foreign procedure. For analogous reasons, those types are disallowed as the result of a `__collect_safe` foreign callable.

Following are the valid parameter types:

`integer-8`: Exact integers from -27 through 28 - 1 are valid. Integers in the range 27 through 28 - 1 are treated as two's complement representations of negative numbers, e.g., `#xff` is treated as -1. The argument is passed to C as an integer of the appropriate size (usually `signed char`).

`unsigned-8`: Exact integers from -27 to 28 - 1 are valid. Integers in the range -27 through -1 are treated as the positive equivalents of their two's complement representation, e.g., -1 is treated as `#xff`. The argument is passed to C as an unsigned integer of the appropriate size (usually `unsigned char`).

`integer-16`: Exact integers from -215 through 216 - 1 are valid. Integers in the range 215 through 216 - 1 are treated as two's complement representations of negative numbers, e.g., `#xffff` is treated as -1. The argument is passed to C as an integer of the appropriate size (usually `short`).

`unsigned-16`: Exact integers from -215 to 216 - 1 are valid. Integers in the range -215 through -1 are treated as the positive equivalents of their two's complement representation, e.g., -1 is treated as `#xffff`. The argument is passed to C as an unsigned integer of the appropriate size (usually `unsigned short`).

`integer-32`: Exact integers from -231 through 232 - 1 are valid. Integers in the range 231 through 232 - 1 are treated as two's complement representations of negative numbers, e.g., `#xffffffff` is treated as -1. The argument is passed to C as an integer of the appropriate size (usually `int`).

`unsigned-32`: Exact integers from -231 to 232 - 1 are valid. Integers in the range -231 through -1 are treated as the positive equivalents of their two's complement representation, e.g., -1 is treated as `#xffffffff`. The argument is passed to C as an unsigned integer of the appropriate size (usually `unsigned int`).

`integer-64`: Exact integers from -263 through 264 - 1 are valid. Integers in the range 263 through 264 - 1 are treated as two's complement representations of negative numbers. The argument is passed to C as an integer of the appropriate size (usually `long long` or, on many 64-bit platforms, `long`).

`unsigned-64`: Exact integers from -263 through 264 - 1 are valid. Integers in the range -263 through -1 are treated as the positive equivalents of their two's complement representation, The argument is passed to C as an integer of the appropriate size (usually `unsigned long long` or, on many 64-bit platforms, `long`).

`double-float`: Only Scheme flonums are valid---other Scheme numeric types are not automatically converted. The argument is passed to C as a double float.

`single-float`: Only Scheme flonums are valid---other Scheme numeric types are not automatically converted. The argument is passed to C as a single float. Since Chez Scheme represents flonums in double-float format, the parameter is first converted into single-float format.

`short`: This type is an alias for the appropriate fixed-size type above, depending on the size of a C `short`.

`unsigned-short`: This type is an alias for the appropriate fixed-size type above, depending on the size of a C `unsigned short`.

`int`: This type is an alias for the appropriate fixed-size type above, depending on the size of a C `int`.

`unsigned`: This type is an alias for the appropriate fixed-size type above, depending on the size of a C `unsigned`.

`unsigned-int`: This type is an alias `unsigned`. fixed-size type above, depending on the size of a C `unsigned`.

`long`: This type is an alias for the appropriate fixed-size type above, depending on the size of a C `long`.

`unsigned-long`: This type is an alias for the appropriate fixed-size type above, depending on the size of a C `unsigned long`.

`long-long`: This type is an alias for the appropriate fixed-size type above, depending on the size of the nonstandard C type `long long`.

`unsigned-long-long`: This type is an alias for the appropriate fixed-size type above, depending on the size of the nonstandard C type `unsigned long long`.

`ptrdiff_t`: This type is an alias for the appropriate fixed-size type above, depending on its definition in the host machine's `stddef.h` include file.

`size_t`: This type is an alias for the appropriate unsigned fixed-size type above, depending on its definition in the host machine's `stddef.h` include file.

`ssize_t`: This type is an alias for the appropriate signed fixed-size type above, depending on its definition in the host machine's `stddef.h` include file.

`iptr`: This type is an alias for the appropriate fixed-size type above, depending on the size of a C pointer.

`uptr`: This type is an alias for the appropriate (unsigned) fixed-size type above, depending on the size of a C pointer.

`void*`: This type is an alias for `uptr`.

`fixnum`: This type is equivalent to `iptr`, except only values in the fixnum range are valid. Transmission of fixnums is slightly faster than transmission of `iptr` values, but the fixnum range is smaller, so some `iptr` values do not have a fixnum representation.

`boolean`: Any Scheme object may be passed as a boolean. `#f` is converted to 0; all other objects are converted to 1. The argument is passed to C as an `int`.

`stdbool`: Any Scheme object may be passed as a boolean. `#f` is converted to 0; all other objects are converted to 1. The argument is passed to C as a `bool` defined by the the host machine's `stdbool.h` include file.

`char`: Only Scheme characters with Unicode scalar values in the range 0 through 255 are valid `char` parameters. The character is converted to its Unicode scalar value, as with `char->integer`, and passed to C as an `unsigned char`.

`wchar_t`: Only Scheme characters are valid `wchar_t` parameters. Under Windows and any other system where `wchar_t` holds only 16-bit values rather than full Unicode scalar values, only characters with 16-bit Unicode scalar values are valid. On systems where `wchar_t` is a full 32-bit value, any Scheme character is valid. The character is converted to its Unicode scalar value, as with `char->integer`, and passed to C as a `wchar_t`.

`wchar`: This type is an alias for `wchar_t`.

`double`: This type is an alias for `double-float`.

`float`: This type is an alias for `single-float`.

`scheme-object`: The argument is passed directly to the foreign procedure; no conversion or type checking is performed. This form of parameter passing should be used with discretion. Scheme objects should not be preserved in foreign variables or data structures since the memory management system may relocate them between foreign procedure calls.

`ptr`: This type is an alias for `scheme-object`.

`u8*`: The argument must be a Scheme bytevector or `#f`. For `#f`, the null pointer (0) is passed to the foreign procedure. For a bytevector, a pointer to the first byte of the bytevector's data is passed. If the C routine to which the data is passed requires the input to be null-terminated, a null (0) byte must be included explicitly in the bytevector. The bytevector should not be retained in foreign variables or data structures, since the memory management system may relocate or discard them between foreign procedure calls, and use their storage for some other purpose.

`u16*`: Arguments of this type are treated just like arguments of type `u8*`. If the C routine to which the data is passed requires the input to be null-terminated, two null (0) bytes must be included explicitly in the bytevector, aligned on a 16-bit boundary.

`u32*`: Arguments of this type are treated just like arguments of type `u8*`. If the C routine to which the data is passed requires the input to be null-terminated, four null (0) bytes must be included explicitly in the bytevector, aligned on a 32-bit boundary.

`utf-8`: The argument must be a Scheme string or `#f`. For `#f`, the null pointer (0) is passed to the foreign procedure. A string is converted into a bytevector, as if via `string->utf8`, with an added null byte, and the address of the first byte of the bytevector is passed to C. The bytevector should not be retained in foreign variables or data structures, since the memory management system may relocate or discard them between foreign procedure calls and use their storage for some other purpose. The `utf-8` argument type is not allowed for a `__collect_safe` foreign procedure.

`utf-16`: Arguments of this type are treated like arguments of type `utf-8`, except they are converted as if via `string->utf16` with endianness `(native-endianness)`, and they are extended by two null bytes rather than one.

`utf-16le`: Arguments of this type are treated like arguments of type `utf-8`, except they are converted as if via `string->utf16` with endianness `little`, and they are extended by two null bytes rather than one.

`utf-16be`: Arguments of this type are treated like arguments of type `utf-8`, except they are converted as if via `string->utf16` with endianness `big`, and they are extended by two null bytes rather than one.

`utf-32`: Arguments of this type are treated like arguments of type `utf-8`, except they are converted as if via `string->utf32` with endianness `(native-endianness)`, and they are extended by four null bytes rather than one.

`utf-32le`: Arguments of this type are treated like arguments of type `utf-8`, except they are converted as if via `string->utf32` with endianness `little`, and they are extended by four null bytes rather than one.

`utf-32be`: Arguments of this type are treated like arguments of type `utf-8`, except they are converted as if via `string->utf32` with endianness `big`, and they are extended by four null bytes rather than one.

`string`: This type is an alias for `utf-8`.

`wstring`: This type is an alias for `utf-16` or `utf-32` as appropriate depending on the size of a C `wchar_t`. For example, `wstring` is equivalent to `utf-16` under Windows running on Intel hardware.

`ftype-name`: Any defined foreign type (ftype) can be used as long as it is equivalent to a type that would be allowed when written directly. See Section 4.5 for a description of foreign types.

`(* ftype-name)`: This type allows a pointer to a foreign type (ftype) to be passed. The argument must be an ftype pointer of the type identified by `ftype-name`, and the actual argument is the address encapsulated in the ftype pointer.

`ftype-pointer`: This type allows any pointer to a foreign type (ftype) to be passed, independent of the foreign type, and including generic foreign pointers and foreign pointers to Scheme objects. In other words, this type is specific to foreign pointers created with `make-ftype-pointer` and similar, but otherwise generic like `void*`.

`ftype-scheme-object-pointer`: This type allows a foreign pointer to a Scheme object, such as one created with `make-ftype-scheme-object-pointer`. The address received on the C side is like the one produced by `object->reference-address` for a non-`#f` value; using the result of `object->reference-address` with `make-ftype-pointer` would produce the result, but only with locked objects that cannot move, while `make-ftype-scheme-object-pointer` defers conversion to an address to a point before the call but after a garbage collection can take place (assuming that `__collect_safe` is not used). Note that `ftype-scheme-object-pointer` differs from `scheme-object` by delivering specifically a reference address to the foreign procedure, which can be useful for communicating bytevector and flvector content.

`(& ftype-name)`: This type allows a foreign type (ftype) to be passed as a value, but represented on the Scheme side as a pointer to the foreign-type data. That is, a `(& ftype-name)` argument is represented on the Scheme side the same as a `(* ftype-name)` argument, but a `(& ftype-name)` argument is passed to the foreign procedure as the content at the foreign pointer's address instead of as the address. For example, if `ftype-name` identifies a `struct` type, then `(& ftype-name)` passes a struct argument instead of a struct-pointer argument. The `ftype-name` cannot refer to an array type. The variant `(& ftype-name ftype-name)` is the same, except that the Scheme-side representation corresponds to the second `ftype-name`, which can be any pointer type.

The result types are similar to the parameter types with the addition of a `void` type. In general, the type conversions are the inverse of the parameter type conversions. No error checking is performed on return, since the system cannot determine whether a foreign result is actually of the indicated type. Particular caution should be exercised with the result types `scheme-object`, `double-float`, `double`, `single-float`, `float`, `ftype-scheme-object-pointer`, and the types that result in the construction of bytevectors or strings, since invalid return values may lead to invalid memory references as well as incorrect computations. Following are the valid result types:

`void`: The result of the foreign procedure call is ignored and an unspecified Scheme object is returned. `void` should be used when foreign procedures are called for effect only.

`integer-8`: The result is interpreted as a signed 8-bit integer and is converted to a Scheme exact integer.

`unsigned-8`: The result is interpreted as an unsigned 8-bit integer and is converted to a Scheme nonnegative exact integer.

`integer-16`: The result is interpreted as a signed 16-bit integer and is converted to a Scheme exact integer.

`unsigned-16`: The result is interpreted as an unsigned 16-bit integer and is converted to a Scheme nonnegative exact integer.

`integer-32`: The result is interpreted as a signed 32-bit integer and is converted to a Scheme exact integer.

`unsigned-32`: The result is interpreted as an unsigned 32-bit integer and is converted to a Scheme nonnegative exact integer.

`integer-64`: The result is interpreted as a signed 64-bit integer and is converted to a Scheme exact integer.

`unsigned-64`: The result is interpreted as an unsigned 64-bit integer and is converted to a Scheme nonnegative exact integer.

`double-float`: The result is interpreted as a double float and is translated into a Chez Scheme flonum.

`single-float`: The result is interpreted as a single float and is translated into a Chez Scheme flonum. Since Chez Scheme represents flonums in double-float format, the result is first converted into double-float format.

`short`: This type is an alias for the appropriate fixed-size type above, depending on the size of a C `short`.

`unsigned-short`: This type is an alias for the appropriate fixed-size type above, depending on the size of a C `unsigned short`.

`int`: This type is an alias for the appropriate fixed-size type above, depending on the size of a C `int`.

`unsigned`: This type is an alias for the appropriate fixed-size type above, depending on the size of a C `unsigned`.

`unsigned-int`: This type is an alias `unsigned`. fixed-size type above, depending on the size of a C `unsigned`.

`long`: This type is an alias for the appropriate fixed-size type above, depending on the size of a C `long`.

`unsigned-long`: This type is an alias for the appropriate fixed-size type above, depending on the size of a C `unsigned long`.

`long-long`: This type is an alias for the appropriate fixed-size type above, depending on the size of the nonstandard C type `long long`.

`unsigned-long-long`: This type is an alias for the appropriate fixed-size type above, depending on the size of the nonstandard C type `unsigned long long`.

`ptrdiff_t`: This type is an alias for the appropriate fixed-size type above, depending on its definition in the host machine's `stddef.h` include file.

`size_t`: This type is an alias for the appropriate unsigned fixed-size type above, depending on its definition in the host machine's `stddef.h` include file.

`ssize_t`: This type is an alias for the appropriate signed fixed-size type above, depending on its definition in the host machine's `stddef.h` include file.

`iptr`: This type is an alias for the appropriate fixed-size type above, depending on the size of a C pointer.

`uptr`: This type is an alias for the appropriate (unsigned) fixed-size type above, depending on the size of a C pointer.

`void*`: This type is an alias for `uptr`.

`boolean`: This type converts a C `int` return value into a Scheme boolean. 0 is converted to `#f`; all other values are converted to `#t`.

`stdbool`: This type converts a C `bool` return value into a Scheme boolean, where `bool` is from the host machine's `stdbool.h` include file. 0 is converted to `#f`; all other values are converted to `#t`.

`char`: This type converts a C `unsigned char` return value into a Scheme character, as if via `integer->char`.

`wchar_t`: This type converts a C `wchar_t` return value into a Scheme character, as if via `integer->char`. The `wchar_t` value must be a valid Unicode scalar value.

`wchar`: This type is an alias for `wchar_t`.

`double`: This type is an alias for `double-float`.

`float`: This type is an alias for `single-float`.

`scheme-object`: The result is assumed to be a valid Scheme object, and no conversion is performed. This type is inherently dangerous, since an invalid Scheme object can corrupt the memory management system with unpredictable (but always unpleasant) results. Since Scheme objects are actually typed pointers, even integers cannot safely be returned as type `scheme-object` unless they were created by the Scheme system.

`ptr`: This type is an alias for `scheme-object`.

`u8*`: The result is interpreted as a pointer to a null-terminated sequence of 8-bit unsigned integers (bytes). If the result is a null pointer, `#f` is returned. Otherwise, the sequence of bytes is stored in a freshly allocated bytevector of the appropriate length, and the bytevector is returned to Scheme.

`u16*`: The result is interpreted as a pointer to a null-terminated sequence of 16-bit unsigned integers. If the result is a null pointer, `#f` is returned. Otherwise, the sequence of 16-bit integers is stored in a freshly allocated bytevector of the appropriate length, and the bytevector is returned to Scheme. The null terminator must be a properly aligned 16-bit word, i.e., two bytes of zero aligned on a 16-bit boundary.

`u32*`: The result is interpreted as a pointer to a null-terminated sequence of 32-bit unsigned integers. If the result is a null pointer, `#f` is returned. Otherwise, the sequence of 16-bit integers is stored in a freshly allocated bytevector of the appropriate length, and the bytevector is returned to Scheme. The null terminator must be a properly aligned 32-bit word, i.e., four bytes of zero aligned on a 32-bit boundary.

`utf-8`: The result is interpreted as a pointer to a null-terminated sequence of 8-bit unsigned character values. If the result is a null pointer, `#f` is returned. Otherwise, the sequence of bytes is converted into a Scheme string, as if via `utf8->string`, and the string is returned to Scheme.

`utf-16`: The result is interpreted as a pointer to a null-terminated sequence of 16-bit unsigned integers. If the result is a null pointer, `#f` is returned. Otherwise, the sequence of integers is converted into a Scheme string, as if via `utf16->string` with endianness `(native-endianness)`, and the string is returned to Scheme. A byte-order mark in the sequence of integers is treated as an ordinary character value and does not affect the byte ordering.

`utf-16le`: The result is interpreted as a pointer to a null-terminated sequence of 16-bit unsigned integers. If the result is a null pointer, `#f` is returned. Otherwise, the sequence of integers is converted into a Scheme string, as if via `utf16->string` with endianness `little`, and the string is returned to Scheme. A byte-order mark in the sequence of integers is treated as an ordinary character value and does not affect the byte ordering.

`utf-16be`: The result is interpreted as a pointer to a null-terminated sequence of 16-bit unsigned integers. If the result is a null pointer, `#f` is returned. Otherwise, the sequence of integers is converted into a Scheme string, as if via `utf16->string` with endianness `big`, and the string is returned to Scheme. A byte-order mark in the sequence of integers is treated as an ordinary character value and does not affect the byte ordering.

`utf-32`: The result is interpreted as a pointer to a null-terminated sequence of 32-bit unsigned integers. If the result is a null pointer, `#f` is returned. Otherwise, the sequence of integers is converted into a Scheme string, as if via `utf32->string` with endianness `(native-endianness)`, and the string is returned to Scheme. A byte-order mark in the sequence of integers is treated as an ordinary character value and does not affect the byte ordering.

`utf-32le`: The result is interpreted as a pointer to a null-terminated sequence of 32-bit unsigned integers. If the result is a null pointer, `#f` is returned. Otherwise, the sequence of integers is converted into a Scheme string, as if via `utf32->string` with endianness `little`, and the string is returned to Scheme. A byte-order mark in the sequence of integers is treated as an ordinary character value and does not affect the byte ordering.

`utf-32be`: The result is interpreted as a pointer to a null-terminated sequence of 32-bit unsigned integers. If the result is a null pointer, `#f` is returned. Otherwise, the sequence of integers is converted into a Scheme string, as if via `utf32->string` with endianness `big`, and the string is returned to Scheme. A byte-order mark in the sequence of integers is treated as an ordinary character value and does not affect the byte ordering.

`string`: This type is an alias for `utf-8`.

`wstring`: This type is an alias for `utf-16` or `utf-32` as appropriate depending on the size of a C `wchar_t`. For example, `wstring` is equivalent to `utf-16` under Windows running on Intel hardware.

`ftype-name`: Any defined foreign type (ftype) can be used as long as it is equivalent to a type that would be allowed when written directly. See Section 4.5 for a description of foreign types.

`(* ftype-name)`: The result is interpreted as the address of a foreign object whose structure is described by the ftype identified by `ftype-name`, and a freshly allocated ftype pointer encapsulating the address is returned.

`ftype-pointer`: The result is interpreted as the address of a foreign object of unspecified content, and a freshly allocated generic ftype pointer encapsulating the address is returned.

`ftype-scheme-object-pointer`: The result is interpreted as the reference address of a Scheme object, and it is returned as encapsulated within a foreign object pointer as created with `make-ftype-scheme-object-pointer`. Note that `ftype-scheme-object-pointer` differs from `scheme-object` in that it receives a reference address from the foreign procedure, which might be the start of bytevector or flvector content. Like `scheme-object`, however, this type is inherently dangerous, and a foreign procedure would rarely be expected to produce such a value.

`(& ftype-name)`: The result is interpreted as a foreign object whose structure is described by the ftype identified by `ftype-name`, where the foreign procedure returns a `ftype-name` result, but the caller must provide an extra `(* ftype-name)` argument before all other arguments to receive the result. An unspecified Scheme object is returned when the foreign procedure is called, since the result is instead written into storage referenced by the extra argument. The `ftype-name` cannot refer to an array type. The variant `(& ftype-name ftype-name)` is the same, except that the foreign pointer passed as an extra initial argument corresponds to the second `ftype-name`, which can be any pointer type.

Consider a C identity procedure:

`int id(x) int x; { return x; }`

After a file containing this procedure has been compiled and loaded (see Section 4.7) it can be accessed as follows:

`(foreign-procedure "id"
(int) int) #<procedure>
((foreign-procedure "id"
(int) int)
1) 1
(define int-id
(foreign-procedure "id"
(int) int))
(int-id 1) 1`

The `"id"` entry can also be interpreted as accepting and returning a boolean:

`(define bool-id
(foreign-procedure "id"
(boolean) boolean))
(bool-id #f) #f
(bool-id #t) #t
(bool-id 1) #t`

As the last example reveals, `bool-id` is actually a conversion procedure. When a Scheme object is passed as type `boolean` it is converted to 0 or 1, and when it is returned it is converted to `#f` or `#t`. As a result objects are converted to normalized boolean values. The `"id"` entry can be used to create other conversion procedures by varying the type specifications:

`(define int->bool
(foreign-procedure "id"
(int) boolean))
(int->bool 0) #f
(int->bool 5) #t
(map (foreign-procedure "id"
(boolean) int)
'(#t #f)) (1 0)
(define void
(foreign-procedure "id"
(int) void))
(void 10) unspecified`

There are, of course, simpler and more efficient ways of accomplishing these conversions directly in Scheme.

A foreign entry is resolved when a `foreign-procedure` expression is evaluated, rather than either when the code is loaded or each time the procedure is invoked. Thus, the following definition is always valid since the `foreign-procedure` expression is not immediately evaluated:

`(define doit
(lambda ()
((foreign-procedure "doit" () void))))`

`doit` should not be invoked, however, before an entry for `"doit"` has been provided. Similarly, an entry for `"doit"` must exist before the following code is evaluated:

`(define doit
(foreign-procedure "doit" () void))`

Although the second definition is more constraining on the load order of foreign files, it is more efficient since the entry resolution need be done only once.

It is often useful to define a template to be used in the creation of several foreign procedures with similar argument types and return values. For example, the following code creates two foreign procedures from a single foreign procedure expression, by abstracting out the foreign procedure name:

`(define double->double
(lambda (proc-name)
(foreign-procedure proc-name
(double)
double)))
(define log10 (double->double "log10"))
(define gamma (double->double "gamma"))`

Both `"log10"` and `"gamma"` must be available as foreign entries (see Section 4.7) before the corresponding definitions. The use of foreign procedure templates can simplify the coding process and reduce the amount of code generated when a large number of foreign procedures are involved, e.g., when an entire library of foreign procedures is imported into Scheme.

### Section 4.3. Calling into Scheme

Section 4.2 describes the `foreign-procedure` form, which permits Scheme code to invoke C or C-compatible foreign procedures. This section describes the `foreign-callable` form, which permits C or C-compatible code to call Scheme procedures. A more primitive mechanism for calling Scheme procedures from C is described in Section 4.10.

As when calling foreign procedures from Scheme, great care must be taken when sharing data between Scheme and foreign code that calls Scheme to avoid corrupting Scheme's memory management system.

A foreign-callable wrapper for a Scheme procedure can also be created by passing the procedure to `make-ftype-pointer` with an appropriate function ftype (Section 4.5).

syntax: `(foreign-callable conv ... proc-exp (param-type ...) res-type)`
returns: a code object
libraries: `(chezscheme)`

`proc-exp` must evaluate to a procedure, the Scheme procedure that is to be invoked by foreign code. The parameter and result types are as described for `foreign-procedure` in Section 4.2, except that the requirements and conversions are effectively reversed, e.g., the conversions described for `foreign-procedure` arguments are performed for `foreign-callable` return values. A `(& ftype)` argument to the callable refers to an address that is valid only during the dynamic extent of the callback invocation. A `(& ftype)` result type for a callable causes the Scheme procedure to receive an extra `(& ftype)` argument before all others; the Scheme procedure should write a result into the extra argument, and the direct result of the Scheme procedure is ignored. A `(& ftype-name ftype-name)` argument or result is the same, except that the Scheme-side representation corresponds to the second `ftype-name`, which can be any pointer type. Type checking is performed for result values but not argument values, since the parameter values are provided by the foreign code and must be assumed to be correct.

Each `conv` adjusts the calling convention to be used. `foreign-callable` supports the same conventions as `foreign-procedure` except for `__com`, `__atomic`, `__alloc`, `__errno`, or `__get_last_error` (which are ignored). The `__collect_safe` convention for a callable activates a calling thread if the thread is not already activated, and the thread's activation state is reverted when the callable returns. If a calling thread is not currently registered with the Scheme system, then reverting the thread's activation state implies destroying the thread's registration (see `Sdestroy_thread`). The `__disable_interrupts` convention for a callable disables interrupts (in the same sense as `disable-interrupts`) before the first point that a callable would otherwise allow interrupt handling, and it reenables interrupts after the last point where the callable would otherwise allow interrupt handling; the `proc-exp` is free to enable interrupts at a point where it is ready to allow them, in which case it should disable interrupts again before returning.

The value produced by `foreign-callable` is a Scheme code object, which contains some header information as well as code that performs the call to the encapsulated Scheme procedure. The code object may be converted into a foreign-callable address via `foreign-callable-entry-point`, which returns an integer representing the address of the entry point within the code object. (The C-callable library function `Sforeign_callable_entry_point`, described in Section 4.10, may be used to obtain the entry point as well.) This is an implicit pointer into an immobile Scheme object, so it will not be relocated by the storage management system, but it may be reclaimed if the code object becomes unreachable on the Scheme side; lock the code object (using `lock-object`) or otherwise retain it if the entry point is, for example, registered as a callback and retained in the "C" side indefinitely.

The following code creates a foreign-callable code object, locks the code object, and returns the entry point.

`(let ([x (foreign-callable
(lambda (x y) (pretty-print (cons x (* y 2))))
(string integer-32)
void)])
(lock-object x)
(foreign-callable-entry-point x))`

Unless the entry point is intended to be permanent, however, a pointer to the code object returned by `foreign-callable` should be retained (in which case locking is unnecessary, since the storage management system will not move the code object as long as it remains reachable on the Scheme side).

Mixed use of `foreign-callable` and `foreign-procedure` may result in nesting of foreign and Scheme calls, and this results in some interesting considerations when continuations are involved, directly or indirectly (as via the default exception handler). See Section 4.4 for a discussion of the interaction between foreign calls and continuations.

The following example demonstrates how the "callback" functions required by many windowing systems might be defined in Scheme with the use of `foreign-callable`. Assume that the following C code has been compiled and loaded (see Section 4.7).

`#include <stdio.h>
typedef void (*CB)(char);
CB callbacks[256];
void cb_init(void) {
int i;
for (i = 0; i < 256; i += 1)
callbacks[i] = (CB)0;
}
void register_callback(char c, CB cb) {
callbacks[c] = cb;
}
void event_loop(void) {
CB f; char c;
for (;;) {
c = getchar();
if (c == EOF) break;
f = callbacks[c];
if (f != (CB)0) f(c);
}
}`

Interfaces to these functions may be defined in Scheme as follows.

`(define cb-init
(foreign-procedure "cb_init" () void))
(define register-callback
(foreign-procedure "register_callback" (char void*) void))
(define event-loop
(foreign-procedure __collect_safe "event_loop" () void))`

A callback for selected characters can then be defined.

`(define callback
(lambda (p)
(let ([code (foreign-callable __collect_safe p (char) void)])
(lock-object code)
(foreign-callable-entry-point code))))
(define ouch
(callback
(lambda (c)
(printf "Ouch! Hit by '~c'~%" c))))
(define rats
(callback
(lambda (c)
(printf "Rats! Received '~c'~%" c))))
(cb-init)
(register-callback #\a ouch)
(register-callback #\c rats)
(register-callback #\e ouch)`

This sets up the following interaction.

`> (event-loop)
a
Ouch! Hit by 'a'
b
c
Rats! Received 'c'
d
e
Ouch! Hit by 'e'`

The `__collect_safe` declarations in this example ensure that other threads can continue working while `event-loop` blocks waiting for input. A more well-behaved version of the example would save each code object returned by `foreign-callable` and unlock it when it is no longer registered as a callback.

procedure: `(foreign-callable-entry-point code)`
returns: the address of the foreign-callable entry point in `code`
libraries: `(chezscheme)`

`code` should be a code object produced by `foreign-callable`.

procedure: `(foreign-callable-code-object address)`
returns: the code object corresponding to the foreign-callable entry point `address`
libraries: `(chezscheme)`

`address` must be an exact integer and should be the address of the entry point of a code object produced by `foreign-callable`.

### Section 4.4. Continuations and Foreign Calls

`foreign-callable` and `foreign-procedure` allow arbitrary nesting of foreign and Scheme calls. Because other languages do not support the fully general first-class continuations of Scheme, the interaction between continuations and nested calls among Scheme and foreign procedures is problematic. Chez Scheme handles this interaction in a general manner by trapping attempts to return to stale foreign contexts rather than by restricting the use of continuations directly. A foreign context is a foreign frame and return point corresponding to a particular call from a foreign language, e.g., C, into Scheme. A foreign context becomes stale after a normal return to the context or after a return to some other foreign context beneath it on the control stack.

As a result of this treatment, Scheme continuations may be used to throw control either upwards or downwards logically through any mix of Scheme and foreign frames. Furthermore, until some return to a foreign context is actually performed, all return points remain valid. In particular, this means that programs that use continuations exclusively for nonlocal exits never attempt to return to a stale foreign context. (Nonlocal exits themselves are no problem and are implemented by the C library function `longjmp` or the equivalent.) Programs that use continuations more generally also function properly as long as they never actually return to a stale foreign context, even if control logically moves past stale foreign contexts via invocation of continuations.

One implication of this mechanism is that the C stack pointer is not automatically restored to its base value when a continuation is used on the Scheme side to perform a nonlocal exit. If the program continues to run after the nonlocal exit, any further build-up of the C stack will add to the existing build up, which might result in a C stack overflow. To avoid this situation, a program can arrange to set up a single C call frame before obtaining the continuation and return to the C frame after the nonlocal exit. The procedure `with-exit-proc` below arranges to do this without involving any C code.

`(define with-exit-proc
(lambda (p)
(define th (lambda () (call/cc p)))
(define-ftype ->ptr (function () ptr))
(let ([fptr (make-ftype-pointer ->ptr th)])
(let ([v ((ftype-ref ->ptr () fptr))])
(unlock-object
(foreign-callable-code-object
(ftype-pointer-address fptr)))
v))))`

`with-exit-proc` behaves like `call/cc` except it resets the C stack when the continuation is invoked. To do this, it creates an ftype-pointer representing a foreign-callable entry point for `th` and creates a Scheme-callable procedure for that entry point. This creates a wrapper for `th` that involves a C call. When a call to the wrapper returns, either by explicit invocation of the continuation passed to `p` or by a normal return from `p`, the C stack is reset to its original value.

### Section 4.5. Foreign Data

The procedures described in this section directly create and manipulate foreign data, i.e., data that resides outside of the Scheme heap. With the exception of `foreign-alloc`, `foreign-sizeof`, and `foreign-alignof`, these procedures are inherently unsafe in the sense that they do not (and cannot) check the validity of the addresses they are passed. Improper use of these procedures can result in invalid memory references, corrupted data, or system crashes.

This section also describes a higher-level syntactic mechanism for manipulating foreign data, including foreign structures, unions, arrays, and bit fields. The syntactic interface is safer than the procedural interface but must still assume that the addresses it's given are appropriate for the types of object being manipulated.

procedure: `(foreign-alloc n)`
returns: the address of a freshly allocated block of foreign data `n` bytes long
libraries: `(chezscheme)`

`n` must be a positive fixnum. The returned value is an exact integer and is guaranteed to be properly aligned for any type of value according to the requirements of the underlying hardware. An exception is raised with condition type `&assertion` if the block of foreign data cannot be allocated.

procedure: `(foreign-free address)`
returns: unspecified
libraries: `(chezscheme)`

This procedure frees the block of storage to which `address` points. `address` must be an exact integer in the range -2w-1 through 2w - 1, where w is the width in bits of a pointer, e.g., 64 for a 64-bit machine. It should be an address returned by an earlier call to `foreign-alloc` and not subsequently passed to `foreign-free`.

procedure: `(foreign-ref type address offset)`
returns: see below
libraries: `(chezscheme)`

`foreign-ref` extracts the value of type `type` from the memory location at `offset` bytes offset from `address`.

`type` must be a symbol identifying the type of value to be extracted. The following types have machine-dependent sizes and correspond to the like-named C types:

- `short`,
- `unsigned-short`,
- `int`,
- `unsigned`,
- `unsigned-int`,
- `long`,
- `unsigned-long`,
- `long-long`,
- `unsigned-long-long`,
- `ptrdiff_t`,
- `size_t`,
- `ssize_t`,
- `char`,
- `wchar_t`,
- `float`,
- `double`, and
- `void*`.

The types `long-long` and `unsigned-long-long` correspond to the C types `long long` and `unsigned long long`. A value of type `char` is referenced as a single byte and converted (as if via `integer->char`) into a Scheme character. A value of type `wchar_t` is converted (as if via `integer->char`) into a Scheme character. The value must be a valid Unicode scalar value.

`wchar` is an alias for `wchar_t`.

Several additional machine-dependent types are recognized:

- `iptr`,
- `uptr`,
- `fixnum`,
- `boolean`, and
- `stdbool`.

`uptr` is equivalent to `void*`; both are treated as unsigned integers the size of a pointer. `iptr` is treated as a signed integer the size of a pointer. `fixnum` is treated as an `iptr`, but with a range limited to the fixnum range. `boolean` is treated as an `int`, with zero converted to the Scheme value `#f` and all other values converted to `#t`. `stdbool` is treated as an `bool` from `stdbool.h`, with zero converted to the Scheme value `#f` and all other values converted to `#t`.

Finally, several fixed-sized types are also supported:

- `integer-8`,
- `unsigned-8`,
- `integer-16`,
- `unsigned-16`,
- `integer-32`,
- `unsigned-32`,
- `integer-64`,
- `unsigned-64`,
- `single-float`, and
- `double-float`.

`address` must be an exact integer in the range -2w-1 through 2w - 1, where w is the width in bits of a pointer, e.g., 64 for a 64-bit machine. `offset` must be an exact fixnum. The sum of `address` and `offset` should address a readable block of memory large enough to hold a value of type `type`, within a block of storage previously returned by `foreign-alloc` and not subsequently freed by `foreign-free` or within a block of storage obtained via some other mechanism, e.g., a foreign call. For multiple-byte values, the native endianness of the machine is assumed.

procedure: `(foreign-set! type address offset value)`
returns: see below
libraries: `(chezscheme)`

`foreign-set!` stores a representation of `value` as type `type` `offset` bytes into the block of foreign data addressed by `address`.

`type` must be a symbol identifying the type of value to be stored, one of those listed in the description of `foreign-ref` above. Scheme characters are converted to type `char` or `wchar_t` as if via `char->integer`. For type `boolean` or `stdbool`, Scheme `#f` is converted to the `int` 0, and any other Scheme object is converted to 1.

`address` must be an exact integer in the range -2w-1 through 2w - 1, where w is the width in bits of a pointer, e.g., 64 for a 64-bit machine. `offset` must be an exact fixnum. The sum of `address` and `offset` should address a writable block of memory large enough to hold a value of type `type`, within a block of storage previously returned by `foreign-alloc` and not subsequently freed by `foreign-free` or within a block of storage obtained via some other mechanism, e.g., a foreign call. `value` must be an appropriate value for `type`, e.g., a floating-point number for the float types or an exact integer within the appropriate range for the integer types. For multiple-byte values, the native endianness of the machine is assumed.

procedure: `(foreign-sizeof type)`
returns: the size in bytes of `type`
libraries: `(chezscheme)`

`type` must be one of the symbols listed in the description of `foreign-ref` above.

procedure: `(foreign-alignof type)`
returns: the alignment in bytes of `type`
libraries: `(chezscheme)`

`type` must be one of the symbols listed in the description of `foreign-ref` above.

syntax: `(define-ftype ftype-name ftype)`
syntax: `(define-ftype ftype-name ftype (nongenerative uid))`
syntax: `(define-ftype (ftype-name ftype maybe-uid-clause) ...)`
returns: unspecified
libraries: `(chezscheme)`

A `define-ftype` form is a definition and can appear anywhere other definitions can appear. It establishes one or more foreign-type (ftype) bindings for the identifier `ftype-name` or identifiers `ftype-name ...` to the foreign type represented `ftype` or the foreign types represented by `ftype ...`. Each `ftype-name` can be used to access foreign objects with the declared shape, and each can be used in the formation of other ftypes. When multiple `ftype-name`s are provided, each `maybe-uid-clause` is either empty or of the form `(nongenerative uid)`. Each `uid` must be a gensym.

An `ftype` must take one of the following forms:

`ftype-name
(struct (field-name ftype) ...)
(union (field-name ftype) ...)
(array length ftype)
(* ftype)
(bits (field-name signedness bits) ...)
(function conv ... (ftype ...) ftype)
(packed ftype)
(unpacked ftype)
(endian endianness ftype)`

where `length` is an exact nonnegative integer, `bits` is an exact positive integer, `field-name` is an identifier, `conv` is `#f` or a string naming a valid convention as described on page 4.2, signedness is either `signed` or `unsigned`, and endianness is one of `native`, `swapped`, `big`, or `little`.

A restriction not reflected above is that `function` ftypes cannot be used as the types of field names or array elements. That is, function ftypes are valid only at the top level of an ftype, e.g,:

`(define-ftype bvcopy_t (function (u8* u8* size_t) void))`

or as the immediate sub-type of a pointer (`*`) ftype, as in the following definitions, which are equivalent assuming the definition of `bvcopy_t` above.

`(define-ftype A
(struct
[x int]
[f (* (function (u8* u8* size_t) void))]))
(define-ftype A
(struct
[x int]
[f (* bvcopy_t)]))`

That is, a function cannot be embedded within a struct, union, or array, but a pointer to a function can be so embedded.

The following definitions establish ftype bindings for `F`, `A`, and `E`.

`(define-ftype F (function (wchar_t int) int))
(define-ftype A (array 10 wchar_t))
(define-ftype E
(struct
[a int]
[b double]
[c (array 25
(struct
[a short]
[_ long]
[b A]))]
[d (endian big
(union
[v1 unsigned-32]
[v2 (bits
[hi unsigned 12]
[lo unsigned 20])]))]
[e (* A)]
[f (* F)]))`

The ftype `F` describes the type of a foreign function that takes two arguments, a wide character and an integer, and returns an integer. The ftype `A` is simply an array of 10 `wchar_t` values, and its size will be 10 times the size of a single `wchar_t`. The ftype `E` is a structure with six fields: an integer `a`, a double-float `b`, an array `c`, a union `d`, a pointer `e`, and a pointer `f`. The array `c` is an array of 25 structs, each of which contains a short integer, a long integer, and a `A` array. The size of the `c` array will be 25 times the size of a single `A` array, plus 25 times the space needed to store each of the short and long integers. The union `d` is either a 32-bit unsigned integer or a 32-bit unsigned integer split into high (12 bits) and low (20 bits) components. The fields of a union overlap so that writing to one effectively overlaps the other. Thus, one can use the `d` union type to split apart an unsigned integer by writing the integer into `v1` and reading the pieces from `hi` and `lo`. The pointer `e` points to an `A` array; it is not itself an array, and its size is just the size of a single pointer. Similarly, `f` points to a function, and its size is also that of a single pointer.

An underscore ( `_` ) can be used as the field name for one or more fields of a `struct`, `union`, or `bits` ftype. Such fields are included in the layout but are considered unnamed and cannot be accessed via the ftype operators described below. Thus, in the example above, the `long` field within the `c` array is inaccessible.

Non-underscore field names are handled symbolically, i.e., they are treated as symbols rather than identifiers. Each symbol must be unique (as a symbol) with respect to the other field names within a single `struct`, `union`, or `bits` ftype but need not be unique with respect to field names in other `struct`, `union`, or `bits` ftypes within the same ftype.

Each `ftype-name` in an `ftype` must either (a) have been defined previously by `define-ftype`, (b) be defined by the current `define-ftype`, (c) be a base-type name, i.e., one of the type names supported by `foreign-ref` and `foreign-set!`, or (d) be `ftype-pointer` or `ftype-scheme-object-pointer`, which indicate a generic ftype pointer or an ftype pointer for a Scheme object, respectively. In case (b), any reference within one `ftype` to the `ftype-name` of one of the earlier bindings is permissible, but a reference to the `ftype-name` of the current or a subsequent binding can appear only within a pointer field.

For example, in:

`(define-ftype
[Qlist (struct
[head int]
[tail (* Qlist)])])`

the reference to `Qlist` is permissible since it appears within a pointer field. Similarly, in:

`(define-ftype
[Qfrob (struct
[head int]
[tail (* Qsnark)])]
[Qsnark (struct
[head int]
[xtra Qfrob]
[tail (* Qfrob)])])`

the mutually recursive references to `Qsnark` and `Qfrob` are permissible. In the following, however:

`(define-ftype
[Qfrob (struct
[head int]
[xtra Qfrob]
[tail (* Qsnark)])]
[Qsnark (struct
[head int]
[tail (* Qfrob)])])`

the reference to `Qfrob` within the `ftype` for `Qfrob` is invalid, and in:

`(define-ftype
[Qfrob (struct
[head int]
[xtra Qsnark]
[tail (* Qsnark)])]
[Qsnark (struct
[head int]
[tail (* Qfrob)])])`

the reference to `Qsnark` is similarly invalid.

By default, padding is inserted where appropriate to maintain proper alignment of multiple-byte scalar values in an attempt to mirror the target machine's C struct layout conventions, where such layouts are adequately documented. For packed ftypes (ftypes wrapped in a `packed` form with no closer enclosing `unpacked` form), this padding is not inserted.

Multiple-byte scalar values are stored in memory using the target machine's native "endianness," e.g., `little` on x86 and x86_64-based platforms and `big` on Sparc-based platforms. Big-endian or little-endian representation can be forced via the `endian` ftype with a `big` or `little` `endianness` specifier. The `native` specifier can be used to force a return back to `native` representation. The `swapped` specifier can be used to swap the representation relative to the default or enclosing representation. Each `endian` form affects only ftypes nested syntactically within it and not nested within a closer `endian` form. The endianness of an ftype is fixed once it is defined.

The total size n of the fields within an ftype bits form must be 8, 16, 24, 32, 40, 48, 56, or 64. Padding must be added manually, if needed. In little-endian representation, the first field occupies the low-order bits of the containing 8, 16, 24, 32, 40, 48, 56, or 64-bit word, with each subsequent field just above the preceding field. In big-endian representation, the first field occupies the high-order bits, with each subsequent field just below the preceding field. For a machine type where endianness is not known at compile time (such as the portable bytecode virtual machine), a bit field must be specified explicitly as `big` or `little` endian by an enclosing declaration.

When `ftype` is an immediate `ftype-name` or an `ftype-name` wrapped only by `packed`, `unpacked`, or `endian`, then the defined `ftype-name` is an alias for the `ftype`. Otherwise, a ftype definition is syntactically generative in the same way as using `(nongenerative)` with `define-record-type`: two ftypes are equivalent only if defined by the same `ftype` binding. In that case, if two ftype definitions look identical but appear in two parts of the same program, the ftypes are not identical, and attempts to access one using the name of the other via the operators described below will fail with a run-time exception. When a `uid` is provided via `(nongenerative uid)`, then the defined ftype is equivalent to any other binding with the same `uid`. A definition with a `uid` is rejected if it is incompatible with a previous definition using the same `uid`. A `uid` can be provided when `ftype-name` is `ftype-pointer`, `ftype-scheme-object-pointer`, or a subtype of one of those; in that case, the definition is generative (instead of defining an alias), and it creates a subtype of the given `ftype-name`.

If `ftype` is `ftype-scheme-object-pointer`, then the defined `ftype-name` is a derived Scheme-object pointer type. If `ftype` is itself a derived Scheme-object pointer type, then so is the defined `ftype-name`.

Array bounds must always be constant. If an array's length cannot be known until run time, the array can be placed at the end of the ftype (and any containing ftype) and declared to have size zero, as illustrated by the example below.

`(define-ftype Vec
(struct
[len int]
[data (array 0 double)]))
(define make-Vec
(lambda (n)
(let ([fptr (make-ftype-pointer Vec
(foreign-alloc
(+ (ftype-sizeof Vec)
(* (ftype-sizeof double) n))))])
(ftype-set! Vec (len) fptr n)
fptr)))
(define x (make-Vec 100))
(/ (- (ftype-pointer-address (ftype-&ref Vec (data 10) x))
(ftype-pointer-address x) 10
(ftype-sizeof int))
(ftype-sizeof double))
(foreign-free (ftype-pointer-address x))`

No array bounds checks are performed for zero-length arrays. Only one variable-sized array can appear in a single foreign object, but one can work around this by treating the object as multiple individual objects.

To avoid specifying the constant length of an array in more than one place, a macro that binds both a variable to the size as well as an ftype name to the ftype can be used. For example,

`(define-syntax define-array
(syntax-rules ()
[(_ array-name type size-name size)
(begin
(define size-name size)
(define-ftype array-name
(array size type)))]))
(define-array A int A-size 100)
A-size 100
(ftype-pointer-ftype
(make-ftype-pointer A
(foreign-alloc (ftype-sizeof A)))) (array 100 int)`

This technique can be used to define arbitrary ftypes with arbitrary numbers of array fields.

A struct ftype is an implicit subtype of the type of the first field of the struct. Similarly, an array ftype is an implicit subtype of the type of its elements. Thus, the struct or array extends the type of first field or element with additional fields or elements. This allows an instance of the struct or array to be treated as an instance of the type of its first field or element, without the need to use `ftype-&ref` to allocate a new pointer to the field or element. A subtype of `ftype-scheme-object-pointer` is never created in this way, however.

syntax: `(ftype-sizeof ftype-name)`
returns: the size in bytes of the ftype identified by `ftype-name`
libraries: `(chezscheme)`

The size includes the sizes of any ftypes directly embedded within the identified ftype but excludes those indirectly embedded via a pointer ftype. In the latter case, the size of the pointer is included.

`ftype-name` must not be defined as a function ftype, since the size of a function cannot generally be determined.

`(define-ftype B
(struct
[b1 integer-32]
[b2 (array 10 integer-32)]))
(ftype-sizeof B) 44
(define-ftype C (* B))
(ftype-sizeof C) 4 ; on 32-bit machines
(ftype-sizeof C) 8 ; on 64-bit machines
(define-ftype BB
(struct
[bb1 B]
[bb2 (* B)]))
(- (ftype-sizeof BB) (ftype-sizeof void*)) 44`

syntax: `(make-ftype-pointer ftype-name expr)`
returns: an ftype-pointer object
libraries: `(chezscheme)`

If `ftype-name` does not describe a function ftype, `expr` must evaluate to an `address` represented as an exact integer in the appropriate range for the target machine. The type identified by `ftype-name` must not be `ftype-scheme-object-pointer` or a derived Scheme-object pointer type.

The ftype-pointer object returned by this procedure encapsulates the address and is tagged with a representation of the type identified by `ftype-name` to enable various forms of checking to be done by the access routines described below.

`(make-ftype-pointer E #x80000000) #<ftype-pointer #x80000000>`

The address will not typically be a constant, as shown. Instead, it might instead come from a call to `foreign-alloc`, e.g.:

`(make-ftype-pointer E (foreign-alloc (ftype-sizeof E)))`

It might also come from source outside of Scheme such as from a C routine called from Scheme via the foreign-procedure interface.

If `ftype-name` describes a function ftype, `expr` must evaluate to an address, procedure, or string. If it evaluates to address, the call behaves like any other call to `make-ftype-pointer` with an address argument.

If it evaluates to a procedure, a foreign-callable code object is created for the procedure, as if via `foreign-callable` (Section 4.3). The address encapsulated in the resulting ftype-pointer object is the address of the procedure's entry point.

`(define fact
(lambda (n)
(if (= n 0) 1 (fact (- n 1)))))
(define-ftype fact_t (function (int) int))
(define fact-fptr (make-ftype-pointer fact_t fact))`

The resulting ftype pointer can be passed to a C routine, if the argument is declared to be a pointer to the same ftype, and the C routine can invoke the function pointer it receives as it would any other function pointer. Thus, `make-ftype-pointer` with a function ftype is an alternative to `foreign-callable` for creating C-callable wrappers for Scheme procedures.

Since the foreign-callable code object can be reclaimed by the garbage collector if it is not otherwise referenced, the implicit foreign-callable's code object is automatically locked, as if via `lock-object`, before it is embedded in the ftype pointer. The code object should be unlocked after its last use from C, since locked objects take up space, cause fragmentation, and increase the cost of collection. Since the system cannot determine automatically when the last use from C occurs, the program must explicitly unlock the code object, which it can do by extracting the address from the ftype-pointer converting the address (back) into a code object, and passing it to `unlock-object`:

`(unlock-object
(foreign-callable-code-object
(ftype-pointer-address fact-fptr)))`

Even after the code object is unlocked, the code object will remain immobile as long as a result of `foreign-callable-code-object` is retained. However, if only the ftype pointer object is retained, then the ftype pointer should not be used again unless it is relocked, e.g., via:

`(lock-object
(foreign-callable-code-object
(ftype-pointer-address fact-fptr)))`

A program can determine whether an object is already locked via the `locked-object?` predicate.

A function ftype can be also used with `make-ftype-pointer` to create an ftype-pointer to a C function, either by providing the address of the C function or its name, represented as a string. For example, with the following definition of `bvcopy_t`,

`(define-ftype bvcopy_t (function (u8* u8* size_t) void))`

the two definitions of `bvcopy-ftpr` below are equivalent.

`(define bvcopy-fptr (make-ftype-pointer bvcopy_t "memcpy"))
(define bvcopy-fptr (make-ftype-pointer bvcopy_t (foreign-entry "memcpy")))`

A library that defines `memcpy` must be loaded first via `load-shared-object`, or `memcpy` must be registered via one of the methods described in Section 4.7.

syntax: `(make-ftype-scheme-object-pointer expr)`
syntax: `(make-ftype-scheme-object-pointer expr offset)`
syntax: `(make-ftype-scheme-object-pointer expr offset ftype-name)`
returns: an ftype-pointer object that refers to a Scheme object
libraries: `(chezscheme)`

Returns an ftype pointer object that can be used in the same places as a generic ftype pointer, and where the address encapsulated by the pointer is a reference address in the sense of `object->reference-address`. The reference address is for the result of `expr`, without special treatment of `#f`. If an `offset` expression is present, then it must produce a `iptr`-compatible integer, and the address encapsulated by the pointer is shifted by that amount.

The resulting ftype pointer is recognized by `ftype-scheme-object-pointer?` as well as `ftype-pointer?` without an `ftype-name`.

If `ftype-name` is provided, it must be `ftype-scheme-object-pointer` or a derived Scheme-object pointer type that is defined using `define-ftype`.

syntax: `(ftype-pointer? obj)`
returns: `#t` if `obj` is an ftype pointer, otherwise `#f`
syntax: `(ftype-pointer? ftype-name obj)`
returns: `#t` if `obj` is an `ftype-name`, otherwise `#f`
syntax: `(ftype-scheme-object-pointer? obj)`
returns: `#t` if `obj` is an ftype pointer for a Scheme object, otherwise `#f`
libraries: `(chezscheme)`

`(define-ftype Widget1 (struct [x int] [y int]))
(define-ftype Widget2 (struct [w Widget1] [b boolean]))
(define x1 (make-ftype-pointer Widget1 #x80000000))
(define x2 (make-ftype-pointer Widget2 #x80000000))
(ftype-pointer? x1) #t
(ftype-pointer? x2) #t
(ftype-pointer? Widget1 x1) #t
(ftype-pointer? Widget1 x2) #t
(ftype-pointer? Widget2 x1) #f
(ftype-pointer? Widget2 x2) #t
(ftype-pointer? #x80000000) #f
(ftype-pointer? Widget1 #x80000000) #f`

procedure: `(ftype-pointer-address fptr)`
returns: the address encapsulated within `fptr`
libraries: `(chezscheme)`

`fptr` must be an ftype-pointer object.

`(define x (make-ftype-pointer E #x80000000))
(ftype-pointer-address x) #x80000000`

procedure: `(ftype-scheme-object-pointer-object fptr)`
returns: the Scheme object whose reference address is encapsulated within `fptr`
procedure: `(ftype-scheme-object-pointer-offset fptr)`
returns: the offset of a reference address encapsulated within `fptr`
libraries: `(chezscheme)`

`fptr` must be an ftype-pointer object that represents a Scheme object.

`(define x (make-ftype-scheme-object-pointer #vu8(1 2 3) 1))
(ftype-scheme-object-pointer-object x) #vu8(1 2 3)
(ftype-scheme-object-pointer-offset x) 1`

syntax: `(ftype-pointer=? fptr1 fptr2)`
returns: `#t` if `fptr1` and `fptr2` have the same address, otherwise `#f`
libraries: `(chezscheme)`

`fptr1` and `fptr2` must be ftype-pointer objects.

`ftype-pointer=?` might be defined as follows:

`(define ftype-pointer=?
(lambda (fptr1 fptr2)
(= (ftype-pointer-address fptr1) (ftype-pointer-address fptr2))))`

It is, however, guaranteed not to allocate bignums for the addresses even if the addresses do not fit in fixnum range.

syntax: `(ftype-pointer-null? fptr)`
returns: `#t` if the address of `fptr` is 0, otherwise `#f`
libraries: `(chezscheme)`

`fptr` must be an ftype-pointer object.

`ftype-pointer-null?` might be defined as follows:

`(define ftype-pointer-null?
(lambda (fptr)
(= (ftype-pointer-address fptr) 0)))`

It is, however, guaranteed not to allocate a bignum for the address even if the address does not fit in fixnum range.

syntax: `(ftype-&ref ftype-name (a ...) fptr-expr)`
syntax: `(ftype-&ref ftype-name (a ...) fptr-expr index)`
returns: an ftype-pointer object
libraries: `(chezscheme)`

The ftype-pointer object returned by `ftype-&ref` encapsulates the address of some object embedded directly or indirectly within the foreign object pointed to by the value of `fptr-expr`, offset by `index`, if present. The value of `fptr-expr` must be an ftype pointer (fptr) of the ftype identified by `ftype-name`, and `index` must either be the identifier `*` or evaluate to a fixnum, possibly negative. The index is automatically scaled by the size of the ftype identified by `ftype-name`, which allows the fptr to be treated as an array of `ftype-name` objects and `index` as an index into that array. An index of `*` or 0 is the same as no index.

The sequence of accessors `a ...` must specify a valid path through the identified ftype. For `struct`, `union`, and `bits` ftypes, an accessor must be a valid field name for the ftype, while for pointer and array ftypes, an accessor must be the identifier `*` or evaluate to a fixnum index. For array ftypes, an index must be nonnegative, and for array ftypes with nonzero length, an index must also be less than the length.

The examples below assume the definitions of `B` and `BB` shown above in the description of `ftype-sizeof`. Fixed addresses are shown for illustrative purposes and are assumed to be valid, although addresses are generally determined at run time via `foreign-alloc` or some other mechanism.

`(define x (make-ftype-pointer B #x80000000))
(ftype-&ref B () x) #<ftype-pointer #x80000000>
(let ([idx 1]) #<ftype-pointer #x8000002C>
(ftype-&ref B () x idx))
(let ([idx -1]) #<ftype-pointer #x7FFFFFD4>
(ftype-&ref B () x idx))
(ftype-&ref B (b1) x) #<ftype-pointer #x80000000>
(ftype-&ref B (b2) x) #<ftype-pointer #x80000004>
(ftype-&ref B (b2 5) x) #<ftype-pointer #x80000018>
(let ([n 5]) (ftype-&ref B (b2 n) x)) #<ftype-pointer #x80000018>
(ftype-&ref B (b1 b2) x) syntax error
(ftype-&ref B (b2 15) x) run-time exception
(define y (make-ftype-pointer BB #x90000000))
(ftype-set! BB (bb2) y x)
(ftype-&ref BB (bb1 b2) y) #<ftype-pointer #x90000004>
(ftype-&ref BB (bb2 * b2) y) #<ftype-pointer #x80000004>
(let ([idx 1]) #<ftype-pointer #x80000030>
(ftype-&ref BB (bb2 idx b2) y))`

With no accessors and no index, as in the first use of `ftype-&ref` above, the returned `ftype-pointer` might be `eq?` to the input. Otherwise, the `ftype-pointer` is freshly allocated.

syntax: `(ftype-set! ftype-name (a ...) fptr-expr val-expr)`
syntax: `(ftype-set! ftype-name (a ...) fptr-expr index val-expr)`
returns: unspecified
syntax: `(ftype-ref ftype-name (a ...) fptr-expr)`
syntax: `(ftype-ref ftype-name (a ...) fptr-expr index)`
returns: a Scheme representation of a foreign object
libraries: `(chezscheme)`

These forms are used to store values into or retrieve values from the object pointed to by the value of `fptr-expr`, offset by `index`, if present. The value of `fptr-expr` must be an ftype pointer (fptr) of the ftype identified by `ftype-name`, and `index` must either be the identifier `*` or evaluate to a fixnum, possibly negative. The index is automatically scaled by the size of the ftype identified by `ftype-name`, which allows the fptr to be treated as an array of `ftype-name` objects and `index` as an index into that array. An index of `*` or 0 is the same as no index.

The sequence of accessors `a ...` must specify a valid path through the identified ftype. For `struct`, `union`, and `bits` ftypes, an accessor must be a valid field name for the ftype, while for pointer and array ftypes, an accessor must be the identifier `*` or evaluate to a fixnum index. For array ftypes, an index must be nonnegative, and for array ftypes with nonzero length, an index must also be less than the length. The field or element specified by the sequence of accessors must be a scalar field, e.g., a pointer field or a field containing a base type such as an `int`, `char`, or `double`.

For `ftype-set!`, `val-expr` must evaluate to a value of the appropriate type for the specified field, e.g., an ftype pointer of the appropriate type or an appropriate base-type value.

For both signed and unsigned integer fields, values in the range -2w-1 through 2w - 1 are accepted, where w is the width in bits of the integer field. For signed integer fields, values in the range 2w-1 through 2w - 1 are treated as two's complement representations of the corresponding negative numbers. For unsigned integer fields, values in the range -2w-1 through -1 are similarly treated as two's complement representations of the corresponding positive numbers.

`char` and `wchar_t` (`wchar`) field values are converted from (`ftype-set!`) or to (`ftype-ref`) Scheme characters, as if with `char->integer` and `integer->char`. Characters stored by `ftype-set!` into a `char` field must have Unicode scalar values in the range 0 through 255. Under Windows and any other system where `wchar_t` (`wchar`) is a 16-bit value, characters stored by `ftype-set!` into a `whar_t` (`wchar`) field must have Unicode scalar values in the range 0 through 216 - 1. On systems where `wchar_t` is a 32-bit value, any character can be stored in a `wchar_t` (`wchar`) field.

The examples below assume that `B` and `C` have been defined as shown in the description of `ftype-sizeof` above.

`(define b
(make-ftype-pointer B
(foreign-alloc
(* (ftype-sizeof B) 3))))
(define c
(make-ftype-pointer C
(foreign-alloc (ftype-sizeof C))))
(ftype-set! B (b1) b 5)
(ftype-set! B (b1) b 1 6)
(ftype-set! B (b1) c 5) exception: ftype mismatch
(ftype-set! B (b2) b 0) exception: not a scalar
(ftype-set! B (b2 -1) b 0) exception: invalid index
(ftype-set! B (b2 0) b 50)
(ftype-set! B (b2 4) b 55)
(ftype-set! B (b2 10) b 55) exception: invalid index
(ftype-set! C () c (ftype-&ref B () b 1))
(= (ftype-pointer-address (ftype-ref C () c)) #t
(+ (ftype-pointer-address b) (ftype-sizeof B)))
(= (ftype-pointer-address (ftype-&ref C (*) c)) #t
(+ (ftype-pointer-address b) (ftype-sizeof B)))
(= (ftype-pointer-address (ftype-&ref C (-1) c)) #t
(ftype-pointer-address b))
(ftype-ref C (-1 b1) c) 5
(ftype-ref C (* b1) c) 6
(ftype-ref C (-1 b2 0) c) 50
(let ([i 4]) (ftype-ref C (-1 b2 i) c)) 55
(ftype-set! C (-1 b2 0) c 75)
(ftype-ref B (b2 0) b) 75
(foreign-free (ftype-pointer-address c))
(foreign-free (ftype-pointer-address b))`

A function ftype pointer can be converted into a Scheme-callable procedure via `ftype-ref`. Assuming that a library defining `memcpy` has been loaded via `load-shared-object` or `memcpy` has been registered via one of the methods described in Section 4.7, A Scheme-callable `memcpy` can be defined as follows.

`(define-ftype bvcopy_t (function (u8* u8* size_t) void))
(define bvcopy-fptr (make-ftype-pointer bvcopy_t "memcpy"))
(define bvcopy (ftype-ref bvcopy_t () bvcopy-fptr))
(define bv1 (make-bytevector 8 0))
(define bv2 (make-bytevector 8 57))
bv1 #vu8(0 0 0 0 0 0 0 0)
bv2 #vu8(57 57 57 57 57 57 57 57)
(bvcopy bv1 bv2 5)
bv1 #vu8(57 57 57 57 57 0 0 0)`

An ftype pointer can also be obtained as a return value from a C function declared to return a pointer to a function ftype.

Thus, `ftype-ref` with a function ftype is an alternative to `foreign-procedure` (Section 4.2) for creating Scheme-callable wrappers for C functions.

syntax: `(ftype-any-set! ftype-name (a ...) fptr-expr val-expr)`
syntax: `(ftype-any-set! ftype-name (a ...) fptr-expr offset val-expr)`
returns: unspecified
syntax: `(ftype-any-ref ftype-name (a ...) fptr-expr)`
syntax: `(ftype-any-ref ftype-name (a ...) fptr-expr offset)`
returns: a Scheme representation of a foreign object
libraries: `(chezscheme)`

These forms are like `ftype-set!` and `ftype-ref`, but with two differences:

1. The ftype pointer produced by `fptr-expr` can be any pointer. It does not have to be specifically for `ftype-name`, even though an `ftype-name` value is written or read.

2. The pointer's address is shifted by an absolute `offset` that is not scaled by the size of `fptr-expr` values.

procedure: `(ftype-pointer-ftype fptr)`
returns: `fptr`'s ftype, represented as an s-expression
libraries: `(chezscheme)`

`fptr` must be an ftype-pointer object.

`(define-ftype Q0
(struct
[x int]
[y int]))
(define-ftype Q1
(struct
[x double]
[y char]
[z (endian big
(bits
[_ unsigned 3]
[a unsigned 9]
[b unsigned 4]))]
[w (* Q0)]))
(define q1 (make-ftype-pointer Q1 0))
(ftype-pointer-ftype q1) (struct
[x double]
[y char]
[z (endian big
(bits
[_ unsigned 3]
[a unsigned 9]
[b unsigned 4]))]
[w (* Q0)])`

procedure: `(ftype-pointer->sexpr fptr)`
returns: an s-expression representation of the object to which `fptr` points
libraries: `(chezscheme)`

`fptr` must be an ftype-pointer object.

For each unnamed field, i.e., each whose field name is an underscore, the corresponding field value in the resulting s-expression is also an underscore. Similarly, if a field is inaccessible, i.e., if its address is invalid, the value is the symbol `invalid`.

`(define-ftype Frob
(struct
[p boolean]
[q char]))
(define-ftype Snurk
(struct
[a Frob]
[b (* Frob)]
[c (* Frob)]
[d (bits
[_ unsigned 15]
[dx signed 17])]
[e (array 5 double)]))
(define x
(make-ftype-pointer Snurk
(foreign-alloc (ftype-sizeof Snurk))))
(ftype-set! Snurk (b) x
(make-ftype-pointer Frob
(foreign-alloc (ftype-sizeof Frob))))
(ftype-set! Snurk (c) x
(make-ftype-pointer Frob 0))
(ftype-set! Snurk (a p) x #t)
(ftype-set! Snurk (a q) x #\A)
(ftype-set! Snurk (b * p) x #f)
(ftype-set! Snurk (b * q) x #\B)
(ftype-set! Snurk (d dx) x -2500)
(do ([i 0 (fx+ i 1)])
((fx= i 5))
(ftype-set! Snurk (e i) x (+ (* i 5.0) 3.0)))
(ftype-pointer->sexpr x) (struct
[a (struct [p #t] [q #\A])]
[b (* (struct [p #f] [q #\B]))]
[c (* (struct [p invalid] [q invalid]))]
[d (bits [_ _] [dx -2500])]
[e (array 5 3.0 8.0 13.0 18.0 23.0)])`

### Section 4.6. Foreign Arrays of Managed Objects

When a bytevector value is passed in a foreign call as an argument of type `u8*`, the foreign procedure receives a pointer to the start of the bytevector's content. The function `object->reference-address` returns that same address, but it is valid only if the bytevector does not move. If the storage manager later moves the bytevector in memory, then the address becomes invalid. The bytevector can be locked with `lock-object` to prevent the storage manage from relocating it, so that the content address will remain valid, but then it must be explicitly unlocked later with `unlock-object`.

Having to explicitly lock and unlock objects can make interoperating with foreign procedures difficult when the foreign procedure expects an array of references (for example, as a `char**` argument). It may seem that a vector of bytevectors could serve that purpose, but although a Scheme vector is an array of references at some level, a vector's reference to a bytevector does not point to the start of the bytevector's content.

A reference bytevector is a type of bytevector that can be used to solve this problem by storing the addresses of the contents of scheme objects using `bytevector-reference-set!`. Reference bytevectors are recognized and treated specially by the storage manager, so that the addresses contained in the reference byte vector are both recognized as a reference to a scheme object and updated appropriately if the storage manager moves that object. A reference bytevector can refer to any type of Scheme object, but the meaning of the address is specified for only three kinds of object: The address for bytevectors and flvectors is the pointer to their content, and the address for `#f` is `0`. In addition, a reference bytevector can contain foreign addresses, which are addresses of memory outside of the storage manager's ranges, but beware that an address range released by a foreign allocator might later become a range that belongs to the Scheme storage manager. During the time that a garbage collection is possible (i.e., when interrupts have not been disabled), every aligned word (i.e. byte offset that is a multiple of `(ftype-sizeof uptr)`) in a reference bytevector must hold a valid object reference or a foreign address.

While all of the procedures described in this section support unaligned access (for consistency with other bytevector operations), use extreme caution when using any byte offset other than an even multiple of the current machine's pointer size (i.e., `(ftype-sizeof uptr)`). Some procedures note cases in which behavior is undefined. Undefined behavior may include invalid memory references, corrupted data, or system crashes.

procedure: `(make-reference-bytevector n)`
returns: a reference bytevector of length `n`
libraries: `(chezscheme)`

Like `make-bytevector`, but the result is a reference bytevector that is initialized with all bytes being `0` (i.e., all referenced objects being `#f` and any trailing bytes after an even multiple of the current machine's pointer size being 0).

procedure: `(make-immobile-reference-bytevector n)`
returns: a reference bytevector of length `n`
libraries: `(chezscheme)`

Like `make-reference-bytevector`, but creates a reference bytevector that will not be relocated in memory by the storage management system until it is reclaimed.

procedure: `(reference-bytevector? obj)`
returns: `#t` if `obj` is an reference bytevector, `#f` otherwise
libraries: `(chezscheme)`

Note that every reference bytevector is also a bytevector that is recognized by `bytevector?`, and `equal?` considers two bytevectors independent of whether either is a reference bytevector.

procedure: `(bytevector-reference-set! bytevector n obj)`
returns: unspecified
libraries: `(chezscheme)`

`bytevector` must be a reference bytevector, and `n` must be a nonnegative fixnum. The sum of `n` and the size of a pointer (i.e., `(ftype-sizeof uptr)`) must not be greater than the length of `bytevector`.

Installs a reference to `obj` at byte offset `n` within `bytevector`. In the case that `obj` is a bytevector or flvector, the reference is represented as the starting address of the bytevector or flvector's content. If `obj` is `#f` then the reference is represented as `0`. The reference occupies `(ftype-sizeof uptr)` bytes.

procedure: `(bytevector-reference-ref bytevector n)`
returns: a Scheme object
libraries: `(chezscheme)`

`bytevector` must be a reference bytevector, and `n` must be a nonnegative fixnum. The sum of `n` and the size of a pointer (i.e., `(ftype-sizeof uptr)`) must not be greater than the length of `bytevector`.

Returns the object that is referenced in `bytevector` at byte offset `n`. If the `(ftype-sizeof uptr)` bytes at offset `n` do not represent a reference to a valid Scheme object then behavior is undefined.

procedure: `(bytevector-reference*-ref bytevector n)`
returns: an allocated Scheme object or a positive exact integer
libraries: `(chezscheme)`

Like `bytevector-reference-ref`, but if `bytevector` at byte offset `n` holds a non-`0` foreign address, `bytevector-reference*-ref` returns an integer. The representation of a reference to a Scheme object can overlap with the representation of a foreign address; see `reference*-address->object`.

procedure: `(object->reference-address obj)`
returns: a nonnegative exact integer
libraries: `(chezscheme)`

The result is `0` if `obj` is `#f`, and it is the address of `objs`'s content in the case that `obj` is a bytevector or flvector---at least, at the point where `object->reference-address` was called. Unless `obj` is a locked or immobile bytevector or flvector, its address can change at any time that a garbage collection is possible (i.e., when interrupts are enabled). For other kinds of `objs`, the representation of the returned address is not specified, except that it is a nonnegative exact integer that is distinct from any other object's representation.

procedure: `(reference-address->object addr)`
returns: a Scheme object
libraries: `(chezscheme)`

`addr` must be a nonnegative exact integer that is a valid reference to a Scheme object, and the result is the referenced Scheme object. If `addr` is not a valid reference to a Scheme object then behavior is undefined.

procedure: `(reference*-address->object addr)`
returns: a Scheme object, possibly `addr` itself
libraries: `(chezscheme)`

`addr` must be a nonnegative exact integer that is a valid reference to a Scheme object, or it must be an foreign address that is outside the range that belongs to the storage manager.

When `addr` is `0`, the result is `#f`. Otherwise, if `addr` is a foreign address, then `addr` itself is returned.

The representation of a reference to a bytevector or flvector is guaranteed to be distinct from any foreign address, but other values may have a representation that overlaps with foreign addresses, in which case `addr` is returned (i.e., the foreign-address interpretation takes precedence). Thus, `reference*-address->object` and `bytevector-reference*-ref` are mainly useful for an address that is known to be either a bytevector reference, an flvector reference, or a foreign address.

If `addr` is not a foreign address and does not reference a valid Scheme object then behavior is undefined.

### Section 4.7. Providing Access to Foreign Procedures

Access to foreign procedures can be provided in several ways:

- Foreign procedures may be loaded from "shared objects" using `load-shared-object`.

- A new Chez Scheme image can be built with additional foreign code linked in. (Consult with the person who installed Chez Scheme at your site for details.) These entries are typically registered via `Sforeign_symbol` or `Sregister_symbol`, documented in Section 4.10.

- Additional entries may be dynamically loaded or otherwise obtained by foreign code. These are also typically registered using `Sforeign_symbol` or `Sregister_symbol`.

- The address of an entry, i.e., a function pointer, may be passed into Scheme and used as the value of the entry expression in a foreign-procedure expression. This allows foreign entry points to be used even when they are not registered by name.

procedure: `(foreign-entry? entry-name)`
returns: `#t` if `entry-name` is an existing foreign procedure entry point, `#f` otherwise
libraries: `(chezscheme)`

`entry-name` must be a string. `foreign-entry?` may be used to determine if an entry exists for a foreign procedure.

The following examples assume that a library that defines `strlen` has been loaded via `load-shared-object` or that `strlen` has been registered via one of the other methods described in this section.

`(foreign-entry? "strlen") #t
((foreign-procedure "strlen"
(string) size_t)
"hey!") 4`

procedure: `(foreign-entry entry-name)`
returns: the address of `entry-name` as an exact integer
libraries: `(chezscheme)`

`entry-name` must be a string naming an existing foreign entry point.

The following examples assume that a library that defines `strlen` has been loaded via `load-shared-object` or that `strlen` has been registered via one of the other methods described in this section.

`(let ([addr (foreign-entry "strlen")])
(and (integer? addr) (exact? addr))) #t
(define-ftype strlen-type (function (string) size_t))
(define strlen
(ftype-ref strlen-type ()
(make-ftype-pointer strlen-type "strlen")))
(strlen "hey!") 4`

procedure: `(foreign-address-name address)`
returns: the entry name corresponding to `address`, if known, otherwise `#f`
libraries: `(chezscheme)`

The following examples assume that a library that defines `strlen` has been loaded via `load-shared-object` or that `strlen` has been registered via one of the other methods described in this section.

`(foreign-address-name (foreign-entry "strlen")) "strlen"`

procedure: `(load-shared-object path)`
returns: unspecified
libraries: `(chezscheme)`

`path` must be a string or `#f`. If `path` is a string, `load-shared-object` loads the shared object named by `path`. Shared objects may be system libraries or files created from ordinary C programs. All external symbols in the shared object, along with external symbols available in other shared objects linked with the shared object, are made available as foreign entries.

This procedure is supported for most platforms upon which Chez Scheme runs.

If `path` does not begin with a "." or "/", the shared object is searched for in a default set of directories determined by the system.

If `path` is `#f`, external symbols in the executable itself (if any), as well as those found in dependent shared objects, are also made available as foreign entries. Usually, executables do not export symbols, but can be instructed to do so with proper compilation flags. One may thus create an executable based on Chez Scheme, and have Scheme code access exported symbols from it and also from any shared object dynamically linked to it.

Because Chez Scheme is usually dynamically linked to the operating system's C library, all built-in C library functions are also accessible after evaluation of `(load-shared-object #f)`. This provides a simple way to gain access to standard C functions (such as `memcpy` or `getenv`), which may be very convenient for Scheme programs intended to be portable across different systems. If Chez Scheme is statically linked however, the standard C functions may only be accessible in this manner if they are present in the executable and exported, otherwise the shared object containing the C library must be explicitly named; see below for examples for some platforms.

On most Unix systems, `load-shared-object` is based on the system routine `dlopen`. Under Windows, `load-shared-object` is based on `LoadLibrary`. Refer to the documentation for these routines and for the C compiler and loader for precise rules for locating and building shared objects.

`load-shared-object` can be used to access built-in C library functions, such as `getenv`. The name of the shared object varies from one system to another.

On Linux systems:

`(load-shared-object "libc.so.6")`

On Solaris, OpenSolaris, FreeBSD, NetBSD, and OpenBSD systems:

`(load-shared-object "libc.so")`

On MacOS X systems:

`(load-shared-object "libc.dylib")`

On Windows:

`(load-shared-object "msvcrt.dll")`

On Haiku:

`(load-shared-object "libroot.so")`

Once the C library has been loaded, `getenv` should be available as a foreign entry.

`(foreign-entry? "getenv") #t`

An equivalent Scheme procedure may be defined and invoked as follows.

`(define getenv
(foreign-procedure "getenv"
(string)
string))
(getenv "HOME") "/home/elmer/fudd"
(getenv "home") #f`

`load-shared-object` can be used to access user-created libraries as well. Suppose the C file `"even.c"` contains

`int even(n) int n; { return n == 0 || odd(n - 1); }`

and the C file `"odd.c"` contains

`int odd(n) int n; { return n != 0 && even(n - 1); }`

The files must be compiled and linked into a shared object before they can be loaded. How this is done depends upon the host system. On Linux, FreeBSD, Haiku, OpenBSD, and OpenSolaris systems:

`(system "cc -fPIC -shared -o evenodd.so even.c odd.c")`

Depending on the host configuration, the `-m32` or `-m64` option might be needed to specify 32-bit or 64-bit compilation as appropriate.

On MacOS X (Intel or PowerPC) systems:

`(system "cc -dynamiclib -o evenodd.so even.c odd.c")`

Depending on the host configuration, the `-m32` or `-m64` option might be needed to specify 32-bit or 64-bit compilation as appropriate.

On 32-bit Sparc Solaris:

`(system "cc -KPIC -G -o evenodd.so even.c odd.c")`

On 64-bit Sparc Solaris:

`(system "cc -xarch=v9 -KPIC -G -o evenodd.so even.c odd.c")`

On Windows, we build a DLL (dynamic link library) file. In order to make the compiler generate the appropriate entry points, we alter `even.c` to read

`#ifdef WIN32
#define EXPORT extern __declspec (dllexport)
#else
#define EXPORT extern
#endif
EXPORT int even(n) int n; { return n == 0 || odd(n - 1); }`

and `odd.c` to read

`#ifdef WIN32
#define EXPORT extern __declspec (dllexport)
#else
#define EXPORT extern
#endif
EXPORT int odd(n) int n; { return n != 0 && even(n - 1); }`

We can then build the DLL as follows, giving it the extension ".so" rather than ".dll" for consistency with the other systems.

`(system "cl -c -DWIN32 even.c")
(system "cl -c -DWIN32 odd.c")
(system "link -dll -out:evenodd.so even.obj odd.obj")`

The resulting ".so" file can be loaded into Scheme and `even` and `odd` made available as foreign procedures:

`(load-shared-object "./evenodd.so")
(let ([odd (foreign-procedure "odd"
(integer-32) boolean)]
[even (foreign-procedure "even"
(integer-32) boolean)])
(list (even 100) (odd 100))) (#t #f)`

The filename is given as `"./evenodd.so"` rather than simply `"evenodd.so"`, because some systems look for shared libraries in a standard set of system directories that does not include the current directory.

procedure: `(remove-foreign-entry entry-name)`
returns: unspecified
libraries: `(chezscheme)`

`remove-foreign-entry` blocks further access to the entry specified by the string `entry-name`. An exception is raised with condition type `&assertion` if the entry does not exist. Since access previously established by `foreign-procedure` is not affected, `remove-foreign-entry` may be used to clean up after the desired interface to a group of foreign procedures has been established.

`remove-foreign-entry` can be used to remove entries registered using `Sforeign_symbol` and `Sregister_symbol` but not entries created as a result of a call to `load-shared-object`.

### Section 4.8. Using Other Foreign Languages

Although the Chez Scheme foreign procedure interface is oriented primarily toward procedures defined in C or available in C libraries, it is possible to invoke procedures defined in other languages that follow C calling conventions. One source of difficulty may be the interpretation of names. Since Unix-based C compilers often prepend an underscore to external names, the foreign interface attempts to interpret entry names in a manner consistent with the host C compiler. Occasionally, such as for assembly coded files, this entry name interpretation may not be desired. It can be prevented by prefixing the entry name with an "=" character. For example, after loading an assembly file containing a procedure `"foo"` one might have:

`(foreign-entry? "foo") #f
(foreign-entry? "=foo") #t`

### Section 4.9. Foreign Error Results

thread parameter: `current-errno-source`
libraries: `(chezscheme)`

`current-errno-source` can be `#f` on any host operating systems. On Windows, the symbols `msvcrt` and `ucrt` are also allowed.

The global C variable `errno` can be linked in different ways on Windows, and it is possible that the `errno` linkage for the Chez Scheme executable differs from the linkage of a dynamically loaded library. When a foreign procedure with convention `__errno` is called, then the value of `current-errno-source` determines which `errno` is used:

- `#f`: `errno` is used as linked to the Chez Scheme executable.

- `msvcrt`: `errno` is accessed from `"msvcrt.dll"` dynamically.

- `ucrt`: `errno` is accessed from `"ucrtbase.dll"` dynamically.

If a dynamic library is linked statically to the C library, then none of these options will work, and the dynamic library must provide a specific way of accessing error results.

### Section 4.10. C Library Routines

Additional foreign interface support is provided via a set of C preprocessor macros and C-callable library functions. Some of these routines allow C programs to examine, allocate, and alter Scheme objects. Others permit C functions to call Scheme procedures via a more primitive interface than that defined in Section 4.3. Still others permit the development of custom executable images and use of the Scheme system as a subordinate program within another program, e.g., for use as an extension language.

C code that uses these routines must include the `"scheme.h"` header file distributed with Chez Scheme and must be linked (statically or dynamically) with the Chez Scheme kernel. The header file contains definitions for the preprocessor macros and `extern` declarations for the library functions. The file is customized to the release of Chez Scheme and machine type with which it is distributed; it should be left unmodified to facilitate switching among Chez Scheme releases, and the proper version of the header file should always be used with C code compiled for use with a particular version of Chez Scheme. The version and machine type are defined in `"scheme.h"` under the names `VERSION` and `MACHINE_TYPE`.

The name of each routine begins with a capital `S`, e.g., `Sfixnump`. Many of the names are simple translations of the names of closely related Scheme procedures, e.g., `Sstring_to_symbol` is the C interface equivalent of `string->symbol`. Most externally visible entries in the Chez Scheme executable that are not documented here begin with capital `S` followed by an underscore (`S_`); their use should be avoided.

In addition to the various macros and external declarations given in `scheme.h`, the header file also defines (`typedefs`) several types used in the header file:

- `ptr`: type of a Scheme value,

- `iptr`: a signed integer the same size as a Scheme value, and

- `uptr`: an unsigned integer the same size as a Scheme value.

- `string_char`: type of a single Scheme string element.

- `octet`: type of a single Scheme bytevector element (unsigned char).

These types may vary depending upon the platform, although `ptr` is typically `void *`, `iptr` is typically `long` `int`, and `uptr` is typically `unsigned` `long` `int`.

Under Windows, defining `SCHEME_IMPORT` before including scheme.h causes scheme.h to declare its entry points using `extern` `declspec` `(dllimport)` rather than `extern` `declspec` `(dllexport)` (the default). Not defining `SCHEME_IMPORT` and instead defining `SCHEME_STATIC` causes scheme.h to declare exports using just `extern`. The static libraries distributed with Chez Scheme are built using `SCHEME_STATIC`.

The remainder of this section describes each of the C interface routines in turn. A declaration for each routine is given in ANSI C function prototype notation to precisely specify the argument and result types. Scheme objects have the C type `ptr`, which is defined in `"scheme.h"`. Where appropriate, C values are accepted as arguments or returned as values in place of Scheme objects.

The preprocessor macros may evaluate their arguments more than once (or not at all), so care should be taken to ensure that this does not cause problems.

Customization. The functions described here are used to initialize the Scheme system, build the Scheme heap, and run the Scheme system from a separate program.

[func] `char *` `Skernel_version``(void)`
[func] `void` `Sscheme_init``(void (*abnormal_exit)(void))`
[func] `void` `Sset_verbose``(int v)`
[func] `void` `Sregister_boot_file``(const char *name)`
[func] `void` `Sregister_boot_executable_relative_file``(const char *execpath,` `const char *name)`
[func] `void` `Sregister_boot_relative_file``(const char *name)`
[func] `void` `Sregister_boot_file_fd``(const char *name, int fd)`
[func] `void` `Sregister_boot_file_fd_region``(const char *name, int fd,` `iptr offset, iptr len, int close_after)`
[func] `void` `Sregister_boot_file_bytes``(const char *name, void *content, iptr len)`
[func] `void` `Sbuild_heap``(const char *exec, void (*custom_init)(void))`
[func] `void` `Senable_expeditor``(const char *history_file)`
[func] `void` `Sretain_static_relocation``(void)`
[func] `int` `Sscheme_start``(int argc, char *argv[])`
[func] `int` `Sscheme_script``(char *scriptfile, int argc, char *argv[])`
[func] `int` `Sscheme_program``(char *programfile, int argc, char *argv[])`
[func] `void` `Scompact_heap``(void)`
[func] `void` `Sscheme_deinit``(void)`

`Skernel_version` returns a string representing the Scheme version. It should be compared against the value of the VERSION preprocessor macro before any of the initialization functions listed above are used to verify that the correct `"scheme.h"` header file has been used.

`Sscheme_init` causes the Scheme system to initialize its static memory in preparation for boot file registration. The `abnormal_exit` parameter should be a (possibly null) pointer to a C function of no arguments that takes appropriate action if the initialization or subsequent heap-building process fails. If null, the default action is to call `exit(1)`.

`Sset_verbose` sets verbose mode on for nonzero values of `v` and off when `v` is zero. In verbose mode, the system displays a trace of the search process for subsequently registered boot files.

`Sregister_boot_file` searches for a boot file and registers it for loading. If the given boot file path is absolute or starts with a `.` or `..` path element, then the path is used relative to the current directory; otherwise, the boot file is found through a search as described in Section 2.9, and `"scheme"` is assumed as the executable name for resolving a `"%x"` escape when the executable path is not otherwise available from the operating system. `Sregister_boot_executable_relative_file` is similar, but accepts a path to the executable instead of assuming `"scheme"`, so it should be preferred when using a boot file path that may be resolved relative to the executable. `Sregister_boot_relative_file` always treats a relative path as relative to the current directory, so the executable path is not needed. In all of those cases, the boot file is opened but not loaded until the heap is built via `Sbuild_heap`. `Sregister_boot_file_fd` provides a specific boot file as a file descriptor, the given file name is used only for error reporting, and the file descriptor is not read until until the heap is built via `Sbuild_heap`. For the first boot file registered only, the system also searches for the boot files upon which the named file depends, either directly or indirectly.

The `Sregister_boot_file_fd_region` function is an alternative to `Sregister_boot_file_fd` that allows the same file descriptor to be used for multiple boot files using different offsets into the file. The `len` argument is used as a hint, but it can be 0 to mean that the size is unknown, or it can be larger than the actual boot content; it must not be non-0 and smaller than the boot content, and the boot content must be self-terminating independent of `len`. No search is performed for dependencies. If the same file descriptor is used for multiple boot files, then `close_after` should be non-zero only for the last one. The boot file content is read only when `Sbuild_heap` is called.

The `Sregister_boot_file_bytes` function is another alternative to the `Sregister_boot_file` functions that registers boot-file content that is already loaded into memory, instead of reading from a file. The registered bytes must remain available until `Sbuild_heap` reads them.

`Sbuild_heap` creates the Scheme heap from the registered boot files. `exec` is assumed to be the name of or path to the executable image and is used when no boot files have been registered as both the base name for the boot-file search process and the executable name for finding boot files relative to the executable. `exec` may be null only if one or more boot files have been registered. `custom_init` must be a (possibly null) pointer to a C function of no arguments; if non-null, it is called before any boot files are loaded.

`Sscheme_start` invokes the interactive startup procedure, i.e., the value of the parameter `scheme-start`, with one Scheme string argument for the first `argc` elements of `argv`, not including `argv[0]`. `Sscheme_script` similarly invokes the script startup procedure, i.e., the value of the parameter `scheme-script`, with one Scheme string argument for `scriptfile` and the first `argc` elements of `argv`, not including `argv[0]`. `Sscheme_program` similarly invokes the program startup procedure, i.e., the value of the parameter `scheme-program`, with one Scheme string argument for `programfile` and the first `argc` elements of `argv`, not including `argv[0]`.

`Senable_expeditor` enables the expression editor (Section 2.2, Chapter 14), which is disabled by default, and determines the history file from which it restores and to which it saves the history. This procedure must be called after the heap is built, or an error will result. It must also be called before `Sscheme_start` in order to be effective. If the `history_file` argument is the null pointer, the history is not restored or saved. The preprocessor variable `FEATURE_EXPEDITOR` is defined in `scheme.h` if support for the expression editor has been compiled into the system.

`Sretain_static_relocation` causes relocation information to be retained for static generation code objects created by heap compaction for the benefit of `compute-size` and related procedures.

`Scompact_heap` compacts the Scheme heap and places all objects currently in the heap into a static generation. Objects in the static generation are never collected. That is, they are never moved during collection and the storage used for them is never reclaimed even if they become inaccessible. `Scompact_heap` is called implicitly after any boot files have been loaded.

`Sscheme_deinit` closes any open files, tears down the Scheme heap, and puts the Scheme system in an uninitialized state.

Predicates. The predicates described here correspond to the similarly named Scheme predicates. A trailing letter `p`, for "predicate," is used in place of the question mark that customarily appears at the end of a Scheme predicate name. Each predicate accepts a single Scheme object and returns a boolean (C integer) value.

[macro] `int` `Sfixnump``(ptr obj)`
[macro] `int` `Scharp``(ptr obj)`
[macro] `int` `Snullp``(ptr obj)`
[macro] `int` `Seof_objectp``(ptr obj)`
[macro] `int` `Sbwp_objectp``(ptr obj)`
[macro] `int` `Sbooleanp``(ptr obj)`
[macro] `int` `Spairp``(ptr obj)`
[macro] `int` `Ssymbolp``(ptr obj)`
[macro] `int` `Sprocedurep``(ptr obj)`
[macro] `int` `Sflonump``(ptr obj)`
[macro] `int` `Svectorp``(ptr obj)`
[macro] `int` `Sbytevectorp``(ptr obj)`
[macro] `int` `Sfxvectorp``(ptr obj)`
[macro] `int` `Sstringp``(ptr obj)`
[macro] `int` `Sbignump``(ptr obj)`
[macro] `int` `Sboxp``(ptr obj)`
[macro] `int` `Sinexactnump``(ptr obj)`
[macro] `int` `Sexactnump``(ptr obj)`
[macro] `int` `Sratnump``(ptr obj)`
[macro] `int` `Sinputportp``(ptr obj)`
[macro] `int` `Soutputportp``(ptr obj)`
[macro] `int` `Srecordp``(ptr obj)`

Accessors. Some of the accessors described here correspond to similarly named Scheme procedures, while others are unique to this interface. `Sfixnum_value`, `Schar_value`, `Sboolean_value`, and `Sflonum_value` return the C equivalents of the given Scheme value.

[macro] `iptr` `Sfixnum_value``(ptr fixnum)`
[macro] `uptr` `Schar_value``(ptr character)`
[macro] `int` `Sboolean_value``(ptr obj)`
[macro] `double` `Sflonum_value``(ptr flonum)`

`Sinteger_value` and `Sunsigned_value` are similar to `Sfixnum_value`, except they accept not only fixnum arguments but bignum arguments in the range of C integer or unsigned values. `Sinteger_value` and `Sunsigned_value` accept the same range of Scheme integer values. They differ only in the result type, and so allow differing interpretations of negative and large unsigned values.

[func] `iptr` `Sinteger_value``(ptr integer)`
[macro] `uptr` `Sunsigned_value``(ptr integer)`

`Sinteger32_value`, `Sunsigned32_value`, `Sinteger64_value`, and `Sunsigned64_value` accept signed or unsigned Scheme integers in the 32- or 64-bit range and return integers of the appropriate type for the machine type.

[func] `Sint32_t` `Sinteger32_value``(ptr integer)`
[macro] `Suint32_t` `Sunsigned32_value``(ptr integer)`
[func] `Sint64_t` `Sinteger64_value``(ptr integer)`
[macro] `Suint64_t` `Sunsigned64_value``(ptr integer)`

The functions `Stry_integer_value`, `Stry_integer32_value`, and `Stry_integer64_value` also convert Scheme values to C integers. Unlike the corresponding `Sinteger_value` functions, these functions do not raise exceptions. Instead, they take additional `result` and `reason` arguments and return 1 or 0 to indicate success or failure. If `integer` can be converted to the requested type, these functions store the value in `result` and return 1. Otherwise, they return 0 without modifying `result`. If `integer` cannot be converted to the requested type and `reason` is not NULL, these functions store a Scheme-style format string in `reason` explaining why the conversion failed. The corresponding functions for unsigned values are `Stry_unsigned_value`, `Stry_unsigned32_value`, and `Stry_unsigned64_value`.

[func] `int` `Stry_integer_value``(ptr integer, iptr* result, const char** reason)`
[func] `int` `Stry_integer32_value``(ptr integer, Sint32_t* result, const char** reason)`
[func] `int` `Stry_integer64_value``(ptr integer, Sint64_t* result, const char** reason)`
[func] `int` `Stry_unsigned_value``(ptr integer, uptr* result, const char** reason)`
[func] `int` `Stry_unsigned32_value``(ptr integer, Suint32_t* result, const char** reason)`
[func] `int` `Stry_unsigned64_value``(ptr integer, Suint64_t* result, const char** reason)`

`Scar`, `Scdr`, `Ssymbol_to_string` (corresponding to `symbol->string`), and `Sunbox` are identical to their Scheme counterparts.

[macro] `ptr` `Scar``(ptr pair)`
[macro] `ptr` `Scdr``(ptr pair)`
[func] `ptr` `Ssymbol_to_string``(ptr sym)`
[macro] `ptr` `Sunbox``(ptr box)`

`Sstring_length`, `Svector_length`, `Sbytevector_length`, and `Sfxvector_length` each return a C integer representing the length (in elements) of the object.

[macro] `iptr` `Sstring_length``(ptr str)`
[macro] `iptr` `Svector_length``(ptr vec)`
[macro] `iptr` `Sbytevector_length``(ptr bytevec)`
[macro] `iptr` `Sfxvector_length``(ptr fxvec)`

`Sstring_ref`, `Svector_ref`, `Sbytevector_u8_ref`, and `Sfxvector_ref` correspond to their Scheme counterparts, except that the index arguments are C integers, the return value for `Sstring_ref` is a C character, and the return value for `Sbytevector_u8_ref` is an octet (unsigned char).

[macro] `char` `Sstring_ref``(ptr str, iptr i)`
[macro] `ptr` `Svector_ref``(ptr vec, iptr i)`
[macro] `octet` `Sbytevector_u8_ref``(ptr fxvec, iptr i)`
[macro] `ptr` `Sfxvector_ref``(ptr fxvec, iptr i)`

A Scheme bytevector is represented as a length field followed by a sequence of octets (unsigned chars). `Sbytevector_data` returns a pointer to the start of the sequence of octets. Extreme care should be taken to stop dereferencing the pointer returned by `Sbytevector_data` or to lock the bytevector into memory (see `Slock_object` below) before any Scheme code is executed, whether by calling into Scheme or returning to a Scheme caller. The storage manager may otherwise relocate or discard the object into which the pointer points and may copy other data over the object.

[macro] `octet *` `Sbytevector_data``(ptr bytevec)`

`Srecord_type` and `Srecord_type_parent` correspond to the Scheme procedures `record-rtd` and `record-type-parent`. `Srecord_type_size` returns the allocation size in bytes of an instance of the record type. The `Srecord_uniform_ref` accessor should be used only on records whose fields are all Scheme values, which is a record for which `Srecord_type_uniformp` composed with `Srecord_type` returns true.

[func] `ptr` `Srecord_type``(ptr rec)`
[func] `ptr` `Srecord_type_parent``(ptr rtd)`
[func] `uptr` `Srecord_type_size``(ptr rtd)`
[func] `int` `Srecord_type_uniformp``(ptr rtd)`
[macro] `ptr` `Srecord_uniform_ref``(ptr rec, iptr i)`

Mutators. Changes to mutable objects that contain pointers, such as pairs and vectors, must be tracked on behalf of the storage manager, as described in one of the references [13]. The operations described here perform this tracking automatically where necessary.

[func] `void` `Sset_box``(ptr box, ptr obj)`
[func] `void` `Sset_car``(ptr pair, ptr obj)`
[func] `void` `Sset_cdr``(ptr pair, ptr obj)`
[macro] `void` `Sstring_set``(ptr str, iptr i, char c)`
[func] `void` `Svector_set``(ptr vec, iptr i, ptr obj)`
[macro] `void` `Sbytevector_u8_set``(ptr bytevec, iptr i, octet n)`
[macro] `void` `Sfxvector_set``(ptr fxvec, iptr i, ptr fixnum)`

Some Scheme objects, such as procedures and numbers, are not mutable, so no operators are provided for altering the contents of those objects.

Constructors. The constructors described here create Scheme objects. Some objects, such as fixnums and the empty list, are represented as immediate values that do not require any heap allocation; others, such as pairs and vectors, are represented as pointers to heap allocated objects.

`Snil`, `Strue`, `Sfalse`, `Sbwp_object`, `Seof_object`, and `Svoid` construct constant immediate values representing the empty list ( `()` ), the boolean values (`#t` and `#f`), the broken-weak-pointer object (`#!bwp`), the eof object (`#!eof`), and the void object.

[macro] `ptr` `Snil`
[macro] `ptr` `Strue`
[macro] `ptr` `Sfalse`
[macro] `ptr` `Sbwp_object`
[macro] `ptr` `Seof_object`
[macro] `ptr` `Svoid`

Fixnums, characters, booleans, flonums, and strings may be created from their C equivalents.

[macro] `ptr` `Sfixnum``(iptr n)`
[macro] `ptr` `Schar``(char c)`
[macro] `ptr` `Sboolean``(int b)`
[func] `ptr` `Sflonum``(double x)`
[func] `ptr` `Sstring``(const char *s)`
[func] `ptr` `Sstring_of_length``(const char *s, iptr n)`
[func] `ptr` `Sstring_utf8``(const char *s, iptr n)`
;

`Sstring` creates a Scheme copy of the C string `s`, while `Sstring_of_length` creates a Scheme string of length `n` and copies the first `n` bytes from `s` into the new Scheme string.

If the C string is encoded in UTF-8, use `Sstring_utf8` instead. Specify the number of bytes to convert as `n` or use -1 to convert until the null terminator.

It is possible to determine whether a C integer is within fixnum range by comparing the fixnum value of a fixnum created from a C integer with the C integer:

`#define fixnum_rangep(x) (Sfixnum_value(Sfixnum(x)) == x)`

`Sinteger` and `Sunsigned` may be used to create Scheme integers whether they are in fixnum range or not.

[func] `ptr` `Sinteger``(iptr n)`
[func] `ptr` `Sunsigned``(uptr n)`

`Sinteger` and `Sunsigned` differ in their treatment of negative C integer values as well as C unsigned integer values that would appear negative if cast to integers. `Sinteger` converts such values into negative Scheme values, whereas `Sunsigned` converts such values into the appropriate positive Scheme values. For example, assuming a 32-bit, two's complement representation for `iptrs`, `Sinteger(-1)` and `Sinteger((iptr)0xffffffff)` both evaluate to the Scheme integer `-1`, whereas `Sunsigned(0xffffffff)` and `Sunsigned((uptr)-1)` both evaluate to the Scheme integer `#xffffffff` (`4294967295`).

Whichever routine is used, `Sinteger_value` and `Sunsigned_value` always reproduce the corresponding C input value, thus the following are all equivalent to `x` if `x` is an iptr.

`Sinteger_value(Sinteger(x))
(iptr)Sunsigned_value(Sinteger(x))
Sinteger_value(Sunsigned((uptr)x))
(iptr)Sunsigned_value(Sunsigned((uptr)x))`

Similarly, the following are all equivalent to `x` if `x` is a uptr.

`(uptr)Sinteger_value(Sinteger((iptr)x))
Sunsigned_value(Sinteger((iptr)x))
(uptr)Sinteger_value(Sunsigned(x))
Sunsigned_value(Sunsigned(x))`

`Sinteger32`, `Sunsigned32`, `Sinteger64`, and `Sunsigned64` are like the generic equivalents but restrict their arguments to the 32- or 64-bit range.

[func] `ptr` `Sinteger32``(Sint32_t n)`
[func] `ptr` `Sunsigned32``(Suint32_t n)`
[func] `ptr` `Sinteger64``(Sint64_t n)`
[func] `ptr` `Sunsigned64``(Suint64_t n)`

`Scons` and `Sbox` are identical to their Scheme counterparts.

[func] `ptr` `Scons``(ptr obj1, ptr obj2)`
[func] `ptr` `Sbox``(ptr obj)`

`Sstring_to_symbol` is similar to its Scheme counterpart, `string->symbol`, except that it takes a C string (character pointer) as input.

[func] `ptr` `Sstring_to_symbol``(const char *s)`

`Smake_string`, `Smake_vector`, `Smake_bytevector`, `Smake_fxvector`, and `Smake_flvector` are similar to their Scheme counterparts.

[func] `ptr` `Smake_string``(iptr n, int c)`
[func] `ptr` `Smake_vector``(iptr n, ptr obj)`
[func] `ptr` `Smake_bytevector``(iptr n, int fill)`
[func] `ptr` `Smake_fxvector``(iptr n, ptr fixnum)`
[func] `ptr` `Smake_flvector``(iptr n, double flonum)`

`Smake_uninitialized_string` is similar to the one-argument `make-string`.

[func] `ptr` `Smake_uninitialized_string``(iptr n)`

Windows-specific helper functions. The following helper functions are provided on Windows only.

[func] `char *` `Sgetenv``(const char *name)`

`Sgetenv` returns the UTF-8-encoded value of UTF-8-encoded environment variable `name` if found and NULL otherwise. Call `free` on the returned value when it is no longer needed.

[func] `wchar_t *` `Sutf8_to_wide``(const char *\s)`
[func] `char *` `Swide_to_utf8``(const wchar_t *\s)`

`Sutf8_to_wide` and `Swide_to_utf8` convert between UTF-8-encoded and UTF-16LE-encoded null-terminated strings. Call `free` on the returned value when it is no longer needed.

Accessing top-level values. Top-level variable bindings may be accessed or assigned via `Stop_level_value` and `Sset_top_level_value`.

[func] `ptr` `Stop_level_value``(ptr sym)`
[func] `void` `Sset_top_level_value``(ptr sym, ptr obj)`

These procedures give fast access to the bindings in the original interaction environment and do not reflect changes to the `interaction-environment` parameter or top-level module imports. To access the current interaction-environment binding for a symbol, it is necessary to call the Scheme `top-level-value` and `set-top-level-value!` procedures instead.

Locking Scheme objects. The storage manager periodically relocates objects in order to reclaim storage and compact the heap. This relocation is completely transparent to Scheme programs, since all pointers to a relocated object are updated to refer to the new location of the object. The storage manager cannot, however, update Scheme pointers that reside outside of the Scheme heap.

As a general rule, all pointers from C variables or data structures to Scheme objects should be discarded before entry (or reentry) into Scheme. That is, if a C procedure receives an object from Scheme or obtains it via the mechanisms described in this section, all pointers to the object should be considered invalid once the C procedure calls into Scheme or returns back to Scheme. Dereferencing an invalid pointer or passing it back to Scheme can have disastrous effects, including unrecoverable memory faults. The foregoing does not apply to immediate objects, e.g., fixnums, characters, booleans, or the empty list. It does apply to all heap-allocated objects, including pairs, vectors, strings, all numbers other than fixnums, ports, procedures, and records.

In practice, the best way to ensure that C code does not retain pointers to Scheme objects is to immediately convert the Scheme objects into C equivalents, if possible. In certain cases, it is not possible to do so, yet retention of the Scheme object is essential to the design of the C portions of the program. In these cases, the object may be locked via the library routine `Slock_object` (or from Scheme, the equivalent procedure `lock-object`).

[func] `void` `Slock_object``(ptr obj)`

Locking an object prevents the storage manager from reclaiming or relocating the object. Locking should be used sparingly, as it introduces memory fragmentation and increases storage management overhead. Locking can also lead to accidental retention of storage if objects are not unlocked. Locking objects that have been made static via heap compaction (see `Scompact_heap` above) is unnecessary but harmless.

Objects may be unlocked via `Sunlock_object` (`unlock-object`).

[func] `void` `Sunlock_object``(ptr obj)`

An object may be locked more than once by successive calls to `Slock_object` or `lock-object`, in which case it must be unlocked by an equal number of calls to `Sunlock_object` or `unlock-object` before it is truly unlocked.

The function `Slocked_objectp` can be used to determine if an object is locked.

[func] `int` `Slocked_objectp``(ptr obj)`

When a foreign procedure call is made into Scheme, a return address pointing into the Scheme code object associated with the foreign procedure is passed implicitly to the C routine. The system therefore locks the code object before calls are made from C back into Scheme and unlocks it upon return from Scheme. This locking is performed automatically; user code should never need to lock such code objects.

An object contained within a locked object, such as an object in the car of a locked pair, need not also be locked unless a separate C pointer to the object exists.

Registering foreign entry points. Foreign entry points may be made visible to Scheme via `Sforeign_symbol` or `Sregister_symbol`.

[func] `void` `Sforeign_symbol``(const char *name, void *addr)`
[func] `void` `Sregister_symbol``(const char *name, void *addr)`

External entry points in object files or shared objects loaded as a result of a call to `load-shared-object` are automatically made visible by the system. Once a foreign entry point is made visible, it may be named in a `foreign-procedure` expression to create a Scheme-callable version of the entry point. `Sforeign_symbol` and `Sregister_symbol` allow programs to register nonexternal entry points, entry points in code linked statically with Chez Scheme, and entry points into code loaded directly from C, i.e., without `load-shared-object`. `Sforeign_symbol` and `Sregister_symbol` differ only in that `Sforeign_symbol` raises an exception when an attempt is made to register an existing name, whereas `Sregister_symbol` permits existing names to be redefined.

Obtaining Scheme entry points. `Sforeign_callable_entry_point` extracts the entry point from a code object produced by `foreign-callable`, performing the same operation as its Scheme counterpart, i.e., the Scheme procedure `foreign-callable-entry-point`.

[macro] `(void (*) (void))` `Sforeign_callable_entry_point``(ptr code)`

This can be used to avoid converting the code object into an address until just when it is needed, which may eliminate the need to lock the code object in some circumstances, assuming that the code object is not saved across any calls back into Scheme.

The inverse translation can be made via `Sforeign_callable_code_object`.

[macro] `ptr` `Sforeign_callable_code_object``((void (*addr)(void)))`

Low-level support for calls into Scheme. Support for calling Scheme procedures from C is provided by the set of routines documented below. Calling a Scheme procedure that expects a small number of arguments (0-3) involves the use of one of the following routines.

[func] `ptr` `Scall0``(ptr procedure)`
[func] `ptr` `Scall1``(ptr procedure, ptr obj1)`
[func] `ptr` `Scall2``(ptr procedure, ptr obj1, ptr obj2)`
[func] `ptr` `Scall3``(ptr procedure, ptr obj1, ptr obj2, ptr obj3)`

In each case, the first argument, `procedure`, should be a Scheme procedure. The remaining arguments, which should be Scheme objects, are passed to the procedure. The tools described earlier in this section may be used to convert C datatypes into their Scheme equivalents. A program that automatically generates conversion code from declarations that are similar to `foreign-procedure` expressions is distributed with Chez Scheme. It can be found in the Scheme library directory on most systems in the file `"foreign.ss"`.

A Scheme procedure may be obtained in a number of ways. For example, it may be received as an argument in a call from Scheme into C, obtained via another call to Scheme, extracted from a Scheme data structure, or obtained from the top-level environment via `Stop_level_value`.

A more general interface involving the following routines is available for longer argument lists.

[func] `void` `Sinitframe``(iptr n)`
[func] `void` `Sput_arg``(iptr i, ptr obj)`
[func] `ptr` `Scall``(ptr procedure, iptr n)`

A C procedure first calls `Sinitframe` with one argument, the number of arguments to be passed to Scheme. It then calls `Sput_arg` once for each argument (in any order), passing `Sput_arg` the argument number (starting with `1`) and the argument. Finally, it calls `Scall` to perform the call, passing it the Scheme procedure and the number of arguments (the same number as in the call to `Sinitframe`). Programmers should ensure a Scheme call initiated via `Sinitframe` is completed via `Scall` before any other calls to Scheme are made and before a return to Scheme is attempted. If for any reason the call is not completed after `Sinitframe` has been called, it may not be possible to return to Scheme.

The following examples serve to illustrate both the simpler and more general interfaces.

`/* a particularly silly way to multiply two floating-point numbers */
double mul(double x, double y) {
ptr times = Stop_level_value(Sstring_to_symbol("*"));
return Sflonum_value(Scall2(times, Sflonum(x), Sflonum(y)));
}`

`/* an equally silly way to call printf with five arguments */
/* it is best to define interfaces such as the one below to handle
* calls into Scheme to prevent accidental attempts to nest frame
* creation and to help ensure that initiated calls are completed
* as discussed above. Specialized versions tailored to particular
* C argument types may be defined as well, with embedded conversions
* to Scheme objects. */
ptr Scall5(ptr p, ptr x1, ptr x2, ptr x3, ptr x4, ptr x5) {
Sinitframe(5);
Sput_arg(1, x1);
Sput_arg(2, x2);
Sput_arg(3, x3);
Sput_arg(4, x4);
Sput_arg(5, x5);
Scall(p, 5);
}
static void dumpem(char *s, int a, double b, ptr c, char *d) {
printf(s, a, b, c, d);
}
static void foo(int x, double y, ptr z, char *s) {
ptr ois, sip, read, expr, eval, c_dumpem;
char *sexpr = "(foreign-procedure \"dumpem\" (string integer-32\
double-float scheme-object string) void)";
/* this series of statements is carefully crafted to avoid referencing
variables holding Scheme objects after calls into Scheme */
ois = Stop_level_value(Sstring_to_symbol("open-input-string"));
sip = Scall1(ois, Sstring(sexpr));
read = Stop_level_value(Sstring_to_symbol("read"));
expr = Scall1(read, sip);
eval = Stop_level_value(Sstring_to_symbol("eval"));
Sforeign_symbol("dumpem", (void *)dumpem);
c_dumpem = Scall1(eval, expr);
Scall5(c_dumpem,
Sstring("x = %d, y = %g, z = %x, s = %s\n"),
Sinteger(x),
Sflonum(y),
z,
Sstring(s));
}`

Calls from C to Scheme should not be made from C interrupt handlers. When Scheme calls into C, the system saves the contents of certain dedicated machine registers in a register save area. When C then calls into Scheme, the registers are restored from the register save area. Because an interrupt can occur at any point in a computation, the contents of the register save locations would typically contain invalid information that would cause the Scheme system to fail to operate properly.

Activating, deactivating, and destroying threads. Three functions are provided by the threaded versions of Scheme to allow C code to notify Scheme when a thread should be activated, deactivated, or destroyed.

[func] `int` `Sactivate_thread``(void)`
[func] `void` `Sdeactivate_thread``(void)`
[func] `int` `Sdestroy_thread``(void)`

A thread created via the Scheme procedure `fork-thread` starts in the active state and need not be activated. Any thread that has been deactivated, and any thread created by some mechanism other than `fork-thread` must, however, be activated before it can access Scheme data or execute Scheme code. A foreign callable that is declared with `__collect_safe` can activate a calling thread. Otherwise, `Sactivate_thread` must be used to activate a thread. It returns 1 the first time the thread is activated and 0 on each subsequent call until the activation is destroyed with `Sdestroy_thread`.

Since active threads operating in C code prevent the storage management system from garbage collecting, a thread should be deactivated via `Sdeactivate_thread` or through a `foreign-procedure` `__collect_safe` declaration whenever the thread may spend a significant amount of time in C code. This is especially important whenever the thread calls a C library function, like `read`, that may block indefinitely. Once deactivated, the thread must not touch any Scheme data or execute any Scheme code until it is reactivated, with one exception. The exception is that the thread may access or even modify a locked Scheme object, such as a locked string, that contains no pointers to other, unlocked Scheme objects. (Objects that are not locked may be relocated by the garbage collector while the thread is inactive.)

`Sdestroy_thread` is used to notify the Scheme system that the thread is shut down and any thread-specific data can be released.

Low-level synchronization primitives. The header file defines several preprocessor macros that can be used to lock memory locations in a manner identical to the corresponding ftype lock operations (sections 15.4 and 15.5).

[macro] `void` `INITLOCK``(void *addr)`
[macro] `void` `SPINLOCK``(void *addr)`
[macro] `void` `UNLOCK``(void *addr)`
[macro] `void` `LOCKED_INCR``(void *addr, int *ret)`
[macro] `void` `LOCKED_DECR``(void *addr, int *ret)`

`LOCKED_INCR` and `LOCKED_DECR` set `ret` to a nonzero (true) value if the incremented or decremented value is 0. Otherwise they set `ret` to 0.

### Section 4.11. Example: Socket Operations

This section presents a simple socket interface that employs a combination of Scheme and C code. The C code defines a set of convenient low-level operating-system interfaces that can be used in the higher-level Scheme code to open, close, read from, and write to sockets.

The C code (csocket.c) is given below, followed by the Scheme code (socket.ss). The code should require little or no modification to run on most Unix systems and can be modified to work under Windows (using the Windows WinSock interface).

A sample session demonstrating the socket interface follows the code. See Section 9.17 for an example that demonstrates how to use the same socket interface to build a process port that allows transparent input from and output to a subprocess via a Scheme port.

C code.

`/* csocket.c */
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/un.h>
#include <string.h>
#include <errno.h>
#include <signal.h>
#include <sys/ioctl.h>
#include <stdio.h>
#include <unistd.h>
/* c_write attempts to write the entire buffer, pushing through
interrupts, socket delays, and partial-buffer writes */
int c_write(int fd, char *buf, ssize_t start, ssize_t n) {
ssize_t i, m;
buf += start;
m = n;
while (m > 0) {
if ((i = write(fd, buf, m)) < 0) {
if (errno != EAGAIN && errno != EINTR)
return i;
} else {
m -= i;
buf += i;
}
}
return n;
}
/* c_read pushes through interrupts and socket delays */
int c_read(int fd, char *buf, size_t start, size_t n) {
int i;
buf += start;
for (;;) {
i = read(fd, buf, n);
if (i >= 0) return i;
if (errno != EAGAIN && errno != EINTR) return -1;
}
}
/* bytes_ready(fd) returns true if there are bytes available
to be read from the socket identified by fd */
int bytes_ready(int fd) {
int n;
(void) ioctl(fd, FIONREAD, &n);
return n;
}
/* socket support */
/* do_socket() creates a new AF_UNIX socket */
int do_socket(void) {
return socket(AF_UNIX, SOCK_STREAM, 0);
}
/* do_bind(s, name) binds name to the socket s */
int do_bind(int s, char *name) {
struct sockaddr_un sun;
int length;
sun.sun_family = AF_UNIX;
(void) strcpy(sun.sun_path, name);
length = sizeof(sun.sun_family) + sizeof(sun.sun_path);
return bind(s, (struct sockaddr*)(&sun), length);
}
/* do_accept accepts a connection on socket s */
int do_accept(int s) {
struct sockaddr_un sun;
socklen_t length;
length = sizeof(sun.sun_family) + sizeof(sun.sun_path);
return accept(s, (struct sockaddr*)(&sun), &length);
}
/* do_connect initiates a socket connection */
int do_connect(int s, char *name) {
struct sockaddr_un sun;
int length;
sun.sun_family = AF_UNIX;
(void) strcpy(sun.sun_path, name);
length = sizeof(sun.sun_family) + sizeof(sun.sun_path);
return connect(s, (struct sockaddr*)(&sun), length);
}
/* get_error returns the operating system's error status */
char* get_error(void) {
extern int errno;
return strerror(errno);
}`

Scheme code.

`;;; socket.ss
;;; Requires csocket.so, built from csocket.c.
(load-shared-object "./csocket.so")
;;; Requires from C library:
;;; close, dup, execl, fork, kill, listen, tmpnam, unlink
(case (machine-type)
[(i3le ti3le a6le ta6le) (load-shared-object "libc.so.6")]
[(i3osx ti3osx a6osx ta6osx) (load-shared-object "libc.dylib")]
[else (load-shared-object "libc.so")])
;;; basic C-library stuff
(define close
(foreign-procedure "close" (int)
int))
(define dup
(foreign-procedure "dup" (int)
int))
(define execl4
(let ((execl-help
(foreign-procedure "execl"
(string string string string void*)
int)))
(lambda (s1 s2 s3 s4)
(execl-help s1 s2 s3 s4 0))))
(define fork
(foreign-procedure "fork" ()
int))
(define kill
(foreign-procedure "kill" (int int)
int))
(define listen
(foreign-procedure "listen" (int int)
int))
(define tmpnam
(foreign-procedure "tmpnam" (void*)
string))
(define unlink
(foreign-procedure "unlink" (string)
int))
;;; routines defined in csocket.c
(define accept
(foreign-procedure "do_accept" (int)
int))
(define bytes-ready?
(foreign-procedure "bytes_ready" (int)
boolean))
(define bind
(foreign-procedure "do_bind" (int string)
int))
(define c-error
(foreign-procedure "get_error" ()
string))
(define c-read
(foreign-procedure "c_read" (int u8* size_t size_t)
ssize_t))
(define c-write
(foreign-procedure "c_write" (int u8* size_t ssize_t)
ssize_t))
(define connect
(foreign-procedure "do_connect" (int string)
int))
(define socket
(foreign-procedure "do_socket" ()
int))
;;; higher-level routines
(define dodup
; (dodup old new) closes old and dups new, then checks to
; make sure that resulting fd is the same as old
(lambda (old new)
(check 'close (close old))
(unless (= (dup new) old)
(error 'dodup
"couldn't set up child process io for fd ~s" old))))
(define dofork
; (dofork child parent) forks a child process and invokes child
; without arguments and parent with the child's pid
(lambda (child parent)
(let ([pid (fork)])
(cond
[(= pid 0) (child)]
[(> pid 0) (parent pid)]
[else (error 'fork (c-error))]))))
(define setup-server-socket
; create a socket, bind it to name, and listen for connections
(lambda (name)
(let ([sock (check 'socket (socket))])
(unlink name)
(check 'bind (bind sock name))
(check 'listen (listen sock 1))
sock)))
(define setup-client-socket
; create a socket and attempt to connect to server
(lambda (name)
(let ([sock (check 'socket (socket))])
(check 'connect (connect sock name))
sock)))
(define accept-socket
; accept a connection
(lambda (sock)
(check 'accept (accept sock))))
(define check
; signal an error if status x is negative, using c-error to
; obtain the operating-system's error message
(lambda (who x)
(if (< x 0)
(error who (c-error))
x)))
(define terminate-process
; kill the process identified by pid
(lambda (pid)
(define sigterm 15)
(kill pid sigterm)
(void)))`

Sample session.

`> (define client-pid)
> (define client-socket)
> (let* ([server-socket-name (tmpnam 0)]
[server-socket (setup-server-socket server-socket-name)])
; fork a child, use it to exec a client Scheme process, and set
; up server-side client-pid and client-socket variables.
(dofork ; child
(lambda ()
; the child establishes the socket input/output fds as
; stdin and stdout, then starts a new Scheme session
(check 'close (close server-socket))
(let ([sock (setup-client-socket server-socket-name)])
(dodup 0 sock)
(dodup 1 sock))
(check 'execl (execl4 "/bin/sh" "/bin/sh" "-c" "exec scheme -q"))
(errorf 'client "returned!"))
(lambda (pid) ; parent
; the parent waits for a connection from the client
(set! client-pid pid)
(set! client-socket (accept-socket server-socket))
(check 'close (close server-socket)))))
> (define put ; procedure to send data to client
(lambda (x)
(let ([s (format "~s~%" x)])
(c-write client-socket s (string-length s)))
(void)))
> (define get ; procedure to read data from client
(let ([buff (make-string 1024)])
(lambda ()
(let ([n (c-read client-socket buff (string-length buff))])
(printf "client:~%~a~%server:~%" (substring buff 0 n))))))
> (get)
server:
> (put '(let ([x 3]) x))
> (get)
client:
3
server:
> (terminate-process client-pid)
> (exit)`

Chez Scheme Version 10 User's Guide
Copyright © 2026 Cisco Systems, Inc.
Licensed under the Apache License Version 2.0 (full copyright notice.). Revised May 2026 for Chez Scheme Version 10.4.1
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