3.6 Function Types

The resulting type can be used to reference foreign functions (usually ffi-objs, but any pointer object can be referenced with this type), generating a matching foreign callout object. Such objects are new primitive procedure objects that can be used like any other Racket procedure. As with other pointer types, #f is treated as a NULL function pointer and vice versa.

A type created with _cprocedure can also be used for passing Racket procedures to foreign functions, which will generate a foreign function pointer that calls to the given Racket callback procedure. There are no restrictions on the Racket procedure; in particular, its lexical context is properly preserved.

The optional abi keyword argument determines the foreign ABI that is used. Supplying #f or 'default indicates the platform-dependent default. The other possible values—'stdcall and 'sysv (i.e., “cdecl”)—are currently supported only for 32-bit Windows; using them on other platforms raises an exception. See also ffi/winapi.

For callouts to foreign functions with the generated type:

• If save-errno is 'posix, then the value of errno is saved (specific to the current thread) immediately after a foreign function callout returns. The saved value is accessible through saved-errno. If save-errno is 'windows, then the value of GetLastError() is saved for later use via saved-errno; the 'windows option is available only on Windows (on other platforms saved-errno will return 0). If save-errno is #f, no error value is saved automatically.

  The error-recording support provided by save-errno is needed because the Racket runtime system may otherwise preempt the current Racket thread and itself call functions that set error values.

• If wrapper is not #f, it takes the callout that would otherwise be generated and returns a replacement procedure. Thus, wrapper acts a hook to perform various argument manipulations before the true callout is invoked, and it can return different results (for example, grabbing a value stored in an “output” pointer and returning multiple values).

• If in-original-place? is true, then when a foreign callout procedure with the generated type is called in any Racket place, the procedure is called from the original Racket place. Use this mode for a foreign function that is not thread-safe at the C level, which means that it is not place-safe at the Racket level. Callbacks from place-unsafe code back into Racket at a non-original place typically will not work, since the place of the Racket code may have a different allocator than the original place.

• Values that are provided to a callout (i.e., the underlying callout, and not the replacement produced by a wrapper, if any) are always considered reachable by the garbage collector until the called foreign function returns. If the foreign function invokes Racket callbacks, however, beware that values managed by the Racket garbage collector might be moved in memory by the garbage collector.

• A callout object is finalized internally. Beware of trying to use a callout object that is reachable only from a finalized object, since the two objects might be finalized in either order.

For callbacks to Racket functions with the generated type:

• The keep argument provides control over reachbility by the garbage collector of the underlying value that foreign code see as a plain C function. Additional care must be taken in case the foreign code might retain the callback function, in which case the callback value must remain reachable or else the held callback will become invalid. The possible values of keep are as follows:

  • #t — the callback stays in memory as long as the converted Racket function is reachable. This mode is the default, as it is fine in most cases. Note that each Racket function can hold onto only one callback value through this mode, so it is not suitable for a function used multiple times as a reatined callback.

  • #f — the callback value is not held. This mode may be useful for a callback that is only used for the duration of the foreign call; for example, the comparison function argument to the standard C library qsort function is only used while qsort is working, and no additional references to the comparison function are kept. Use this option only in such cases, when no holding is necessary and you want to avoid the extra cost.

  • A box holding #f or any other non-list value — the callback value is stored in the box, overriding any non-list value that was in the box (making it useful for holding a single callback value). When you know that the callback is no longer needed, you can “release” the callback value by changing the box contents or by allowing the box itself to become unreachable. This mode can be useful if the box is held for a dynamic extent that corresponds to when the callback is needed; for example, you might encapsulate some foreign functionality in a Racket class or a unit, and keep the callback box as a field in new instances or instantiations of the unit.

  • A box holding null (or any list) — similar to a box holding a non-list value, except that new callback values are consed onto the contents of the box. This mode is therefore useful in cases when a Racket function is used in multiple callbacks (that is, sent to foreign code to hold onto multiple times) and all callbacks should be retained together.

  • A one-argument function — the function is invoked with the callback value when it is generated. This mode allows you to explicitly manage reachability of the generated callback closure.

• If wrapper is not #f, it takes the procedure to be converted into a callback and returns a replacement procedure to be invoked as the callback. Thus, wrapper acts a hook to perform various argument manipulations before a Racket callback function is called, and it can return different results to the foreign caller.

  The callback value’s reachability (and its interaction with keep is based on the original function for the callback, not the result of wrapper.

• If atomic? is true, then when a Racket procedure is given this procedure type and called as a callback from foreign code, then the Racket process is put into atomic mode while evaluating the Racket procedure body.

  In atomic mode, other Racket threads do not run, so the Racket code must not call any function that potentially blocks on synchronization with other threads, or else it may lead to deadlock. In addition, the Racket code must not perform any potentially blocking operation (such as I/O), it must not raise an uncaught exception, it must not perform any escaping continuation jumps, and its non-tail recursion must be minimal to avoid C-level stack overflow; otherwise, the process may crash or misbehave.

• If an async-apply procedure is provided, then a Racket callback procedure with the generated procedure type can be applied in a foreign thread (i.e., an OS-level thread other than the one used to run Racket). The call in the foreign thread is transferred to the OS-level thread that runs Racket, but the Racket-level thread (in the sense of thread) is unspecified; the job of the provided async-apply procedure is to arrange for the callback procedure to be run in a suitable Racket thread.

  The given async-apply procedure is applied to a thunk that encapsulates the specific callback invocation, and the foreign OS-level thread blocks until the thunk is called and completes; the thunk must be called exactly once, and the callback invocation must return normally. The given async-apply procedure itself is called in atomic mode (see atomic? above).

  If the callback is known to complete quickly, requires no synchronization, and works independent of the Racket thread in which it runs, then it is safe for the given async-apply procedure to apply the thunk directly. Otherwise, the given async-apply procedure must arrange for the thunk to be applied in a suitable Racket thread sometime after the given async-apply procedure itself returns; if the thunk raises an exception or synchronizes within an unsuitable Racket-level thread, it can deadlock or otherwise damage the Racket process.

  Foreign-thread detection to trigger async-apply works only when Racket is compiled with OS-level thread support, which is the default for many platforms. If a callback with an async-apply is called from foreign code in the same OS-level thread that runs Racket, then the async-apply wrapper is not used.

For example,

(_fun _int _string -> _int)

specifies a function that receives an integer and a string, and returns an integer.

In its full form, the _fun syntax provides an IDL-like language that can be used to create a wrapper function around the primitive foreign function. These wrappers can implement complex foreign interfaces given simple specifications. The full form of each of the type specifications can include an optional label and an expression. If a = value-expr is provided, then the resulting function will be a wrapper that calculates the argument for that position itself, meaning that it does not expect an argument for that position. The expression can use previous arguments if they were labeled with id :. In addition, the result of a function call need not be the value returned from the foreign call: if the optional output-expr is specified, or if an expression is provided for the output type, then this specifies an expression that will be used as a return value. This expression can use any of the previous labels, including a label given for the output which can be used to access the actual foreign return value.

In rare cases where complete control over the input arguments is needed, the wrapper’s argument list can be specified as maybe-args, in any form (including a “rest” argument). Identifiers in this place are related to type labels, so if an argument is there is no need to use an expression.

For example,

(_fun (n s) :: (s : _string) (n : _int) -> _int)

specifies a function that receives an integer and a string, but the foreign function receives the string first.

3.6.1 Custom Function Types

The behavior of the _fun type can be customized via custom function types, which are pieces of syntax that can behave as C types and C type constructors, but they can interact with function calls in several ways that are not possible otherwise. When the _fun form is expanded, it tries to expand each of the given type expressions, and ones that expand to certain keyword-value lists interact with the generation of the foreign function wrapper. This expansion makes it possible to construct a single wrapper function, avoiding the costs involved in compositions of higher-order functions.

Custom function types are macros that expand to a sequence (key: val ...), where each key: is from a short list of known keys. Each key interacts with generated wrapper functions in a different way, which affects how its corresponding argument is treated:

• type: specifies the foreign type that should be used, if it is #f then this argument does not participate in the foreign call.

• expr: specifies an expression to be used for arguments of this type, removing it from wrapper arguments.

• bind: specifies a name that is bound to the original argument if it is required later (e.g., _box converts its associated value to a C pointer, and later needs to refer back to the original box).

• 1st-arg: specifies a name that can be used to refer to the first argument of the foreign call (good for common cases where the first argument has a special meaning, e.g., for method calls).

• prev-arg: similar to 1st-arg:, but refers to the previous argument.

• pre: a pre-foreign code chunk that is used to change the argument’s value.

• post: a similar post-foreign code chunk.

• keywords: specifies keyword/value expressions that will be used with the surrounding _fun form. (Note: the keyword/value sequence follows keywords:, not parenthesized.)

The pre: and post: bindings can be of the form (id => expr) to use the existing value. Note that if the pre: expression is not (id => expr), then it means that there is no input for this argument to the _fun-generated procedure. Also note that if a custom type is used as an output type of a function, then only the post: code is used.

Most custom types are meaningful only in a _fun context, and will raise a syntax error if used elsewhere. A few such types can be used in non-_fun contexts: types which use only type:, pre:, post:, and no others. Such custom types can be used outside a _fun by expanding them into a usage of make-ctype, using other keywords makes this impossible, because it means that the type has specific interaction with a function call.

For instance, the following defines a new type that automatically coerces the input number to an inexact form which is compatible with the _float type.

(define-fun-syntax _float*

(syntax-id-rules (_float*)

[(_float*) (type: _float pre: (x => (+ 0.0 x)))]))

(_fun _float* -> _bool)

• i — indicates an input pointer argument: the wrapper arranges for the function call to receive a value that can be used with the type and to send a pointer to this value to the foreign function. After the call, the value is discarded.

• o — indicates an output pointer argument: the foreign function expects a pointer to a place where it will save some value, and this value is accessible after the call, to be used by an extra return expression. If _ptr is used in this mode, then the generated wrapper does not expect an argument since one will be freshly allocated before the call.

• io — combines the above into an input/output pointer argument: the wrapper gets the Racket value, allocates and set a pointer using this value, and then references the value after the call. The “_ptr” name can be confusing here: it means that the foreign function expects a pointer, but the generated wrapper uses an actual value. (Note that if this is used with structs, a struct is created when calling the function, and a copy of the return value is made too—which is inefficient, but ensures that structs are not modified by C code.)

For example, the _ptr type can be used in output mode to create a foreign function wrapper that returns more than a single argument. The following type:

(_fun (i : (_ptr o _int))

-> (d : _double)

-> (values d i))

creates a function that calls the foreign function with a fresh integer pointer, and use the value that is placed there as a second return value.

There is no need for other modes: input or input/output would be just like _bytes, since the string carries its size information (there is no real need for the o part of the syntax, but it is present for consistency with the above macros).