2.5 The Closure API

libffi also provides a way to write a generic function – a function that can accept and decode any combination of arguments. This can be useful when writing an interpreter, or to provide wrappers for arbitrary functions.

This facility is called the closure API. Closures are not supported on all platforms; you can check the FFI_CLOSURES define to determine whether they are supported on the current platform.

Because closures work by assembling a tiny function at runtime, they require special allocation on platforms that have a non-executable heap. Memory management for closures is handled by a pair of functions:

Function: void *ffi_closure_alloc (size_t size, void **code)

Allocate a chunk of memory holding size bytes. This returns a pointer to the writable address, and sets *code to the corresponding executable address.

size should be sufficient to hold a ffi_closure object.

Function: void ffi_closure_free (void *writable)

Free memory allocated using ffi_closure_alloc. The argument is the writable address that was returned.

Once you have allocated the memory for a closure, you must construct a ffi_cif describing the function call. Finally you can prepare the closure function:

Function: ffi_status ffi_prep_closure_loc (ffi_closure *closure, ffi_cif *cif, void (*fun) (ffi_cif *cif, void *ret, void **args, void *user_data), void *user_data, void *codeloc)

Prepare a closure function. The arguments to ffi_prep_closure_loc are:

closure

The address of a ffi_closure object; this is the writable address returned by ffi_closure_alloc.

cif

The ffi_cif describing the function parameters. Note that this object, and the types to which it refers, must be kept alive until the closure itself is freed.

user_data

An arbitrary datum that is passed, uninterpreted, to your closure function.

codeloc

The executable address returned by ffi_closure_alloc.

fun

The function which will be called when the closure is invoked. It is called with the arguments:

cif

The ffi_cif passed to ffi_prep_closure_loc.

ret

A pointer to the memory used for the function’s return value.

If the function is declared as returning void, then this value is garbage and should not be used.

Otherwise, fun must fill the object to which this points, following the same special promotion behavior as ffi_call. That is, in most cases, ret points to an object of exactly the size of the type specified when cif was constructed. However, integral types narrower than the system register size are widened. In these cases your program may assume that ret points to an ffi_arg object.

args

A vector of pointers to memory holding the arguments to the function.

user_data

The same user_data that was passed to ffi_prep_closure_loc.

ffi_prep_closure_loc will return FFI_OK if everything went ok, and one of the other ffi_status values on error.

After calling ffi_prep_closure_loc, you can cast codeloc to the appropriate pointer-to-function type.

You may see old code referring to ffi_prep_closure. This function is deprecated, as it cannot handle the need for separate writable and executable addresses.