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.Dd February 15, 2008

.Dt ffi_call 3

.Sh NAME

.Nm ffi_call

.Nd Invoke a foreign function.

.Sh SYNOPSIS

.In ffi.h

.Ft void

.Fo ffi_call

.Fa "ffi_cif *cif"

.Fa "void (*fn)(void)"

.Fa "void *rvalue"

.Fa "void **avalue"

.Fc

.Sh DESCRIPTION

The

.Nm ffi_call

function provides a simple mechanism for invoking a function without

requiring knowledge of the function's interface at compile time.

.Fa fn

is called with the values retrieved from the pointers in the

.Fa avalue

array. The return value from

.Fa fn

is placed in storage pointed to by

.Fa rvalue .

.Fa cif

contains information describing the data types, sizes and alignments of the

arguments to and return value from

.Fa fn ,

and must be initialized with

.Nm ffi_prep_cif

before it is used with

.Nm ffi_call .

.Pp

.Fa rvalue

must point to storage that is sizeof(ffi_arg) or larger for non-floating point

types. For smaller-sized return value types, the

.Nm ffi_arg

or

.Nm ffi_sarg

integral type must be used to hold

the return value.

.Sh EXAMPLES

.Bd -literal

#include 

#include 

unsigned char

foo(unsigned int, float);

int

main(int argc, const char **argv)

{

    ffi_cif cif;

    ffi_type *arg_types[2];

    void *arg_values[2];

    ffi_status status;

    // Because the return value from foo() is smaller than sizeof(long), it

    // must be passed as ffi_arg or ffi_sarg.

    ffi_arg result;

    // Specify the data type of each argument. Available types are defined

    // in .

    arg_types[0] = &ffi_type_uint;

    arg_types[1] = &ffi_type_float;

    // Prepare the ffi_cif structure.

    if ((status = ffi_prep_cif(&cif, FFI_DEFAULT_ABI,

        2, &ffi_type_uint8, arg_types)) != FFI_OK)

    {

        // Handle the ffi_status error.

    }

    // Specify the values of each argument.

    unsigned int arg1 = 42;

    float arg2 = 5.1;

    arg_values[0] = &arg1;

    arg_values[1] = &arg2;

    // Invoke the function.

    ffi_call(&cif, FFI_FN(foo), &result, arg_values);

    // The ffi_arg 'result' now contains the unsigned char returned from foo(),

    // which can be accessed by a typecast.

    printf("result is %hhu", (unsigned char)result);

    return 0;

}

// The target function.

unsigned char

foo(unsigned int x, float y)

{

    unsigned char result = x - y;

    return result;

}

.Ed

.Sh SEE ALSO

.Xr ffi 3 ,

.Xr ffi_prep_cif 3