ffi\_call(3) Library Functions Manual ffi\_call(3)

# [NAME](#NAME)

`ffi_call` — Invoke a foreign function.

# [SYNOPSIS](#SYNOPSIS)

`#include <ffi.h>`

`void`  
`ffi_call`(`ffi_cif *cif`, `void (*fn)(void)`, `void *rvalue`, `void **avalue`);

# [DESCRIPTION](#DESCRIPTION)

The `ffi_call` function provides a simple mechanism for invoking a function without requiring knowledge of the function's interface at compile time. `fn` is called with the values retrieved from the pointers in the `avalue` array. The return value from `fn` is placed in storage pointed to by `rvalue`. `cif` contains information describing the data types, sizes and alignments of the arguments to and return value from `fn`, and must be initialized with `ffi_prep_cif` before it is used with `ffi_call`.

`rvalue` must point to storage that is sizeof(ffi\_arg) or larger for non-floating point types. For smaller-sized return value types, the `ffi_arg` or `ffi_sarg` integral type must be used to hold the return value.

# [EXAMPLES](#EXAMPLES)

```
#include <ffi.h>
#include <stdio.h>

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 <ffi/ffi.h>.
    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;
}
```

# [SEE ALSO](#SEE_ALSO)

[ffi(3)](/man/os/libffi/ffi.3), [ffi\_prep\_cif(3)](/man/os/libffi/ffi_prep_cif.3)

February 15, 2008

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Document: ffi_call(3) from libffi 3.8.0-r2 (Chainguard OS)
Source: https://github.com/libffi/libffi.git