This manual is for Libffi, a portable foreign-function interface library.
Copyright © 2008, 2010 Red Hat, Inc.
Permission is granted to copy, distribute and/or modify this document under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2, or (at your option) any later version. A copy of the license is included in the section entitled “GNU General Public License”.
Compilers for high level languages generate code that follow certain conventions. These conventions are necessary, in part, for separate compilation to work. One such convention is the calling convention. The calling convention is a set of assumptions made by the compiler about where function arguments will be found on entry to a function. A calling convention also specifies where the return value for a function is found. The calling convention is also sometimes called the ABI or Application Binary Interface. Some programs may not know at the time of compilation what arguments are to be passed to a function. For instance, an interpreter may be told at run-time about the number and types of arguments used to call a given function. ‘Libffi’ can be used in such programs to provide a bridge from the interpreter program to compiled code.
The ‘libffi’ library provides a portable, high level programming interface to various calling conventions. This allows a programmer to call any function specified by a call interface description at run time.
FFI stands for Foreign Function Interface. A foreign function interface is the popular name for the interface that allows code written in one language to call code written in another language. The ‘libffi’ library really only provides the lowest, machine dependent layer of a fully featured foreign function interface. A layer must exist above ‘libffi’ that handles type conversions for values passed between the two languages.
‘Libffi’ assumes that you have a pointer to the function you wish to call and that you know the number and types of arguments to pass it, as well as the return type of the function.
The first thing you must do is create an ffi_cif object that
matches the signature of the function you wish to call. This is a
separate step because it is common to make multiple calls using a
single ffi_cif. The cif in ffi_cif stands for
Call InterFace. To prepare a call interface object, use the function
ffi_prep_cif.
This initializes cif according to the given parameters.
abi is the ABI to use; normally
FFI_DEFAULT_ABIis what you want. Multiple ABIs for more information.nargs is the number of arguments that this function accepts. ‘libffi’ does not yet handle varargs functions; see Missing Features for more information.
rtype is a pointer to an
ffi_typestructure that describes the return type of the function. See Types.argtypes is a vector of
ffi_typepointers. argtypes must have nargs elements. If nargs is 0, this argument is ignored.
ffi_prep_cifreturns alibffistatus code, of typeffi_status. This will be eitherFFI_OKif everything worked properly;FFI_BAD_TYPEDEFif one of theffi_typeobjects is incorrect; orFFI_BAD_ABIif the abi parameter is invalid.
To call a function using an initialized ffi_cif, use the
ffi_call function:
This calls the function fn according to the description given in cif. cif must have already been prepared using
ffi_prep_cif.rvalue is a pointer to a chunk of memory that will hold the result of the function call. This must be large enough to hold the result and must be suitably aligned; it is the caller's responsibility to ensure this. If cif declares that the function returns
void(usingffi_type_void), then rvalue is ignored. If rvalue is ‘NULL’, then the return value is discarded.avalues is a vector of
void *pointers that point to the memory locations holding the argument values for a call. If cif declares that the function has no arguments (i.e., nargs was 0), then avalues is ignored. Note that argument values may be modified by the callee (for instance, structs passed by value); the burden of copying pass-by-value arguments is placed on the caller.
Here is a trivial example that calls puts a few times.
#include <stdio.h>
#include <ffi.h>
int main()
{
ffi_cif cif;
ffi_type *args[1];
void *values[1];
char *s;
int rc;
/* Initialize the argument info vectors */
args[0] = &ffi_type_pointer;
values[0] = &s;
/* Initialize the cif */
if (ffi_prep_cif(&cif, FFI_DEFAULT_ABI, 1,
&ffi_type_uint, args) == FFI_OK)
{
s = "Hello World!";
ffi_call(&cif, puts, &rc, values);
/* rc now holds the result of the call to puts */
/* values holds a pointer to the function's arg, so to
call puts() again all we need to do is change the
value of s */
s = "This is cool!";
ffi_call(&cif, puts, &rc, values);
}
return 0;
}
Libffi provides a number of built-in type descriptors that can
be used to describe argument and return types:
ffi_type_voidvoid. This cannot be used for argument types, only
for return values.
ffi_type_uint8ffi_type_sint8ffi_type_uint16ffi_type_sint16ffi_type_uint32ffi_type_sint32ffi_type_uint64ffi_type_sint64ffi_type_floatfloat type.
ffi_type_doubledouble type.
ffi_type_ucharunsigned char type.
ffi_type_scharsigned char type. (Note that there is not an exact
equivalent to the C char type in libffi; ordinarily you
should either use ffi_type_schar or ffi_type_uchar
depending on whether char is signed.)
ffi_type_ushortunsigned short type.
ffi_type_sshortshort type.
ffi_type_uintunsigned int type.
ffi_type_sintint type.
ffi_type_ulongunsigned long type.
ffi_type_slonglong type.
ffi_type_longdoublelong double type, this is defined.
On other platforms, it is not.
ffi_type_pointervoid * pointer. You should use this for all
pointers, regardless of their real type.
Each of these is of type ffi_type, so you must take the address
when passing to ffi_prep_cif.
Although ‘libffi’ has no special support for unions or
bit-fields, it is perfectly happy passing structures back and forth.
You must first describe the structure to ‘libffi’ by creating a
new ffi_type object for it.
The
ffi_typehas the following members:
size_t size- This is set by
libffi; you should initialize it to zero.unsigned short alignment- This is set by
libffi; you should initialize it to zero.unsigned short type- For a structure, this should be set to
FFI_TYPE_STRUCT.ffi_type **elements- This is a ‘NULL’-terminated array of pointers to
ffi_typeobjects. There is one element per field of the struct.
The following example initializes a ffi_type object
representing the tm struct from Linux's time.h.
Here is how the struct is defined:
struct tm {
int tm_sec;
int tm_min;
int tm_hour;
int tm_mday;
int tm_mon;
int tm_year;
int tm_wday;
int tm_yday;
int tm_isdst;
/* Those are for future use. */
long int __tm_gmtoff__;
__const char *__tm_zone__;
};
Here is the corresponding code to describe this struct to
libffi:
{
ffi_type tm_type;
ffi_type *tm_type_elements[12];
int i;
tm_type.size = tm_type.alignment = 0;
tm_type.elements = &tm_type_elements;
for (i = 0; i < 9; i++)
tm_type_elements[i] = &ffi_type_sint;
tm_type_elements[9] = &ffi_type_slong;
tm_type_elements[10] = &ffi_type_pointer;
tm_type_elements[11] = NULL;
/* tm_type can now be used to represent tm argument types and
return types for ffi_prep_cif() */
}
A given platform may provide multiple different ABIs at once. For instance, the x86 platform has both ‘stdcall’ and ‘fastcall’ functions.
libffi provides some support for this. However, this is
necessarily platform-specific.
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:
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_closureobject.
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:
Prepare a closure function.
closure is the address of a
ffi_closureobject; this is the writable address returned byffi_closure_alloc.cif is the
ffi_cifdescribing the function parameters.user_data is an arbitrary datum that is passed, uninterpreted, to your closure function.
codeloc is the executable address returned by
ffi_closure_alloc.fun is the function which will be called when the closure is invoked. It is called with the arguments:
- cif
- The
ffi_cifpassed toffi_prep_closure_loc.- ret
- A pointer to the memory used for the function's return value. fun must fill this, unless the function is declared as returning
void.- 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_locwill returnFFI_OKif everything went ok, and something else 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.
A trivial example that creates a new puts by binding
fputs with stdin.
#include <stdio.h>
#include <ffi.h>
/* Acts like puts with the file given at time of enclosure. */
void puts_binding(ffi_cif *cif, unsigned int *ret, void* args[],
FILE *stream)
{
*ret = fputs(*(char **)args[0], stream);
}
int main()
{
ffi_cif cif;
ffi_type *args[1];
ffi_closure *closure;
int (*bound_puts)(char *);
int rc;
/* Allocate closure and bound_puts */
closure = ffi_closure_alloc(sizeof(ffi_closure), &bound_puts);
if (closure)
{
/* Initialize the argument info vectors */
args[0] = &ffi_type_pointer;
/* Initialize the cif */
if (ffi_prep_cif(&cif, FFI_DEFAULT_ABI, 1,
&ffi_type_uint, args) == FFI_OK)
{
/* Initialize the closure, setting stream to stdout */
if (ffi_prep_closure_loc(closure, &cif, puts_binding,
stdout, bound_puts) == FFI_OK)
{
rc = bound_puts("Hello World!");
/* rc now holds the result of the call to fputs */
}
}
}
/* Deallocate both closure, and bound_puts */
ffi_closure_free(closure);
return 0;
}
libffi is missing a few features. We welcome patches to add
support for these.
ffi_call: The Basicsffi_closure_alloc: The Closure APIffi_closure_free: The Closure APIFFI_CLOSURES: The Closure APIffi_prep_cif: The Basicsffi_prep_closure_loc: The Closure APIffi_status: The Closure APIffi_status: The Basicsffi_type: Structuresffi_type_double: Primitive Typesffi_type_float: Primitive Typesffi_type_longdouble: Primitive Typesffi_type_pointer: Primitive Typesffi_type_schar: Primitive Typesffi_type_sint: Primitive Typesffi_type_sint16: Primitive Typesffi_type_sint32: Primitive Typesffi_type_sint64: Primitive Typesffi_type_sint8: Primitive Typesffi_type_slong: Primitive Typesffi_type_sshort: Primitive Typesffi_type_uchar: Primitive Typesffi_type_uint: Primitive Typesffi_type_uint16: Primitive Typesffi_type_uint32: Primitive Typesffi_type_uint64: Primitive Typesffi_type_uint8: Primitive Typesffi_type_ulong: Primitive Typesffi_type_ushort: Primitive Typesffi_type_void: Primitive Typesvoid: The Closure APIvoid: The Basics