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libffi

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”.


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1 What is libffi?

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.


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2 Using libffi


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2.1 The Basics

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.

— Function: ffi_status ffi_prep_cif (ffi_cif *cif, ffi_abi abi, unsigned int nargs, ffi_type *rtype, ffi_type **argtypes)

This initializes cif according to the given parameters.

abi is the ABI to use; normally FFI_DEFAULT_ABI is 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_type structure that describes the return type of the function. See Types.

argtypes is a vector of ffi_type pointers. argtypes must have nargs elements. If nargs is 0, this argument is ignored.

ffi_prep_cif returns a libffi status code, of type ffi_status. This will be either FFI_OK if everything worked properly; FFI_BAD_TYPEDEF if one of the ffi_type objects is incorrect; or FFI_BAD_ABI if the abi parameter is invalid.

To call a function using an initialized ffi_cif, use the ffi_call function:

— Function: void ffi_call (ffi_cif *cif, void *fn, void *rvalue, void **avalues)

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 (using ffi_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.


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2.2 Simple Example

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;
     }


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2.3 Types


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2.3.1 Primitive Types

Libffi provides a number of built-in type descriptors that can be used to describe argument and return types:

ffi_type_void
The type void. This cannot be used for argument types, only for return values.
ffi_type_uint8
An unsigned, 8-bit integer type.
ffi_type_sint8
A signed, 8-bit integer type.
ffi_type_uint16
An unsigned, 16-bit integer type.
ffi_type_sint16
A signed, 16-bit integer type.
ffi_type_uint32
An unsigned, 32-bit integer type.
ffi_type_sint32
A signed, 32-bit integer type.
ffi_type_uint64
An unsigned, 64-bit integer type.
ffi_type_sint64
A signed, 64-bit integer type.
ffi_type_float
The C float type.
ffi_type_double
The C double type.
ffi_type_uchar
The C unsigned char type.
ffi_type_schar
The C signed 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_ushort
The C unsigned short type.
ffi_type_sshort
The C short type.
ffi_type_uint
The C unsigned int type.
ffi_type_sint
The C int type.
ffi_type_ulong
The C unsigned long type.
ffi_type_slong
The C long type.
ffi_type_longdouble
On platforms that have a C long double type, this is defined. On other platforms, it is not.
ffi_type_pointer
A generic void * 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.


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2.3.2 Structures

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.

— ffi_type:

The ffi_type has 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_type objects. There is one element per field of the struct.


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2.3.3 Type Example

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() */
         }


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2.4 Multiple ABIs

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.


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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.

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

cif is the ffi_cif describing 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_cif passed to ffi_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_loc will return FFI_OK if 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.


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2.6 Closure Example

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;
     }
     


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3 Missing Features

libffi is missing a few features. We welcome patches to add support for these.


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Index