share/info/libffi.info

This is libffi.info, produced by makeinfo version 7.0.3 from
libffi.texi.

This manual is for libffi, a portable foreign function interface
library.

   Copyright © 2008–2024 Anthony Green and Red Hat, Inc.

   Permission is hereby granted, free of charge, to any person obtaining
a copy of this software and associated documentation files (the
“Software”), to deal in the Software without restriction, including
without limitation the rights to use, copy, modify, merge, publish,
distribute, sublicense, and/or sell copies of the Software, and to
permit persons to whom the Software is furnished to do so, subject to
the following conditions:

   The above copyright notice and this permission notice shall be
included in all copies or substantial portions of the Software.

   THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND,
EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

INFO-DIR-SECTION Development
START-INFO-DIR-ENTRY
* libffi: (libffi).             Portable foreign function interface library.
END-INFO-DIR-ENTRY


File: libffi.info,  Node: Top,  Next: Introduction,  Up: (dir)

libffi
******

This manual is for libffi, a portable foreign function interface
library.

   Copyright © 2008–2024 Anthony Green and Red Hat, Inc.

   Permission is hereby granted, free of charge, to any person obtaining
a copy of this software and associated documentation files (the
“Software”), to deal in the Software without restriction, including
without limitation the rights to use, copy, modify, merge, publish,
distribute, sublicense, and/or sell copies of the Software, and to
permit persons to whom the Software is furnished to do so, subject to
the following conditions:

   The above copyright notice and this permission notice shall be
included in all copies or substantial portions of the Software.

   THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND,
EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

* Menu:

* Introduction::                What is libffi?
* Using libffi::                How to use libffi.
* Memory Usage::                Where memory for closures comes from.
* Missing Features::            Things libffi can’t do.
* Index::                       Index.


File: libffi.info,  Node: Introduction,  Next: Using libffi,  Prev: Top,  Up: Top

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.


File: libffi.info,  Node: Using libffi,  Next: Memory Usage,  Prev: Introduction,  Up: Top

2 Using libffi
**************

* Menu:

* The Basics::                  The basic libffi API.
* Simple Example::              A simple example.
* Types::                       libffi type descriptions.
* Multiple ABIs::               Different passing styles on one platform.
* The Closure API::             Writing a generic function.
* Closure Example::             A closure example.
* Thread Safety::               Thread safety.


File: libffi.info,  Node: The Basics,  Next: Simple Example,  Up: Using libffi

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.
     *note Multiple ABIs:: for more information.

     NARGS is the number of arguments that this function accepts.

     RTYPE is a pointer to an ‘ffi_type’ structure that describes the
     return type of the function.  *Note 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.

   If the function being called is variadic (varargs) then
‘ffi_prep_cif_var’ must be used instead of ‘ffi_prep_cif’.

 -- Function: ffi_status ffi_prep_cif_var (ffi_cif *CIF, ffi_abi ABI,
          unsigned int NFIXEDARGS, unsigned int NTOTALARGS, ffi_type
          *RTYPE, ffi_type **ARGTYPES)
     This initializes CIF according to the given parameters for a call
     to a variadic function.  In general its operation is the same as
     for ‘ffi_prep_cif’ except that:

     NFIXEDARGS is the number of fixed arguments, prior to any variadic
     arguments.  It must be greater than zero.

     NTOTALARGS the total number of arguments, including variadic and
     fixed arguments.  ARGTYPES must have this many elements.

     ‘ffi_prep_cif_var’ will return ‘FFI_BAD_ARGTYPE’ if any of the
     variable argument types are ‘ffi_type_float’ (promote to
     ‘ffi_type_double’ first), or any integer type small than an int
     (promote to an int-sized type first).

     Note that, different cif’s must be prepped for calls to the same
     function when different numbers of arguments are passed.

     Also note that a call to ‘ffi_prep_cif_var’ with
     NFIXEDARGS=NOTOTALARGS is NOT equivalent to a call to
     ‘ffi_prep_cif’.

   Note that the resulting ‘ffi_cif’ holds pointers to all the
‘ffi_type’ objects that were used during initialization.  You must
ensure that these type objects have a lifetime at least as long as that
of the ‘ffi_cif’.

   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, no smaller than the system register size (generally 32 or
     64 bits), 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.

     In most situations, ‘libffi’ will handle promotion according to the
     ABI. However, for historical reasons, there is a special case with
     return values that must be handled by your code.  In particular,
     for integral (not ‘struct’) types that are narrower than the system
     register size, the return value will be widened by ‘libffi’.
     ‘libffi’ provides a type, ‘ffi_arg’, that can be used as the return
     type.  For example, if the CIF was defined with a return type of
     ‘char’, ‘libffi’ will try to store a full ‘ffi_arg’ into the return
     value.

     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 while the return value must be register-sized, arguments
     should exactly match their declared type.  For example, if an
     argument is a ‘short’, then the entry in AVALUES should point to an
     object declared as ‘short’; but if the return type is ‘short’, then
     RVALUE should point to an object declared as a larger type –
     usually ‘ffi_arg’.


File: libffi.info,  Node: Simple Example,  Next: Types,  Prev: The Basics,  Up: Using libffi

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;
       ffi_arg 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_sint, 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;
     }


File: libffi.info,  Node: Types,  Next: Multiple ABIs,  Prev: Simple Example,  Up: Using libffi

2.3 Types
=========

* Menu:

* Primitive Types::             Built-in types.
* Structures::                  Structure types.
* Size and Alignment::          Size and alignment of types.
* Arrays Unions Enums::         Arrays, unions, and enumerations.
* Type Example::                Structure type example.
* Complex::                     Complex types.
* Complex Type Example::        Complex type example.


File: libffi.info,  Node: Primitive Types,  Next: Structures,  Up: Types

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.

‘ffi_type_complex_float’
     The C ‘_Complex float’ type.

‘ffi_type_complex_double’
     The C ‘_Complex double’ type.

‘ffi_type_complex_longdouble’
     The C ‘_Complex long double’ type.  On platforms that have a C
     ‘long double’ type, this is defined.  On other platforms, it is
     not.

   Each of these is of type ‘ffi_type’, so you must take the address
when passing to ‘ffi_prep_cif’.


File: libffi.info,  Node: Structures,  Next: Size and Alignment,  Prev: Primitive Types,  Up: Types

2.3.2 Structures
----------------

‘libffi’ 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.

 -- Data type: 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.

          Note that ‘libffi’ has no special support for bit-fields.  You
          must manage these manually.

   The ‘size’ and ‘alignment’ fields will be filled in by ‘ffi_prep_cif’
or ‘ffi_prep_cif_var’, as needed.


File: libffi.info,  Node: Size and Alignment,  Next: Arrays Unions Enums,  Prev: Structures,  Up: Types

2.3.3 Size and Alignment
------------------------

‘libffi’ will set the ‘size’ and ‘alignment’ fields of an ‘ffi_type’
object for you.  It does so using its knowledge of the ABI.

   You might expect that you can simply read these fields for a type
that has been laid out by ‘libffi’.  However, there are some caveats.

   • The size or alignment of some of the built-in types may vary
     depending on the chosen ABI.

   • The size and alignment of a new structure type will not be set by
     ‘libffi’ until it has been passed to ‘ffi_prep_cif’ or
     ‘ffi_get_struct_offsets’.

   • A structure type cannot be shared across ABIs.  Instead each ABI
     needs its own copy of the structure type.

   So, before examining these fields, it is safest to pass the
‘ffi_type’ object to ‘ffi_prep_cif’ or ‘ffi_get_struct_offsets’ first.
This function will do all the needed setup.

     ffi_type *desired_type;
     ffi_abi desired_abi;
     ...
     ffi_cif cif;
     if (ffi_prep_cif (&cif, desired_abi, 0, desired_type, NULL) == FFI_OK)
       {
         size_t size = desired_type->size;
         unsigned short alignment = desired_type->alignment;
       }

   ‘libffi’ also provides a way to get the offsets of the members of a
structure.

 -- Function: ffi_status ffi_get_struct_offsets (ffi_abi abi, ffi_type
          *struct_type, size_t *offsets)
     Compute the offset of each element of the given structure type.
     ABI is the ABI to use; this is needed because in some cases the
     layout depends on the ABI.

     OFFSETS is an out parameter.  The caller is responsible for
     providing enough space for all the results to be written – one
     element per element type in STRUCT_TYPE.  If OFFSETS is ‘NULL’,
     then the type will be laid out but not otherwise modified.  This
     can be useful for accessing the type’s size or layout, as mentioned
     above.

     This function returns ‘FFI_OK’ on success; ‘FFI_BAD_ABI’ if ABI is
     invalid; or ‘FFI_BAD_TYPEDEF’ if STRUCT_TYPE is invalid in some
     way.  Note that only ‘FFI_STRUCT’ types are valid here.


File: libffi.info,  Node: Arrays Unions Enums,  Next: Type Example,  Prev: Size and Alignment,  Up: Types

2.3.4 Arrays, Unions, and Enumerations
--------------------------------------

2.3.4.1 Arrays
..............

‘libffi’ does not have direct support for arrays or unions.  However,
they can be emulated using structures.

   To emulate an array, simply create an ‘ffi_type’ using
‘FFI_TYPE_STRUCT’ with as many members as there are elements in the
array.

     ffi_type array_type;
     ffi_type **elements
     int i;

     elements = malloc ((n + 1) * sizeof (ffi_type *));
     for (i = 0; i < n; ++i)
       elements[i] = array_element_type;
     elements[n] = NULL;

     array_type.size = array_type.alignment = 0;
     array_type.type = FFI_TYPE_STRUCT;
     array_type.elements = elements;

   Note that arrays cannot be passed or returned by value in C –
structure types created like this should only be used to refer to
members of real ‘FFI_TYPE_STRUCT’ objects.

   However, a phony array type like this will not cause any errors from
‘libffi’ if you use it as an argument or return type.  This may be
confusing.

2.3.4.2 Unions
..............

A union can also be emulated using ‘FFI_TYPE_STRUCT’.  In this case,
however, you must make sure that the size and alignment match the real
requirements of the union.

   One simple way to do this is to ensue that each element type is laid
out.  Then, give the new structure type a single element; the size of
the largest element; and the largest alignment seen as well.

   This example uses the ‘ffi_prep_cif’ trick to ensure that each
element type is laid out.

     ffi_abi desired_abi;
     ffi_type union_type;
     ffi_type **union_elements;

     int i;
     ffi_type element_types[2];

     element_types[1] = NULL;

     union_type.size = union_type.alignment = 0;
     union_type.type = FFI_TYPE_STRUCT;
     union_type.elements = element_types;

     for (i = 0; union_elements[i]; ++i)
       {
         ffi_cif cif;
         if (ffi_prep_cif (&cif, desired_abi, 0, union_elements[i], NULL) == FFI_OK)
           {
             if (union_elements[i]->size > union_type.size)
               {
                 union_type.size = union_elements[i];
                 size = union_elements[i]->size;
               }
             if (union_elements[i]->alignment > union_type.alignment)
               union_type.alignment = union_elements[i]->alignment;
           }
       }

2.3.4.3 Enumerations
....................

‘libffi’ does not have any special support for C ‘enum’s.  Although any
given ‘enum’ is implemented using a specific underlying integral type,
exactly which type will be used cannot be determined by ‘libffi’ – it
may depend on the values in the enumeration or on compiler flags such as
‘-fshort-enums’.  *Note (gcc)Structures unions enumerations and
bit-fields implementation::, for more information about how GCC handles
enumerations.


File: libffi.info,  Node: Type Example,  Next: Complex,  Prev: Arrays Unions Enums,  Up: Types

2.3.5 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.type = FFI_TYPE_STRUCT;
           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() */
         }


File: libffi.info,  Node: Complex,  Next: Complex Type Example,  Prev: Type Example,  Up: Types

2.3.6 Complex Types
-------------------

‘libffi’ supports the complex types defined by the C99 standard
(‘_Complex float’, ‘_Complex double’ and ‘_Complex long double’ with the
built-in type descriptors ‘ffi_type_complex_float’,
‘ffi_type_complex_double’ and ‘ffi_type_complex_longdouble’.

   Custom complex types like ‘_Complex int’ can also be used.  An
‘ffi_type’ object has to be defined to describe the complex type to
‘libffi’.

 -- Data type: ffi_type
     ‘size_t size’
          This must be manually set to the size of the complex type.

     ‘unsigned short alignment’
          This must be manually set to the alignment of the complex
          type.

     ‘unsigned short type’
          For a complex type, this must be set to ‘FFI_TYPE_COMPLEX’.

     ‘ffi_type **elements’

          This is a ‘NULL’-terminated array of pointers to ‘ffi_type’
          objects.  The first element is set to the ‘ffi_type’ of the
          complex’s base type.  The second element must be set to
          ‘NULL’.

   The section *note Complex Type Example:: shows a way to determine the
‘size’ and ‘alignment’ members in a platform independent way.

   For platforms that have no complex support in ‘libffi’ yet, the
functions ‘ffi_prep_cif’ and ‘ffi_prep_args’ abort the program if they
encounter a complex type.


File: libffi.info,  Node: Complex Type Example,  Prev: Complex,  Up: Types

2.3.7 Complex Type Example
--------------------------

This example demonstrates how to use complex types:

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

     void complex_fn(_Complex float cf,
                     _Complex double cd,
                     _Complex long double cld)
     {
       printf("cf=%f+%fi\ncd=%f+%fi\ncld=%f+%fi\n",
              (float)creal (cf), (float)cimag (cf),
              (float)creal (cd), (float)cimag (cd),
              (float)creal (cld), (float)cimag (cld));
     }

     int main()
     {
       ffi_cif cif;
       ffi_type *args[3];
       void *values[3];
       _Complex float cf;
       _Complex double cd;
       _Complex long double cld;

       /* Initialize the argument info vectors */
       args[0] = &ffi_type_complex_float;
       args[1] = &ffi_type_complex_double;
       args[2] = &ffi_type_complex_longdouble;
       values[0] = &cf;
       values[1] = &cd;
       values[2] = &cld;

       /* Initialize the cif */
       if (ffi_prep_cif(&cif, FFI_DEFAULT_ABI, 3,
                        &ffi_type_void, args) == FFI_OK)
         {
           cf = 1.0 + 20.0 * I;
           cd = 300.0 + 4000.0 * I;
           cld = 50000.0 + 600000.0 * I;
           /* Call the function */
           ffi_call(&cif, (void (*)(void))complex_fn, 0, values);
         }

       return 0;
     }

   This is an example for defining a custom complex type descriptor for
compilers that support them:

     /*
      * This macro can be used to define new complex type descriptors
      * in a platform independent way.
      *
      * name: Name of the new descriptor is ffi_type_complex_<name>.
      * type: The C base type of the complex type.
      */
     #define FFI_COMPLEX_TYPEDEF(name, type, ffitype)             \
       static ffi_type *ffi_elements_complex_##name [2] = {      \
         (ffi_type *)(&ffitype), NULL                             \
       };                                                        \
       struct struct_align_complex_##name {                      \
         char c;                                                  \
         _Complex type x;                                         \
       };                                                        \
       ffi_type ffi_type_complex_##name = {                      \
         sizeof(_Complex type),                                   \
         offsetof(struct struct_align_complex_##name, x),         \
         FFI_TYPE_COMPLEX,                                        \
         (ffi_type **)ffi_elements_complex_##name                 \
       }

     /* Define new complex type descriptors using the macro: */
     /* ffi_type_complex_sint */
     FFI_COMPLEX_TYPEDEF(sint, int, ffi_type_sint);
     /* ffi_type_complex_uchar */
     FFI_COMPLEX_TYPEDEF(uchar, unsigned char, ffi_type_uint8);

   The new type descriptors can then be used like one of the built-in
type descriptors in the previous example.


File: libffi.info,  Node: Multiple ABIs,  Next: The Closure API,  Prev: Types,  Up: Using libffi

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.


File: libffi.info,  Node: The Closure API,  Next: Closure Example,  Prev: Multiple ABIs,  Up: Using libffi

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.


File: libffi.info,  Node: Closure Example,  Next: Thread Safety,  Prev: The Closure API,  Up: Using libffi

2.6 Closure Example
===================

A trivial example that creates a new ‘puts’ by binding ‘fputs’ with
‘stdout’.

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

     /* Acts like puts with the file given at time of enclosure. */
     void puts_binding(ffi_cif *cif, void *ret, void* args[],
                       void *stream)
     {
       *(ffi_arg *)ret = fputs(*(char **)args[0], (FILE *)stream);
     }

     typedef int (*puts_t)(char *);

     int main()
     {
       ffi_cif cif;
       ffi_type *args[1];
       ffi_closure *closure;

       void *bound_puts;
       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_sint, 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 = ((puts_t)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;
     }



File: libffi.info,  Node: Thread Safety,  Prev: Closure Example,  Up: Using libffi

2.7 Thread Safety
=================

‘libffi’ is not completely thread-safe.  However, many parts are, and if
you follow some simple rules, you can use it safely in a multi-threaded
program.

   • ‘ffi_prep_cif’ may modify the ‘ffi_type’ objects passed to it.  It
     is best to ensure that only a single thread prepares a given
     ‘ffi_cif’ at a time.

   • On some platforms, ‘ffi_prep_cif’ may modify the size and alignment
     of some types, depending on the chosen ABI. On these platforms, if
     you switch between ABIs, you must ensure that there is only one
     call to ‘ffi_prep_cif’ at a time.

     Currently the only affected platform is PowerPC and the only
     affected type is ‘long double’.


File: libffi.info,  Node: Memory Usage,  Next: Missing Features,  Prev: Using libffi,  Up: Top

3 Memory Usage
**************

Note that memory allocated by ‘ffi_closure_alloc’ and freed by
‘ffi_closure_free’ does not come from the same general pool of memory
that ‘malloc’ and ‘free’ use.  To accomodate security settings, ‘libffi’
may aquire memory, for example, by mapping temporary files into multiple
places in the address space (once to write out the closure, a second to
execute it).  The search follows this list, using the first that works:

   • A anonymous mapping (i.e.  not file-backed)

   • ‘memfd_create()’, if the kernel supports it.

   • A file created in the directory referenced by the environment
     variable ‘LIBFFI_TMPDIR’.

   • Likewise for the environment variable ‘TMPDIR’.

   • A file created in ‘/tmp’.

   • A file created in ‘/var/tmp’.

   • A file created in ‘/dev/shm’.

   • A file created in the user’s home directory (‘$HOME’).

   • A file created in any directory listed in ‘/etc/mtab’.

   • A file created in any directory listed in ‘/proc/mounts’.

   If security settings prohibit using any of these for closures,
‘ffi_closure_alloc’ will fail.


File: libffi.info,  Node: Missing Features,  Next: Index,  Prev: Memory Usage,  Up: Top

4 Missing Features
******************

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

   • Variadic closures.

   • There is no support for bit fields in structures.

   • The “raw” API is undocumented.

   • The Go API is undocumented.


File: libffi.info,  Node: Index,  Prev: Missing Features,  Up: Top

Index
*****

[index]
* Menu:

* ABI:                                   Introduction.         (line 13)
* Application Binary Interface:          Introduction.         (line 13)
* calling convention:                    Introduction.         (line 13)
* cif:                                   The Basics.           (line 14)
* closure API:                           The Closure API.      (line 13)
* closures:                              The Closure API.      (line 13)
* FFI:                                   Introduction.         (line 31)
* ffi_call:                              The Basics.           (line 72)
* FFI_CLOSURES:                          The Closure API.      (line 13)
* ffi_closure_alloc:                     The Closure API.      (line 19)
* ffi_closure_free:                      The Closure API.      (line 26)
* ffi_get_struct_offsets:                Size and Alignment.   (line 39)
* ffi_prep_cif:                          The Basics.           (line 16)
* ffi_prep_cif_var:                      The Basics.           (line 39)
* ffi_prep_closure_loc:                  The Closure API.      (line 34)
* ffi_status:                            The Basics.           (line 16)
* ffi_status <1>:                        The Basics.           (line 39)
* ffi_status <2>:                        Size and Alignment.   (line 39)
* ffi_status <3>:                        The Closure API.      (line 34)
* ffi_type:                              Structures.           (line 10)
* ffi_type <1>:                          Structures.           (line 10)
* ffi_type <2>:                          Complex.              (line 15)
* ffi_type <3>:                          Complex.              (line 15)
* ffi_type_complex_double:               Primitive Types.      (line 82)
* ffi_type_complex_float:                Primitive Types.      (line 79)
* ffi_type_complex_longdouble:           Primitive Types.      (line 85)
* ffi_type_double:                       Primitive Types.      (line 41)
* ffi_type_float:                        Primitive Types.      (line 38)
* ffi_type_longdouble:                   Primitive Types.      (line 71)
* ffi_type_pointer:                      Primitive Types.      (line 75)
* ffi_type_schar:                        Primitive Types.      (line 47)
* ffi_type_sint:                         Primitive Types.      (line 62)
* ffi_type_sint16:                       Primitive Types.      (line 23)
* ffi_type_sint32:                       Primitive Types.      (line 29)
* ffi_type_sint64:                       Primitive Types.      (line 35)
* ffi_type_sint8:                        Primitive Types.      (line 17)
* ffi_type_slong:                        Primitive Types.      (line 68)
* ffi_type_sshort:                       Primitive Types.      (line 56)
* ffi_type_uchar:                        Primitive Types.      (line 44)
* ffi_type_uint:                         Primitive Types.      (line 59)
* ffi_type_uint16:                       Primitive Types.      (line 20)
* ffi_type_uint32:                       Primitive Types.      (line 26)
* ffi_type_uint64:                       Primitive Types.      (line 32)
* ffi_type_uint8:                        Primitive Types.      (line 14)
* ffi_type_ulong:                        Primitive Types.      (line 65)
* ffi_type_ushort:                       Primitive Types.      (line 53)
* ffi_type_void:                         Primitive Types.      (line 10)
* Foreign Function Interface:            Introduction.         (line 31)
* void:                                  The Basics.           (line 72)
* void <1>:                              The Closure API.      (line 19)
* void <2>:                              The Closure API.      (line 26)



Tag Table:
Node: Top1399
Node: Introduction2933
Node: Using libffi4593
Node: The Basics5122
Node: Simple Example10240
Node: Types11275
Node: Primitive Types11786
Node: Structures14103
Node: Size and Alignment15214
Node: Arrays Unions Enums17489
Node: Type Example20470
Node: Complex21779
Node: Complex Type Example23295
Node: Multiple ABIs26347
Node: The Closure API26730
Node: Closure Example30648
Node: Thread Safety32292
Node: Memory Usage33125
Node: Missing Features34402
Node: Index34783

End Tag Table


Local Variables:
coding: utf-8
End: