kc3-lang/libffi/doc/libffi.info

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  Date : 2013-11-13T16:55:36
  

  This enshrines the current testsuite practice of using ffi_arg for returned values. It would be reasonable and logical to use the actual return argument type as passed to ffi_prep_cif, but this would mean changing a large number of tests that use ffi_arg and all backends that write results to an ffi_arg.

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• doc/libffi.info

• This is libffi.info, produced by makeinfo version 5.1 from libffi.texi.
  
  This manual is for Libffi, a portable foreign-function interface
  library.
  
     Copyright (C) 2008, 2010, 2011 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".
  
  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 (C) 2008, 2010, 2011 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".
  
  * Menu:
  
  * Introduction::                What is libffi?
  * Using libffi::                How to use libffi.
  * 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: Missing Features,  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.
  
  
  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 varabi,
            unsigned int NFIXEDARGS, unsigned int varntotalargs, ffi_type
            *RTYPE, ffi_type **ARGTYPES)
       This initializes CIF according to the given parameters for a call
       to a variadic function.  In general it's 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.
  
       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'.
  
     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.
  
       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.
  
  
  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.
  * Type Example::                Structure 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.
  
     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: Type Example,  Prev: Primitive Types,  Up: Types
  
  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.
  
   -- 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.
  
  
  File: libffi.info,  Node: Type Example,  Prev: Structures,  Up: Types
  
  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.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: 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.
  
       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.
  
  
  File: libffi.info,  Node: Closure Example,  Prev: The Closure API,  Up: Using libffi
  
  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, ffi_arg *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_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 = 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: Missing Features,  Next: Index,  Prev: Using libffi,  Up: Top
  
  3 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 closure API is
  
     * The "raw" API is undocumented.
  
     Note that variadic support is very new and tested on a relatively
  small number of platforms.
  
  
  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 62)
  * FFI_CLOSURES:                          The Closure API.      (line 13)
  * ffi_closure_alloc:                     The Closure API.      (line 19)
  * ffi_closure_free:                      The Closure API.      (line 26)
  * 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>:                        The Closure API.      (line 34)
  * ffi_type:                              Structures.           (line 11)
  * ffi_type <1>:                          Structures.           (line 11)
  * 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 62)
  * void <1>:                              The Closure API.      (line 19)
  * void <2>:                              The Closure API.      (line 26)
  
  
  
  Tag Table:
  Node: Top682
  Node: Introduction1429
  Node: Using libffi3061
  Node: The Basics3547
  Node: Simple Example7198
  Node: Types8229
  Node: Primitive Types8512
  Node: Structures10333
  Node: Type Example11207
  Node: Multiple ABIs12473
  Node: The Closure API12844
  Node: Closure Example15788
  Node: Missing Features17342
  Node: Index17795
  
  End Tag Table
  

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