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### 6.5 Constructing Function Calls

Using the built-in functions described below, you can record the arguments a function received, and call another function with the same arguments, without knowing the number or types of the arguments.

You can also record the return value of that function call, and later return that value, without knowing what data type the function tried to return (as long as your caller expects that data type).

However, these built-in functions may interact badly with some sophisticated features or other extensions of the language. It is, therefore, not recommended to use them outside very simple functions acting as mere forwarders for their arguments.

Built-in Function: void * __builtin_apply_args ()

  This built-in function returns a pointer to data describing how to perform a call with the same arguments as were passed to the current function.

  The function saves the arg pointer register, structure value address, and all registers that might be used to pass arguments to a function into a block of memory allocated on the stack. Then it returns the address of that block.

Built-in Function: void * __builtin_apply (void (*function)(), void *arguments, size_t size)

  This built-in function invokes function with a copy of the parameters described by arguments and size.

  The value of arguments should be the value returned by `__builtin_apply_args`. The argument size specifies the size of the stack argument data, in bytes.

  This function returns a pointer to data describing how to return whatever value was returned by function. The data is saved in a block of memory allocated on the stack.

  It is not always simple to compute the proper value for size. The value is used by `__builtin_apply` to compute the amount of data that should be pushed on the stack and copied from the incoming argument area.

Built-in Function: void __builtin_return (void *result)

  This built-in function returns the value described by result from the containing function. You should specify, for result, a value returned by `__builtin_apply`.

Built-in Function: __builtin_va_arg_pack ()

  This built-in function represents all anonymous arguments of an inline function. It can be used only in inline functions which will be always inlined, never compiled as a separate function, such as those using `__attribute__ ((__always_inline__))` or `__attribute__ ((__gnu_inline__))` extern inline functions. It must be only passed as last argument to some other function with variable arguments. This is useful for writing small wrapper inlines for variable argument functions, when using preprocessor macros is undesirable. For example:

      extern int myprintf (FILE *f, const char *format, ...);
      extern inline __attribute__ ((__gnu_inline__)) int
      myprintf (FILE *f, const char *format, ...)
      {
        int r = fprintf (f, "myprintf: ");
        if (r < 0)
          return r;
        int s = fprintf (f, format, __builtin_va_arg_pack ());
        if (s < 0)
          return s;
        return r + s;
      }

Built-in Function: size_t __builtin_va_arg_pack_len ()

  This built-in function returns the number of anonymous arguments of an inline function. It can be used only in inline functions which will be always inlined, never compiled as a separate function, such as those using `__attribute__ ((__always_inline__))` or `__attribute__ ((__gnu_inline__))` extern inline functions. For example following will do link or runtime checking of open arguments for optimized code:

      #ifdef __OPTIMIZE__
      extern inline __attribute__((__gnu_inline__)) int
      myopen (const char *path, int oflag, ...)
      {
        if (__builtin_va_arg_pack_len () > 1)
          warn_open_too_many_arguments ();

        if (__builtin_constant_p (oflag))
          {
            if ((oflag & O_CREAT) != 0 && __builtin_va_arg_pack_len () < 1)
              {
                warn_open_missing_mode ();
                return __open_2 (path, oflag);
              }
            return open (path, oflag, __builtin_va_arg_pack ());
          }

        if (__builtin_va_arg_pack_len () < 1)
          return __open_2 (path, oflag);

        return open (path, oflag, __builtin_va_arg_pack ());
      }
      #endif

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