Constructing Function Calls
***************************

.. index:: constructing calls

.. index:: forwarding 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.

.. index:: __builtin_apply_args

  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 are 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.

.. index:: __builtin_apply

  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 is 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.

.. index:: __builtin_return

  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``.

.. index:: __builtin_va_arg_pack

  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 that are 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:

.. code-block:: c++

  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;
  }

.. index:: __builtin_va_arg_pack_len

  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 that
are 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 does link- or run-time checking of open
arguments for optimized code:

.. code-block:: c++

  #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