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| <a name="Constructing-Calls"></a> |
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| |
| <h3 class="section">6.5 Constructing Function Calls</h3> |
| |
| <p><a name="index-constructing-calls-2194"></a><a name="index-forwarding-calls-2195"></a> |
| 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. |
| |
| <p>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). |
| |
| <p>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. |
| |
| <div class="defun"> |
| — Built-in Function: void * <b>__builtin_apply_args</b> ()<var><a name="index-g_t_005f_005fbuiltin_005fapply_005fargs-2196"></a></var><br> |
| <blockquote><p>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. |
| |
| <p>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. |
| </p></blockquote></div> |
| |
| <div class="defun"> |
| — Built-in Function: void * <b>__builtin_apply</b> (<var>void </var>(<var>*function</var>)()<var>, void *arguments, size_t size</var>)<var><a name="index-g_t_005f_005fbuiltin_005fapply-2197"></a></var><br> |
| <blockquote><p>This built-in function invokes <var>function</var> |
| with a copy of the parameters described by <var>arguments</var> |
| and <var>size</var>. |
| |
| <p>The value of <var>arguments</var> should be the value returned by |
| <code>__builtin_apply_args</code>. The argument <var>size</var> specifies the size |
| of the stack argument data, in bytes. |
| |
| <p>This function returns a pointer to data describing |
| how to return whatever value was returned by <var>function</var>. The data |
| is saved in a block of memory allocated on the stack. |
| |
| <p>It is not always simple to compute the proper value for <var>size</var>. The |
| value is used by <code>__builtin_apply</code> to compute the amount of data |
| that should be pushed on the stack and copied from the incoming argument |
| area. |
| </p></blockquote></div> |
| |
| <div class="defun"> |
| — Built-in Function: void <b>__builtin_return</b> (<var>void *result</var>)<var><a name="index-g_t_005f_005fbuiltin_005freturn-2198"></a></var><br> |
| <blockquote><p>This built-in function returns the value described by <var>result</var> from |
| the containing function. You should specify, for <var>result</var>, a value |
| returned by <code>__builtin_apply</code>. |
| </p></blockquote></div> |
| |
| <div class="defun"> |
| — Built-in Function: __builtin_va_arg_pack <b>(</b>)<var><a name="index-g_t_0028-2199"></a></var><br> |
| <blockquote><p>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 |
| <code>__attribute__ ((__always_inline__))</code> or |
| <code>__attribute__ ((__gnu_inline__))</code> 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: |
| <pre class="smallexample"> 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; |
| } |
| </pre> |
| </blockquote></div> |
| |
| <div class="defun"> |
| — Built-in Function: __builtin_va_arg_pack_len <b>(</b>)<var><a name="index-g_t_0028-2200"></a></var><br> |
| <blockquote><p>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 <code>__attribute__ ((__always_inline__))</code> or |
| <code>__attribute__ ((__gnu_inline__))</code> extern inline functions. |
| For example following will do link or runtime checking of open |
| arguments for optimized code: |
| <pre class="smallexample"> #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 |
| </pre> |
| </blockquote></div> |
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