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| |
| <h4 class="subsection">12.12.9 Variable Arguments Output Functions</h4> |
| |
| <p>The functions <code>vprintf</code> and friends are provided so that you can |
| define your own variadic <code>printf</code>-like functions that make use of |
| the same internals as the built-in formatted output functions. |
| |
| <p>The most natural way to define such functions would be to use a language |
| construct to say, “Call <code>printf</code> and pass this template plus all |
| of my arguments after the first five.” But there is no way to do this |
| in C, and it would be hard to provide a way, since at the C language |
| level there is no way to tell how many arguments your function received. |
| |
| <p>Since that method is impossible, we provide alternative functions, the |
| <code>vprintf</code> series, which lets you pass a <code>va_list</code> to describe |
| “all of my arguments after the first five.” |
| |
| <p>When it is sufficient to define a macro rather than a real function, |
| the GNU C compiler provides a way to do this much more easily with macros. |
| For example: |
| |
| <pre class="smallexample"> #define myprintf(a, b, c, d, e, rest...) \ |
| printf (mytemplate , ## rest) |
| </pre> |
| <p class="noindent">See <a href="../cpp/Variadic-Macros.html#Variadic-Macros">Variadic Macros</a>, for details. |
| But this is limited to macros, and does not apply to real functions at all. |
| |
| <p>Before calling <code>vprintf</code> or the other functions listed in this |
| section, you <em>must</em> call <code>va_start</code> (see <a href="Variadic-Functions.html#Variadic-Functions">Variadic Functions</a>) to initialize a pointer to the variable arguments. Then you |
| can call <code>va_arg</code> to fetch the arguments that you want to handle |
| yourself. This advances the pointer past those arguments. |
| |
| <p>Once your <code>va_list</code> pointer is pointing at the argument of your |
| choice, you are ready to call <code>vprintf</code>. That argument and all |
| subsequent arguments that were passed to your function are used by |
| <code>vprintf</code> along with the template that you specified separately. |
| |
| <p>In some other systems, the <code>va_list</code> pointer may become invalid |
| after the call to <code>vprintf</code>, so you must not use <code>va_arg</code> |
| after you call <code>vprintf</code>. Instead, you should call <code>va_end</code> |
| to retire the pointer from service. However, you can safely call |
| <code>va_start</code> on another pointer variable and begin fetching the |
| arguments again through that pointer. Calling <code>vprintf</code> does not |
| destroy the argument list of your function, merely the particular |
| pointer that you passed to it. |
| |
| <p>GNU C does not have such restrictions. You can safely continue to fetch |
| arguments from a <code>va_list</code> pointer after passing it to |
| <code>vprintf</code>, and <code>va_end</code> is a no-op. (Note, however, that |
| subsequent <code>va_arg</code> calls will fetch the same arguments which |
| <code>vprintf</code> previously used.) |
| |
| <p>Prototypes for these functions are declared in <samp><span class="file">stdio.h</span></samp>. |
| <a name="index-stdio_002eh-1036"></a> |
| <!-- stdio.h --> |
| <!-- ISO --> |
| |
| <div class="defun"> |
| — Function: int <b>vprintf</b> (<var>const char *template, va_list ap</var>)<var><a name="index-vprintf-1037"></a></var><br> |
| <blockquote><p>This function is similar to <code>printf</code> except that, instead of taking |
| a variable number of arguments directly, it takes an argument list |
| pointer <var>ap</var>. |
| </p></blockquote></div> |
| |
| <!-- wchar.h --> |
| <!-- ISO --> |
| <div class="defun"> |
| — Function: int <b>vwprintf</b> (<var>const wchar_t *template, va_list ap</var>)<var><a name="index-vwprintf-1038"></a></var><br> |
| <blockquote><p>This function is similar to <code>wprintf</code> except that, instead of taking |
| a variable number of arguments directly, it takes an argument list |
| pointer <var>ap</var>. |
| </p></blockquote></div> |
| |
| <!-- stdio.h --> |
| <!-- ISO --> |
| <div class="defun"> |
| — Function: int <b>vfprintf</b> (<var>FILE *stream, const char *template, va_list ap</var>)<var><a name="index-vfprintf-1039"></a></var><br> |
| <blockquote><p>This is the equivalent of <code>fprintf</code> with the variable argument list |
| specified directly as for <code>vprintf</code>. |
| </p></blockquote></div> |
| |
| <!-- wchar.h --> |
| <!-- ISO --> |
| <div class="defun"> |
| — Function: int <b>vfwprintf</b> (<var>FILE *stream, const wchar_t *template, va_list ap</var>)<var><a name="index-vfwprintf-1040"></a></var><br> |
| <blockquote><p>This is the equivalent of <code>fwprintf</code> with the variable argument list |
| specified directly as for <code>vwprintf</code>. |
| </p></blockquote></div> |
| |
| <!-- stdio.h --> |
| <!-- ISO --> |
| <div class="defun"> |
| — Function: int <b>vsprintf</b> (<var>char *s, const char *template, va_list ap</var>)<var><a name="index-vsprintf-1041"></a></var><br> |
| <blockquote><p>This is the equivalent of <code>sprintf</code> with the variable argument list |
| specified directly as for <code>vprintf</code>. |
| </p></blockquote></div> |
| |
| <!-- wchar.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Function: int <b>vswprintf</b> (<var>wchar_t *s, size_t size, const wchar_t *template, va_list ap</var>)<var><a name="index-vswprintf-1042"></a></var><br> |
| <blockquote><p>This is the equivalent of <code>swprintf</code> with the variable argument list |
| specified directly as for <code>vwprintf</code>. |
| </p></blockquote></div> |
| |
| <!-- stdio.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Function: int <b>vsnprintf</b> (<var>char *s, size_t size, const char *template, va_list ap</var>)<var><a name="index-vsnprintf-1043"></a></var><br> |
| <blockquote><p>This is the equivalent of <code>snprintf</code> with the variable argument list |
| specified directly as for <code>vprintf</code>. |
| </p></blockquote></div> |
| |
| <!-- stdio.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Function: int <b>vasprintf</b> (<var>char **ptr, const char *template, va_list ap</var>)<var><a name="index-vasprintf-1044"></a></var><br> |
| <blockquote><p>The <code>vasprintf</code> function is the equivalent of <code>asprintf</code> with the |
| variable argument list specified directly as for <code>vprintf</code>. |
| </p></blockquote></div> |
| |
| <!-- stdio.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Function: int <b>obstack_vprintf</b> (<var>struct obstack *obstack, const char *template, va_list ap</var>)<var><a name="index-obstack_005fvprintf-1045"></a></var><br> |
| <blockquote><p>The <code>obstack_vprintf</code> function is the equivalent of |
| <code>obstack_printf</code> with the variable argument list specified directly |
| as for <code>vprintf</code>. |
| </p></blockquote></div> |
| |
| <p>Here's an example showing how you might use <code>vfprintf</code>. This is a |
| function that prints error messages to the stream <code>stderr</code>, along |
| with a prefix indicating the name of the program |
| (see <a href="Error-Messages.html#Error-Messages">Error Messages</a>, for a description of |
| <code>program_invocation_short_name</code>). |
| |
| <pre class="smallexample"> #include <stdio.h> |
| #include <stdarg.h> |
| |
| void |
| eprintf (const char *template, ...) |
| { |
| va_list ap; |
| extern char *program_invocation_short_name; |
| |
| fprintf (stderr, "%s: ", program_invocation_short_name); |
| va_start (ap, template); |
| vfprintf (stderr, template, ap); |
| va_end (ap); |
| } |
| </pre> |
| <p class="noindent">You could call <code>eprintf</code> like this: |
| |
| <pre class="smallexample"> eprintf ("file `%s' does not exist\n", filename); |
| </pre> |
| <p>In GNU C, there is a special construct you can use to let the compiler |
| know that a function uses a <code>printf</code>-style format string. Then it |
| can check the number and types of arguments in each call to the |
| function, and warn you when they do not match the format string. |
| For example, take this declaration of <code>eprintf</code>: |
| |
| <pre class="smallexample"> void eprintf (const char *template, ...) |
| __attribute__ ((format (printf, 1, 2))); |
| </pre> |
| <p class="noindent">This tells the compiler that <code>eprintf</code> uses a format string like |
| <code>printf</code> (as opposed to <code>scanf</code>; see <a href="Formatted-Input.html#Formatted-Input">Formatted Input</a>); |
| the format string appears as the first argument; |
| and the arguments to satisfy the format begin with the second. |
| See <a href="../gcc/Function-Attributes.html#Function-Attributes">Declaring Attributes of Functions</a>, for more information. |
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