#### 12.12.9 Variable Arguments Output Functions

The functions `vprintf` and friends are provided so that you can define your own variadic `printf`-like functions that make use of the same internals as the built-in formatted output functions.

The most natural way to define such functions would be to use a language construct to say, “Call `printf` 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.

Since that method is impossible, we provide alternative functions, the `vprintf` series, which lets you pass a `va_list` to describe “all of my arguments after the first five.”

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:

         #define myprintf(a, b, c, d, e, rest...) \
         	    printf (mytemplate , ## rest)

See Variadic Macros, for details. But this is limited to macros, and does not apply to real functions at all.

Before calling `vprintf` or the other functions listed in this section, you must call `va_start` (see Variadic Functions) to initialize a pointer to the variable arguments. Then you can call `va_arg` to fetch the arguments that you want to handle yourself. This advances the pointer past those arguments.

Once your `va_list` pointer is pointing at the argument of your choice, you are ready to call `vprintf`. That argument and all subsequent arguments that were passed to your function are used by `vprintf` along with the template that you specified separately.

In some other systems, the `va_list` pointer may become invalid after the call to `vprintf`, so you must not use `va_arg` after you call `vprintf`. Instead, you should call `va_end` to retire the pointer from service. However, you can safely call `va_start` on another pointer variable and begin fetching the arguments again through that pointer. Calling `vprintf` does not destroy the argument list of your function, merely the particular pointer that you passed to it.

GNU C does not have such restrictions. You can safely continue to fetch arguments from a `va_list` pointer after passing it to `vprintf`, and `va_end` is a no-op. (Note, however, that subsequent `va_arg` calls will fetch the same arguments which `vprintf` previously used.)

Prototypes for these functions are declared in `stdio.h`.

— Function: int vprintf (const char *template, va_list ap)

> This function is similar to `printf` except that, instead of taking a variable number of arguments directly, it takes an argument list pointer ap.

— Function: int vwprintf (const wchar_t *template, va_list ap)

> This function is similar to `wprintf` except that, instead of taking a variable number of arguments directly, it takes an argument list pointer ap.

— Function: int vfprintf (FILE *stream, const char *template, va_list ap)

> This is the equivalent of `fprintf` with the variable argument list specified directly as for `vprintf`.

— Function: int vfwprintf (FILE *stream, const wchar_t *template, va_list ap)

> This is the equivalent of `fwprintf` with the variable argument list specified directly as for `vwprintf`.

— Function: int vsprintf (char *s, const char *template, va_list ap)

> This is the equivalent of `sprintf` with the variable argument list specified directly as for `vprintf`.

— Function: int vswprintf (wchar_t *s, size_t size, const wchar_t *template, va_list ap)

> This is the equivalent of `swprintf` with the variable argument list specified directly as for `vwprintf`.

— Function: int vsnprintf (char *s, size_t size, const char *template, va_list ap)

> This is the equivalent of `snprintf` with the variable argument list specified directly as for `vprintf`.

— Function: int vasprintf (char **ptr, const char *template, va_list ap)

> The `vasprintf` function is the equivalent of `asprintf` with the variable argument list specified directly as for `vprintf`.

— Function: int obstack_vprintf (struct obstack *obstack, const char *template, va_list ap)

> The `obstack_vprintf` function is the equivalent of `obstack_printf` with the variable argument list specified directly as for `vprintf`.

Here's an example showing how you might use `vfprintf`. This is a function that prints error messages to the stream `stderr`, along with a prefix indicating the name of the program (see Error Messages, for a description of `program_invocation_short_name`).

         #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);
         }

You could call `eprintf` like this:

         eprintf ("file `%s' does not exist\n", filename);

In GNU C, there is a special construct you can use to let the compiler know that a function uses a `printf`-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 `eprintf`:

         void eprintf (const char *template, ...)
         	__attribute__ ((format (printf, 1, 2)));

This tells the compiler that `eprintf` uses a format string like `printf` (as opposed to `scanf`; see Formatted Input); the format string appears as the first argument; and the arguments to satisfy the format begin with the second. See Declaring Attributes of Functions, for more information.