VPRINTF(3C) Standard C Library Functions VPRINTF(3C)

vprintf, vfprintf, vsprintf, vsnprintf, vasprintf —
print formatted output of a variable argument list

Standard C Library (libc, -lc)

#include <stdarg.h>
#include <stdio.h>

int
vprintf(const char *format, va_list);

int
vfprintf(FILE *stream, const char *format, va_list ap);

int
vsprintf(char *s, const char *format, va_list ap);

int
vsnprintf(char *s, size_t n, const char *format, va_list ap);

int
vasprintf(char **ret, const char *format, va_list ap);

The vprintf(), vfprintf(), vsprintf(), vsnprintf(), and vasprintf() functions are the same as printf(), fprintf(), sprintf(), snprintf(), and asprintf(), respectively, except that instead of being called with a variable number of arguments, they are called with an argument list as defined in the <stdarg.h> header. See printf(3C).

The <stdarg.h> header defines the type va_list and a set of macros for advancing through a list of arguments whose number and types may vary. The argument ap to the vprint family of functions is of type va_list. This argument is used with the <stdarg.h> header file macros va_start(), va_arg(), and va_end() (see stdarg(3EXT)). The EXAMPLES section below demonstrates the use of va_start() and va_end() with vprintf().

The macro va_alist() is used as the parameter list in a function definition, as in the function called error() in the example below. The macro ‘va_start(ap, name)’, where ap is of type va_list and name is the rightmost parameter (just before ...), must be called before any attempt to traverse and access unnamed arguments is made. The ‘va_end(ap)’ macro must be invoked when all desired arguments have been accessed. The argument list in ap can be traversed again if va_start() is called again after va_end(). In the example below, the error() arguments (arg1, arg2, ...) are passed to vfprintf() in the argument ap.

Refer to printf(3C).

Example 1 Using vprintf() to write an error routine.

The following demonstrates how vfprintf() could be used to write an error routine:

#include <stdarg.h>
#include <stdio.h>

/*
 * error should be called like
 *	error(function_name, format, arg1, ...);
 */
void
error(char *function_name, char *format, ...)
{
	va_list ap;

	va_start(ap, format);

	/* Print out name of function causing error */
	(void) fprintf(stderr, "ERR in %s: ", function_name);

	/* Print out remainder of message */
	(void) vfprintf(stderr, format, ap);

	va_end(ap);

	(void) abort();
}

The vfprintf() function will fail if either the stream is unbuffered or the stream's buffer needed to be flushed and:
The file is a regular file and an attempt was made to write at or beyond the offset maximum.

Committed

All of these functions can be used safely in multithreaded applications, as long as setlocale(3C) is not being called to change the locale.

printf(3C), stdarg(3EXT), attributes(5), standards(5)

See standards(5) for the standards conformance of vprintf(), vfprintf(), vsprintf(), and vsnprintf(). The vasprintf() function is modeled on the one that appears in the FreeBSD, NetBSD, and GNU C libraries.

The vsnprintf() return value when n is 0 was changed in the Solaris 10 release. The change was based on the SUSv3 specification. The previous behavior was based on the initial SUSv2 specification, where vsnprintf() when n is 0 returns an unspecified value less than 1.
July 10, 2020 illumos