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

NAME

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

SYNOPSIS

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

int vprintf(const char *format, va_list ap);


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);

DESCRIPTION

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.

RETURN VALUES

Refer to printf(3C).

ERRORS

The vprintf() and vfprintf() functions will fail if either the stream is
unbuffered or the stream's buffer needed to be flushed and:

EFBIG
The file is a regular file and an attempt was made to write at
or beyond the offset maximum.

EXAMPLES

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

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


#include <stdio.h>
#include <stdarg.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();
}

ATTRIBUTES

See attributes(7) for descriptions of the following attributes:


+--------------------+-----------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+--------------------+-----------------+
|Interface Stability | Committed |
+--------------------+-----------------+
|MT-Level | See below. |
+--------------------+-----------------+
|Standard | See below. |
+--------------------+-----------------+


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


See standards(7) 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.

SEE ALSO

printf(3C), stdarg(3EXT), attributes(7), standards(7)

NOTES

The vsnprintf() return value when n = 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 = 0 returns an unspecified value less than 1.

February 21, 2023 VPRINTF(3C)