asprintf, vasprintf — print to allocated string


#define _GNU_SOURCE         /* See feature_test_macros(7) */
#include <stdio.h>

int asprintf(char **strp, const char *fmt, ...);

int vasprintf(char **strp, const char *fmt, va_list ap);


The functions asprintf() and vasprintf() are analogs of sprintf(3) and vsprintf(3), except that they allocate a string large enough to hold the output including the terminating null byte ('\0'), and return a pointer to it via the first argument. This pointer should be passed to free(3) to release the allocated storage when it is no longer needed.

Return Value

When successful, these functions return the number of bytes printed, just like sprintf(3). If memory allocation wasn't possible, or some other error occurs, these functions will return -1, and the contents of strp are undefined.


For an explanation of the terms used in this section, see attributes(7).

Interface Attribute Value
asprintf(), vasprintf() Thread safety MT-Safe locale

Conforming to

These functions are GNU extensions, not in C or POSIX. They are also available under *BSD. The FreeBSD implementation sets strp to NULL on error.

See Also

free(3), malloc(3), printf(3)


This page is part of release 5.04 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at

Referenced By

mandoc_malloc(3), printf(3), talloc_string(3), tracef(3), tracelog(3).

The man page vasprintf(3) is an alias of asprintf(3).

2019-03-06 GNU Linux Programmer's Manual