vasprintf

       #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);

DESCRIPTION
       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.

ATTRIBUTES
       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)

COLOPHON
       This  page  is  part of release 4.04 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
       http://www.kernel.org/doc/man-pages/.

GNU                               2015-04-19                       ASPRINTF(3)
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