"int asprintf(char **"strp", const char *"fmt", ...);"
"int vasprintf(char **"strp", const char *"fmt", va_list "ap);
The functions asprintf and vasprintf are analogues of sprintf and vsprintf, except that they allocate a string large enough to hold the output including the terminating NUL, and return a pointer to it via the first parameter. This pointer should be passed to free(3) to release the allocated storage when it is no longer needed.
When successful, these functions return the number of bytes printed, just like sprintf(3). If memory allocation wasnt possible, or some other error occurs, these functions will return -1, and the contents of strp is undefined.
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.
free(3), malloc(3), printf(3)