Keep It Real

C++ Stickies : snprintf() [Safe Version of sprintf() that doesn’t suffer from buffer overruns]

   int snprintf(char *str, size_t size, const char *format, ...);
    int vsnprintf(char *str, size_t size, const char *format, va_list args);

Writes output to the string str, under control of the format string format, that specifies how subsequent arguments are converted for output. It is similar to sprintf(3), except that size specifies the maximum number of characters to produce. The trailing nul character is counted towards this limit, so you must allocate at least size characters for str.

If size is zero, nothing is written and str may be null. Otherwise, output characters beyond the n-1st are discarded rather than being written to str, and a nul character is written at the end of the characters actually written to str. If copying takes place between objects that overlap, the behaviour is undefined.

Example,
    #include <slack/std.h>
    #ifndef HAVE_SNPRINTF
    #include <slack/snprintf.h>
    #endif

    int main(int ac, char **av)
    {
        char buf[16];
        char *str = buf;
        char *extra = NULL;
        int len;

        if (!av[1])
            return EXIT_FAILURE;

        if ((len = snprintf(buf, 16, "%s", av[1])) >= 16)
            if (extra = malloc((len + 1) * sizeof(char)))
                snprintf(str = extra, len + 1, "%s", av[1]);

        printf("%s\n", str);

        if (extra)
            free(extra);

        return EXIT_SUCCESS;
    }