strftime(3) with proper return values

xstrftime.c

int
xstrftime(char *s, size_t max, const char *format, const struct tm *tm) {
        char *fmt;
        int slen, flen;

        flen = strlen(format);
        if(!(fmt = malloc(flen + 1)))
                return -1;
        fmt[0] = 'x';
        memcpy(&fmt[1], format, flen);
        slen = strftime(s, max, fmt, tm) - 1;
        if(slen > 0)
                memmove(s, &s[1], max);
        free(fmt);
        return slen;
}