Increase text buffer

void *buffer_alloc(void *buf, char *str) {
    void *tmp = NULL;

    if (buf == NULL) {
        tmp = (char*)calloc(strlen(str) + 1, sizeof(*buf));
        if (tmp == NULL) {
            printf("calloc(): failed. Memory full ?\n");
            return ;
        }
        strncat(tmp, str, strlen(str) + 1);
    } else {
        if (print_debug() == true) printf("buffer_alloc: realloc\n");
        if ((tmp = (char *)realloc(buf, strlen(buf) + strlen(str) + 1)) == NULL) {
            printf("realloc(): failed. Memory full ?\n");
            return ;
        } else {
            strncat(tmp, str, strlen(str) + 1);
        }
    }

    buf = tmp;
    return buf;
}