static void save_pid(const pid_t pid, const char *pid_file) {
    FILE *fp;
    if (pid_file == NULL)
        return;

    if ((fp = fopen(pid_file, "w")) == NULL) {
        fprintf(stderr, "Could not open the pid file %s for writing\n", pid_file);
        return;
    }

    fprintf(fp,"%ld\n", (long)pid);
    if (fclose(fp) == -1) {
        fprintf(stderr, "Could not close the pid file %s.\n", pid_file);
        return;
    }
}

The product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly.