static void save_pid(const char *pid_file) {
    FILE *fp;
    if (access(pid_file, F_OK) == 0) {
        if ((fp = fopen(pid_file, "r")) != NULL) {
            char buffer[1024];
            if (fgets(buffer, sizeof(buffer), fp) != NULL) {
                unsigned int pid;
                if (safe_strtoul(buffer, &pid) && kill((pid_t)pid, 0) == 0) {
                    fprintf(stderr, "WARNING: The pid file contained the following (running) pid: %u\n", pid);
                }
            }
            fclose(fp);
        }
    }

    /* Create the pid file first with a temporary name, then
     * atomically move the file to the real name to avoid a race with
     * another process opening the file to read the pid, but finding
     * it empty.
     */
    char tmp_pid_file[1024];
    snprintf(tmp_pid_file, sizeof(tmp_pid_file), "%s.tmp", pid_file);

    if ((fp = fopen(tmp_pid_file, "w")) == NULL) {
        vperror("Could not open the pid file %s for writing", tmp_pid_file);
        return;
    }

    fprintf(fp,"%ld\n", (long)getpid());
    if (fclose(fp) == -1) {
        vperror("Could not close the pid file %s", tmp_pid_file);
    }

    if (rename(tmp_pid_file, pid_file) != 0) {
        vperror("Could not rename the pid file from %s to %s",
                tmp_pid_file, pid_file);
    }
}

The product performs a calculation that can produce an integer overflow or wraparound when the logic assumes that the resulting value will always be larger than the original value. This occurs when an integer value is incremented to a value that is too large to store in the associated representation. When this occurs, the value may become a very small or negative number.