关于 execlp()和 waitpid()的一个问题

我在读一个 OJ 判题核心的代码，里面关于特判的代码，fork 了一个子进程，调用 execlp()去执行 spj，没有报错，但是父进程里 waitpid()就报错了，返回值是-1，错误码 10，no child processes，这是怎么回事

    int status = 0;
    pid_t pid_spj = fork();
    if (pid_spj < 0) {
        LOG_WARNING("error for spj failed, %d:%s", errno, strerror(errno));
        output_result(judge_conf::OJ_SE, -errno, judge_conf::EXIT_COMPARE_SPJ_FORK);
        exit(judge_conf::EXIT_COMPARE_SPJ_FORK);
    } else if (pid_spj == 0) {
        freopen(file_spj.c_str(), "w", stdout);
        if (EXIT_SUCCESS == malarm(ITIMER_REAL, judge_conf::spj_time_limit)) {
            log_close();
            if (execlp((spj_path + "/" + problem::spj_exe_file).c_str(),
                       problem::spj_exe_file.c_str(), input_file.c_str(),
                       output_file.c_str(), output_file_std.c_str(), NULL) < 0) {
                printf("spj execlp error\n");
            }
        }
    } else {
        //no child process
        if (waitpid(pid_spj, &status, 0) < 0) {
            LOG_BUG("waitpid failed, %d:%s", errno, strerror(errno));
            output_result(judge_conf::OJ_SE, -errno, judge_conf::EXIT_COMPARE_SPJ_WAIT);
            exit(judge_conf::EXIT_COMPARE_SPJ_WAIT);
        }
    }