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 errorn");             }         }     } 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);         }     }