c - execve '/bin/sleep'命令卡住

Question

我有一个客户端服务器程序，其中服务器是一个简单的 shell

命令运行良好，参数传递正常，但我认为 execve 卡住

所以这是简单的 shell 代码 -- 前半部分用于验证该命令是否在此 shell 中被允许

  while((n = Rio_readlineb(&rio, buf, MAXLINE)) != 0){ //loop until connection has been terminated
      if (!strcmp(buf, "quit\n")){
        printf("User %s disconnected.\n", username);
        flag1 = 0; //the user will need to login again
      }
      if (flag1 == 1){ //the user is loged in
        bg = p3parseline(buf, argv_for_shell);
        strtok(buf, "\n");
        printf("User %s sent the command '%s' to be executed.\n", username, buf);
        // temp = argv_for_shell[0];
        // strcat(temp, "\n");
    //check if the command is allowed
    flag2 = 0; //set command allowed? flag back to false
    file = Fopen("rrshcommands.txt", "r");
    while (Fgets(command, MAXLINE, file) != NULL){
      strtok(command, "\n");
      if (!strcmp(argv_for_shell[0], command)){
        flag2 = 1;
      }
    }
    Fclose(file);

    if (flag2 == 0){ //case where the command is not allowed
      printf("The command '%s' is not allowed.\n", buf);
      strcpy(buf, "Command not allowed\n");
      Rio_writen(connfd, buf, strlen(buf));
    }
    else{
      if ((pid = fork()) == 0) {   /* Child runs user job */
        printf("Fork/Execing the command %s on behalf of the user.\n", argv_for_shell[0]);
        Dup2(connfd, 1);

        if (execve(argv_for_shell[0], argv_for_shell, environ) < 0) {
          printf("%s: Command not found.\n", argv_for_shell[0]);
          exit(0);
        }
        printf("Finished execing\n");
      }
      /* Parent waits for foreground job to terminate */
      if (!bg) {
        int status;
        if (waitpid(pid, &status, 0) < 0)
          unix_error("waitfg: waitpid error");
        memset(&buf[0], 0, sizeof(buf)); //flush the buffer
        Rio_writen(connfd, buf, strlen(buf));
      }
      else{
        memset(&buf[0], 0, sizeof(buf)); //flush the buffer
        Rio_writen(connfd, buf, strlen(buf));
      }
      signal(SIGCHLD, reap_background);
    }
  } 

我之前发布了一个与此相关的问题，但是由一个单独的问题引起的——也请随意阅读那个问题

我在 execve 下添加了一个 printf 语句来确认程序是否到达那个点，但它没有

shell 的输出看起来像

User k logging in from 127.0.0.1 at TCP port 1024.
User k successfully logged in.
User k sent the command '/bin/sleep 1&' to be executed.
Fork/Execing the command /bin/sleep on behalf of the user.

有什么想法吗？

Answer 1

据我所知，这是意料之中的事情。首先你 fork()，然后你打印“Fork/Execing the command/bin/sleep behalf of the user.”，然后你调用 execve()。对 execve() 的调用永远不会返回(假设它成功)，因为 execve() 替换了当前程序。

因此这一行:

printf("Finished execing\n");

永远无法到达。

这正是您(大概)首先执行 fork() 的原因。您应该做的是 wait() 等待 fork() 的进程完成。

还有，这条线是做什么用的？

signal(SIGCHLD, reap_background);