如何在后台运行的 c++ 中编写 cp 函数（在 linux shell 中）？

Question

我正在尝试写一个自己的小linux shell，我想写函数cp，函数格式如下：

cp <old-file-path> <new-file-path>

它将第一个文件复制到第二个文件（覆盖它），如果第二个文件不存在，它将创建一个新文件。如果文件没有打开或任何系统调用没有成功，它将打印一条错误消息。但是，有时我想复制大文件，所以我想在后台运行这个 cp 命令（使用 fork 而不等待它完成）。

我的问题是：如何使用 fork 而不等待进程完成？目前，子进程变成了僵尸进程。

这是我的代码：

// num_args contains the number of arguments sent to cp
class CopyCommand : public BuiltInCommand {
 public:
  CopyCommand(const char* cmd_line) : BuiltInCommand(cmd_line){}
  virtual ~CopyCommand() {}
  void execute() override{
      if(this->num_args < 1){  // if no arguments were send to cp
          perror("invalid arguments");
          return;
      }
      char* buff;
      int fd1 = open(args[1], O_RDONLY);   
      if(fd1 == -1){
            perror("open failed");
            return;
      }
      if(this->num_args==2){          // copy file1 into file2 (overrite file 1)
          int fd2 = open(args[2], O_TRUNC);
            if (fd2 == -1) {                // if we couldn't open the file then create a new one (not sure if we supposed to this ?)
                fd2 = open(args[2], O_CREAT, 0666);
                if (fd2 == -1) {
                    perror("open failed");
                    return;
                }
            }
          pid_t PID = fork();
          if(PID == -1){
              perror("fork failed");
              return;
          }
          else if(PID == 0){

             // i need to use fork here :( before i start to write
            int read_res = read(fd1, &buff, 1);  /// read from the file fd1 into fd2
            while (read_res != -1) {
                if (!read_res) {
                    break;
                }
                if (write(fd2, buff, 1) == -1) {
                    perror("write failed");
                }
                read_res = read(fd1, buff, 1);
            }
            if (read_res == -1) {
                perror("read failed");
            }     
}   
      }
      else if(this->num_args==1){               // create file2 and copy file1 into file2

          // don't know how to do this yet 
          // i need to use fork here :(

      }
  }
};

Answer 1

首先，我稍微重写了您的代码。特别要注意，子分支 (PID == 0) 在完成后退出。父级在分叉后关闭传递的文件描述符，以防出现错误。

if (this->num_args == 2) {
    int fd1 = open(args[1], O_RDONLY);
    if (fd1 == -1) {
        perror("open failed");
        return;
    }

  int fd2 = open(args[2], O_TRUNC);
  if (fd2 == -1) {
    fd2 = open(args[2], O_CREAT, 0666);
    if (fd2 == -1) {
      perror("open failed");
      close(fd1);
      return;
    }
  }

  pid_t PID = fork();
  if (PID == -1) {
    perror("fork failed");
  } else if (PID == 0) {
    char buff[1024];
    int read_res = read(fd1, &buff, 1024); /// read from the file fd1 into fd2
    while (read_res != -1) {
      if (!read_res) {
        break;
      }
      if (write(fd2, buff, read_res) == -1) {
        perror("write failed");
      }
      read_res = read(fd1, buff, 1024);
    }
    if (read_res == -1) {
      perror("read failed");
    }
    exit(0);
  } else {
    printf("Copy running in background (pid: %d)\n", PID);
  }

  close(fd1);
  close(fd2);
  return
}

当子进程调用exit时，进程会停留在"Zombie"状态。此状态允许父进程（您）调用 wait 或 waitpid 来检索退出代码。

作为进程结束的次要影响，内核将向您的进程发送一个 SIGCHLD，让您知道您实际上可以调用 wait 而不会阻塞。在你的情况下，你不关心退出代码，所以你可以在程序开始时设置一个 "don't care" 信号处理程序，让内核清理进程：

signal(SIGCHLD, SIG_IGN);

这在 signal(2) 中有记录：

If a process explicitly specifies SIG_IGN as the action for the signal SIGCHLD, the system will not create zombie processes when children of the calling process exit. As a consequence, the system will discard the exit status from the child processes.

如何在后台运行的 c++ 中编写 cp 函数（在 linux shell 中）？

how to write a cp function (in linux shell ) in c++ that runs in background?

c++

fork