什么是相关和不相关的过程？

Question

有人可以澄清相关和不相关流程的定义吗？

我知道 fork 创建了两个相关进程

fork()

但是我不确定我们调用exec*家族函数替换程序镜像时是否仍然认为进程是相关的：

if (child) {
   exec("path to binary", ...) <- Is it still related process
}

我问的原因是想弄清楚在什么场景下可以使用哪种IPC方式。例如，pipes 只允许在相关进程之间使用。所以我在上面要求澄清我编写的新程序（可能使用不同的语言）是否可以访问管道文件描述符。

我们是否可以说任何使用 fork() 创建的进程，无论使用的是 exec 还是原始程序映像，总是相关的，而其他所有进程都不相关？

谢谢！

ref: mark mitchell: 高级 linux 编程

A call to pipe creates file descriptors, which are valid only within that process and its children. A process’s file descriptors cannot be passed to unrelated processes; however, when the process calls fork, file descriptors are copied to the new child process.Thus, pipes can connect only related processes.

Answer 1

Can we say that any process created with fork() regardless of whether exec or original program image is used is always related?

是的，我们可以。

无论您是否在子进程中调用 exec，父进程和子进程仍然可以使用管道相互通信。请参阅下面创建子进程的示例，它将自身更改为 echo 然后父进程仍然可以读取其 echo 参数。

#include <iostream>
#include <unistd.h>
#include <sys/wait.h>
#include <errno.h>
#include <assert.h>

int main() {

  int filedes[2];
  int const pipeRes = ::pipe(filedes);
  assert(pipeRes != -1) ;

  pid_t pid = ::fork();
  assert(pid != -1);

  if (pid == 0) // child code                                                                                                                                                                                                               
    {
      // connect the entrance of the pipe to STDOUT_FILENO within the child process                                                                                                                                                         
      while ((::dup2(filedes[1], STDOUT_FILENO) == -1) && (errno == EINTR)) {}
      // close both input and output of new pipe in child process                                                                                                                                                                           
      close(filedes[1]);
      close(filedes[0]);
      execl("/bin/echo", "echo", "child-says-hello", (char*)0);
      assert(false); // execl failed                                                                                                                                                                                                        
    }

  // parent code                                                                                                                                                                                                                            
  close(filedes[1]); // close input of new pipe in parent, it will only read from it                                                                                                                                                        

  char buffer[1024];
  while (1)
    {
      ssize_t count = read(filedes[0], buffer, sizeof(buffer));
      if (count == -1)
        {
          if (errno == EINTR) continue;
          else assert(false);
        }
      else if (count == 0)
        {
      // read everything                                                                                                                                                                                                                
          break;
        }
      else
        {
          std::cout << "received count:" << count << " bytes with: " << buffer << "\n";
        }
    }

  close(filedes[0]);
  wait(0);
  return 0;
}

输出为

eceived count:17 bytes with: child-says-hello

上面的代码示例基于 this tutorial。

什么是相关和不相关的过程？

What is related and unrelated processes?

c++

linux

ipc

pipe

systems-programming