循环 fork()

Question

这是我想做的一些管道的代码示例。问题是 pid2[0] 没有提供 child 进程。我如何解决它？ pid2[1] 和 pid2[2] 等提供 parent 和 children。

int numCommands = numPipes + 1;      /// not worrying about '>' and '<' right now
int *pipes = newint[numPipes*2];     /// two ends for each pipe
for(int i = 0; i < numPipes*2; i+=2) /// Offset by two since each pipe has two ends
   pipe(pipes + i);
int *pid2 = new int [numCommands];

for(int i = 0; i < numCommands; i++)
{
   pid2[i] = fork();
   if(pid2[i] < 0)   
   {
      std::cerr << "Failure to fork..." << std:endl;
      return EXIT_FAILURE
   }

   if(pid2[i] == 0)    /// Child process
   {
      if(i == 0)       /// First Command
      {
         dup2(pipes[1], 1);
      }

      else if(i == numCommands -1)  /// Last Command
      {
         dup2(pipes[2*(numCommands-1)-1], 0);
      }

      else             /// Middle commands
      {
         dup2(pipes[2*(i-1)], 0);    
         dup2(pipes[(2*i)+1],1);
      }
      for(int j = 0; j < numPipes*2;j++)
         close(pipes[j]);
      execvp(pipeCommands[i][0], pipeCommands[i].data());  ///pipeCommands is a vector<vector<char*>>
      perror("exec failed");
      return EXIT_SUCCESS;
   }
   else        /// The parent
   {
      for(int j = 0; j <numPipes*2;j++)
          close(pipes[j]);
      for(int k = 0; k < numCommands; k++)
         waitpid(pid2[k],nullptr,0);
   }
}

Answer 1

您对 pipes 数组的索引有点问题。对于 sort file.txt | head | wc，我假设 numPipes 为 2。让我们针对 i.

的每个值遍历您的 for 循环

我==0

dup2(pipes[1], 1); // Send stdout to pipes[1]

我==1

dup2(pipes[2*(i-1)], 0); // dup2(pipes[0], 0), stdin from pipes[0]
dup2(pipes[(2*i)+1],1);  // dup2(pipes[3], 1), stdout to pipes[3]

我 == 2

dup2(pipes[2*(numCommands-1)-1], 0); //dup2(pipes[3], stdin from pipes[3]

换句话说，来自进程 0 的任何标准输出都将进入死胡同。第二个进程永远不会从它的标准输出中读取。所以你放在那里的调试语句（cout == stdout，记住）也会丢失。

循环 fork()

Looping a fork()

linux

shell

ubuntu

fork

pid