Fork和pipe文件描述符错误

我试图写一个基本的shell，为了方便管道，我写了下面的函数，其中tokens是要执行的行，index是tokens中管道字符的位置(目前，管道需要两边都有空白)。每次我在shell中运行管道命令时，两个命令都按预期运行，但无法写入/读取管道，错误为"错误的文件描述符"。这令人困惑的原因是我的文件重定向工作得很好，所以我不明白为什么它不能与管道一起工作。我读了文档，看了一些示例代码，我想不出我做错了什么。几个小时后，我不知所措，我哪里做错了?

int pipeit(vector<string> tokens,unsigned int index){
    //separate the entire line into two delimited by the pipe
    vector<string> firstline;
    vector<string> secondline;
    for(unsigned int i=0;i<tokens.size();i++){
        if(i==index){
            continue;
        }
        else if(i<index){
            firstline.push_back(tokens[i]);
        }
        else{
            secondline.push_back(tokens[i]);
        }
    }
    //make sure the lines aren't empty
    if(secondline.size()==0||firstline.size()==0){
      fprintf(stderr, "ERROR, expected at least one command at both ends of pipen");
    }
    //open a pipe, check that it was successfully created
    int pip[2];
    if(pipe(pip)<0){
      perror("pipe failure");
      return -1;
    }
    //attempt to fork
    int parentstatus=0;
    int childstatus=0;
    switch(fork()){
      //fork failed, close the pipe and return -1 (failure)
      case -1:
        {
          close(pip[0]);
          close(pip[1]);
          perror("fork failure");
          return -1;
        }
      //pid of 0 indicates this is the child process
      case 0:
        {
          //close the write end of the pipe and redirect stdin to the read end
          close(pip[0]);
          int stdIn=dup(0);
          dup2(pip[1],0);
          //once redirect is done, can close the other end of the pipe
          close(pip[1]);
          //parse the second line as a list of commands (not important for question)
          //then restore stdin, close the old file descriptor pointing to the pipe
          execute_line(secondline,builtins);
          dup2(stdIn,0);
          close(stdIn);
          break;
        }
      //pid other than 0 indicates this is the parent process
      default:
        {
          //close the read end of the pipe
          close(pip[1]);
          //redirect stdout to the write end of the pipe
          int stdOut=dup(1);
          dup2(pip[0],1);
          close(pip[0]);
          //execute this line (not important for question)
          parentstatus=execute_line(firstline,builtins);
          //restore stdout and close the temporary fd pointing to the pipe
          dup2(stdOut,1);
          close(stdOut);
          //wait for child process to exit and store it's return value in 'status' (then childstatus)
          int status;
          wait(&status);
          childstatus=status;
          break;
        }
    }
    //return a combination of the parent and child's return status
    //(not standard, I know, just easy and irrelevant for the question)
    return childstatus & parentstatus;
  }

tokens只是一个空格分隔的std::vector<string>，包含要执行的命令(是的，你需要用空格填充'|'才能工作，不要担心)，index是列表中发现'|'字符的地方，让事情变得更容易。对于可运行的源文件，只需将上述函数定义粘贴到:

#include <unistd.h> //pipe, dup2, fork
#include <iostream>
#include <string>
int builtins = 0xf00; //not what this actually is, but that doesn't matter atm
using namespace std;
int pipeit(vector<string> tokens, unsigned int index);
void execute_line(vector<string> cmds, int biltins){
    cout << "Executing: " << endl;
    for(unsigned int i = 0; i < cmds.size(); ++i){
        cout<<cmds[i]<< " ";
    }
    cout << endl;
}
int main(){
    vector<string> cmds;
    cmds.push_back(new string("cmd1"));
    cmds.push_back(new string("|"));
    cmds.push_back(new string("cmd2"));
    return pipeit(cmds, 1);
}