等待（NULL）似乎會導致孩子停止執行

Question

我正在寫一個簡單的管道程序會要求兩個命令，然後運行程序，就好像他們進入bash作爲cmd1 | cmd2。然後它應該循環並再次詢問，直到其中一個命令是quit。

我已經寫了這麼多：

#include<iostream> 
#include<string> 
#include<sys/types.h> 
#include<sys/wait.h> 
#include<string.h> 
#include<unistd.h> 
#include<stdlib.h> 
#include<stdio.h> 
#include<signal.h> 

using namespace std; 

int main(int argc, char *argv[]) 
{ 
    int pid1, pid2, errchk; 
    int pip[2]; 
    char cmd1[128]; 
    char cmd2[128]; 
    int i = 0; 
    int status; 

    errchk = pipe(pip);//make pipe 
    if(errchk == -1)//check for error in pipe 
    { 
     perror("pipe"); 
     exit(1); 
    } 

    while(i<3) 
    { 
     i++; 
     //Enter commands here 
     cout<<"Enter cmd1: "; 
     cin>>cmd1; 
     cout<<"Enter cmd2: "; 
     cin>>cmd2; 
     //if a command is quit... quit 
     if(strcmp(cmd1,"quit")==0 || strcmp(cmd2,"quit") == 0) 
     { 
      cout<<"Quitting...\n"; 
      exit(1); 
     } 

     pid1 = fork(); 
     cout<<"first fork makes pids: "<<pid1<<endl; 
     if(pid1 < 0) 
     { 
      perror("fork"); 
      exit(1); 
     } 

     else if(pid1 == 0) //MAIN CHILD 
     { 
      cout<<"In main child with pid: "<<pid1<<endl; 
      pid2 = fork(); 
      cout<<"second fork makes pids: "<<pid2<<endl; 
      if(pid2 == 0)//SUB CHILD 2 to run cmd2 
      { 
       cout<<"In child of second fork"<<endl; 
       close(0); 
       close(pip[1]); 
       dup(pip[0]); 
       execvp(cmd2,argv);//execute command2 and die 
      } 
      else if(pid2 > 0)//SUB CHILD 1 to run cmd1 
      { 
       cout<<"In parent of second fork"<<endl; 
       close(1); 
       close(pip[0]); 
       dup(pip[1]); 
       execvp(cmd1,argv);//execute command1 and die 
      } 
     } 
     else if(pid1 > 0) //MAIN PARENT 
     { 
      wait(NULL);//wait for cmd1 
      wait(NULL);//wait for cmd2 
      cout<<"DONE executing"<<endl; //keep going 
     } 
    } 
    return 0; 
} 

當我運行它，並進入ls和wc 我得到的輸出是：

Enter cmd1: ls 
Enter cmd2: wc 
first fork makes pids: 5785 
first fork makes pids: 0 
In main child with pid: 0 
second fork makes pids: 5786 
In parent of second fork 
second fork makes pids: 0 
In child of second fork 
DONE executing 
     5  5  54 

我的主要問題是，我應後ls|wc而不是其他的方式都Done executing來。 我想了wait(NULL)不工作，但我不知道。

請指教。 並謝謝。

Answer 1

您有：

parent 
    1. child1 
    2. child2 
    2. exec() 
    1. exec() 

當您在child1 Exec時，得到的child2到重設父初始化，您可以不再等待的child2終止。

所以我們需要做重構的一點點得到的東西看起來像：

parent 
    1. child1 
    1. exec() 
    2. child2 
    2. exec() 
wait() x 2 

在你自己的代碼立足這一點，與重構來完成類似的外觀類似的代碼（一些評論在直插式）：

#include<iostream> 
#include<string> 
#include<sys/types.h> 
#include<sys/wait.h> 
#include<string.h> 
#include<unistd.h> 
#include<stdlib.h> 
#include<stdio.h> 
#include<signal.h> 

using namespace std; 

int main(int argc, char *argv[]) 
{ 
    int pid1, pid2, errchk; 
    int pip[2]; 
    char cmd1[128]; 
    char cmd2[128]; 
    int status; 

    while(true) 
    { 
     errchk = pipe(pip);//make pipe 
     if(errchk == -1)//check for error in pipe 
     { 
      perror("pipe"); 
      exit(1); 
     } 

     //Enter commands here 
     cout<<"Enter cmd1: "; 
     cin>>cmd1; 
     cout<<"Enter cmd2: "; 
     cin>>cmd2; 
     //if a command is quit... quit 
     if(strcmp(cmd1,"quit")==0 || strcmp(cmd2,"quit") == 0) 
     { 
      cout<<"Quitting...\n"; 
      exit(1); 
     } 

     pid1 = fork(); 
     cout<<"first fork makes pids: "<<pid1<<endl; 
     if(pid1 < 0) 
     { 
      perror("fork"); 
      exit(1); 
     } 
     if (pid1 == 0) // in child 
     { 
      cout<<"In child of first fork"<<endl; 
      close(pip[0]); // close read-end of pipe 
      close(0); // close stdin 
      close(1); // close stdout 
      dup2(pip[1], 1); // write-end of pipe is stdout of cmd1 
      argv[0] = cmd1; // make it look like the command in the ps output 
      execvp(cmd1,argv);//execute command1 and die 
      fprintf(stderr, "execvp(1): `%s': %s\n", cmd1, strerror(errno)); 
      return 0; 
     } 
     pid2 = fork(); 
     cout<<"second fork makes pids: "<<pid2<<endl; 
     if (pid2 < 0) 
     { 
      perror("fork2"); 
      exit(1); 
     } 
     if (pid2 == 0) 
     { 
      cout<<"In child of second fork"<<endl; 
      close(pip[1]); // close write-end of pipe 
      close(0); // close stdin 
      dup2(pip[0], 0); // read-end of pipe is stdin of cmd2 
      argv[0] = cmd2; // update ps output 
      execvp(cmd2,argv);//execute command2 and die 
      fprintf(stderr, "execvp(1): `%s': %s\n", cmd2, strerror(errno)); 
      return 0; 
     } 
     if(pid1 > 0) //MAIN PARENT 
     { 
      // close remaining pipe handles in parent 
      close(pip[0]); 
      close(pip[1]); 
      wait(0);//wait for one command to run 
      wait(0);//wait for a second command to run 
      cout<<"DONE executing"<<endl; //keep going 
     } 
    } 
    return 0; 
}