wait(NULL) seems to cause child to stop executing - c++

Question

I'm writing a simple pipe program that will ask for two commands then run the programs as if they were entered into bash as cmd1 | cmd2. Then it should loop and ask again until one of the commands is quit.
I've already written this much:
#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;
}
When I run it and enter ls and wc
The output I get is:
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
The main problem I have is that I should have Done executing come after ls|wc not the other way around.
I'm thinking the wait(NULL) isn't working but I'm not sure.
Please advise.
And Thank you.

Answer 1

You have:
parent
1. child1
2. child2
2. exec()
1. exec()
When you exec in child1, child2 gets reparented to init, and you can no longer wait for child2 to terminate.
So we need to do a little bit of refactoring to get something looking like:
parent
1. child1
1. exec()
2. child2
2. exec()
wait() x 2
Basing this on your own code, with the refactoring to accomplish something like the look-alike code (some comments in-line):
#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;
}