Using pipes to connect child processes in a shell

Question

I'm trying to child process communication in my shell. Like this:

ls | ls -l
ls -l | wc -l

The problem is, when I do "ls | ls -l" it works, and I get what was expected, only "ls -l". But if I do "ls -l | wc -l" it just stays like waiting like the program was in a cycle.

Here's my code:

void execute_commands(COMMAND *commlist) {

pid_t process;
COMMAND *first = commlist;
int fd_before[2];
int fd_actual[2];
int status;

while(commlist != NULL) {       

  if(commlist->next != NULL) pipe(fd_actual);

  if ((process = fork()) < 0){ }         // FAILED

  else if (process == 0) {               

    if(first != commlist) {
      dup2(fd_before[0], PIPE_READ);
    }

    if(commlist->next != NULL) {
      dup2(fd_actual[1], PIPE_WRITE);
    }

    /* You can ignore this, it's from input files /*
    if(inputfile != NULL) {  
      int in = open(inputfile, 0);      
      if(in < 0) exit(1);
      dup2(in, 0);
      close(in);
    }
    /* You can ignore this, it's from input files /*
    if(outputfile != NULL) {  
      int out = creat(outputfile, 0644);
      dup2(out, 1);
      close(out);
    }

    if(execvp(commlist->argv[0], commlist->argv)< 0) exit(1);
  }
  else {
    wait(&status);
    commlist = commlist->next;
    fd_before[0] = fd_actual[0];
    fd_before[1] = fd_actual[1];
  }                                    
  close(fd_before[PIPE_READ]);
  close(fd_before[PIPE_WRITE]);
  close(fd_actual[PIPE_READ]);
  close(fd_actual[PIPE_WRITE]);
}

/*wait(&status); Should wait be here, or where it currently is? */

}

--

I have another doubt, if I change my code and put this, within the child process:

close(fd_before[PIPE_READ]);
close(fd_before[PIPE_WRITE]);
close(fd_actual[PIPE_READ]);
close(fd_actual[PIPE_WRITE]);

It's going to close my current input and output, and not receive or output anything.

But when I do that inside the inputsfile "close(in);", it still works. What's the difference?

score 2 · Answer 1 · edited May 23 '17 at 10:28

This is working for ls -l | ls -l because ls in that form doesn't read standard input, so it will run even if you neglect to set up stdin properly.

For ls -l | wc -l, wc needs stdin to count lines, and it's not receiving any input.

Several other things are missing

Without the definition of PIPE_READ and PIPE_WRITE we can't see where you're expecting to the duplicate descriptors to appear
There is an unclosed comment in the code so it's clearly not complete
At some point you will need to exec() your commands (ls, wc, et cetera)

Since this looks like an assignment, I'll direct you to existing posts showing this in greater detail:

Understanding how to connect pipes and wait for them in a custom C shell

UNIX Pipes Between Child Processes

Using pipes to connect child processes in a shell

1 Answers1