Question

I am working on an assignment for my Operating System class (Posix & C), building a mini-shell, and I don’t know how to solve the following problem:

My mini-shell has to accept two commands, for example ls | grep a. For that I create a pipe of size two and a child. The child closes all that it has to close and opens all that it has to open (standard/pipe’s in & out). It then executes “ls,” using execvp. I am sure this works well. After that, the parent shuts and opens inputs and outputs (I am sure I do it well), and then executes grep a.

Now, the problem is that the process grep a never finishes. Same for tail -1, e.g.. Yet it does work for head -1. I think that happens because grep and tail, which are executed by the parent, wait for more input, even though the child has finished its operation. ls | grep a produces the right output, displayed on the console (The pipe’s output is set as default output), but, as I’ve said, grep a does not finish.

So, my question is: how can I inform the parent that the pipe has finished writing, so it can finish the execution of grep a for example?

Thank you.

Here’s the code:

[fd is the pipe, it is initialized previously in the code. If you can see any incongruous thing, please let me know; I’ve cleaned the code a bit, and this is only the problematic part, as you can see.]

   int fd[2];
   pipe(fd);

   if ((pid = fork()) != -1){ 
     if(pid == 0){         /*Child executing */
        close(fd[0]);
        close(1);
        dup(fd[1]);
        close(fd[1]);
        execvp(argvv[0][0], argvv[0]); /* Here's stored the first instruction */

      } else{             /* Parent executing */
        wait(&status);
        close(fd[1]);
        close(0);
        dup(fd[0]); 
        close(fd[0]);
        execvp(argvv[1][0], argvv[1]); /* Here's stored the second instruction */
       }
   }  

Answer 1

If the grep continues to run after the ls has exited, that indicates that you have not closed all the pipes that you need to close.

In particular, the write end of the pipe whose read end is attached to the grep process is still open in another process. You will need to show your code to know more.

---

The code you have pasted works correctly (when expanded to a full program, as per the below). Both subprocesses exit just fine.

This means that you’ve eliminated the code that has the problem when you created your cut-down version here – perhaps you have another fork() between the pipe() call and this fork()?

#include <unistd.h>
#include <sys/wait.h>

int main()
{
   pid_t pid;
   char *argvv[2][3] = { { "ls", 0, 0}, { "grep", "a", 0 } };
   int status;

   int fd[2];
   pipe(fd);

   if ((pid = fork()) != -1) {
     if(pid == 0){         /*Child executing */
        close(fd[0]);
        close(1);
        dup(fd[1]);
        close(fd[1]);
        execvp(argvv[0][0], argvv[0]); /* Here's stored the first instruction */

      } else{             /* Parent executing */
        wait(&status);
        close(fd[1]);
        close(0);
        dup(fd[0]);
        close(fd[0]);
        execvp(argvv[1][0], argvv[1]); /* Here's stored the second instruction */
       }
   }

   return 0;
}