Cause of broken pipe in c

Question

I am building a simple shell in c using fork and execlp. I will be given a set of commands separated by pipes. eg: ls -l | wc -l . I am trying to implement a simple shell program in c using. For some strange reason, I am getting a broken pipe error.

void excueteCommand(commandNode* head, int input) {

    int pfds[2] = { -1, -1 };

    if (head->next != NULL) {

        pipe(pfds);
    }

    if (fork() == 0) { /* child */

        if (input != -1) {

            dup2(input, 0);
            close(input);
        }

        if (pfds[1] != -1) {

            dup2(pfds[1], 1);
            close(pfds[1]);
        }

        if (pfds[0] != -1) {

            close(pfds[0]);
        }

        execlp(head->command, head->args, NULL);
        exit(1);
    }

    else { /* parent */

        if (input != -1) {
            close(input);
        }

        if (pfds[1] != -1) {
            close(pfds[1]);
        }

        if (head->next != NULL) {

            thePipenizer(head->next, pfds[0]);
        }
    }
}

Answer 1

Hi i don't know what index is. I made a simple prog that work to show you some mistake that you make.

int main()
{
  int   pfds[2] = { -1, -1 };
  int   input;
  char  *argvv = "";
  char  *tab = malloc(500);
  int   i;
  int   nbRead;
  int   stat;

  input = dup(0);
  if (pipe(pfds) == -1)
     exit(-1);
  if (fork() == 0)
    { /* child */
      dup2(pfds[1], 1);
      close(pfds[0]);
      execlp("ls", argvv, NULL);
      exit(-1);
    }
  else
    { /* parent */
      close(pfds[1]);
      wait(&stat);
      do
        {
          nbRead = read(pfds[0], tab, 500);
          for (i = 0; i < nbRead; i++)
            printf("%c", tab[i]);
        } while (nbRead > 0);
      close(pfds[0]);
    }
  return (0);
}

First : Instead of regarding pfds[0] and pfds[1] value, look at the return value of the function pipe. Function pipe return -1 in case of error (see man pipe).

Second : Don't forget to close all open fd, it's important for the system.

If you can told us more about

input

maybe we can found your problem.

Answer 2

In the parent, try resetting pfds. That is:

if( pfds[1] != -1 ) {
    close( pfds[1] );
    pfds[1] = -1;   /* Add this line */
}

By leaving it set to its previous value, it is very likely that subsequent children are closing file descriptors that you do not expect them to be closing.