pipe.c

/**
 * Author:          Samuel Marsh
 * Date:            30/10/2015
 */

#include "mysh.h"

#define FD_READ     0
#define FD_WRITE    1

/**
 * Executes a command tree node, passing one child's stdout to the other's stdin
 * (piping the execution).
 *
 * @param  t the command tree, with root node of type N_PIPE
 * @return   the exit status of the right hand side of the tree (the process
 *           which is receiving stdin from the other side's stdout)
 */
int execute_pipe(CMDTREE *t)
{
  int fd[2];

  //index 0 (FD_READ) is for reading, index 1 (FD_WRITE) is for writing
  if (pipe(fd) == -1)
  {
    MYSH_PERROR("execute_pipe");
    return EXIT_FAILURE;
  }

  switch (fork())
  {
    case FORK_FAILURE:
      {
        MYSH_PERROR("execute_pipe");
        return EXIT_FAILURE;
      }
    case FORK_CHILD:
      {
        //set stdout to write to the pipe
        if (dup2(fd[FD_WRITE], fileno(stdout)) == -1)
        {
          MYSH_PERROR("execute_pipe");
          exit(EXIT_FAILURE);
        }

        //close both piped files - we have stdout now which is all we need
        close(fd[FD_READ]);
        close(fd[FD_WRITE]);

        //not really important what status we exit with, this value is
        //ignored anyway
        exit(execute_cmdtree(t->left));
        break;
      }
    default:
      {
        //after much debugging, should fork again in parent... was forking again
        //in child, but led to weird race condition (?) where WEXITSTATUS in
        //execute_external_command would return exit status of wrong command
        switch (fork())
        {
          case FORK_FAILURE:
            {
              MYSH_PERROR("execute_pipe");
              return EXIT_FAILURE;
            }
          case FORK_CHILD:
            {
              //set stdin to read from the pipe
              if (dup2(fd[FD_READ], fileno(stdin)) == -1)
              {
                MYSH_PERROR("execute_pipe");
                exit(EXIT_FAILURE);
              }

              //close both piped files - we have stdin now which is all we need
              close(fd[FD_READ]);
              close(fd[FD_WRITE]);

              exit(execute_cmdtree(t->right));
              break;
            }
          default:
            {
              //close both in parent as well
              close(fd[FD_READ]);
              close(fd[FD_WRITE]);

              int exit_status;
              //we are in the original shell - just wait for the children to finish
              while (wait(&exit_status) > 0);
              exit_status = WEXITSTATUS(exit_status);
              return exit_status;
            }
        }
      }
  }

  //we are only here if one of the children somehow failed to exit, (probably) won't ever happen
  fprintf(stderr, "failed to exit process\n");
  return EXIT_FAILURE;
}