PIPES

[Brian Bliss]

I've done this before, and ran into this problem.
well, you perfrom the fork, test whether you are child or
parent, and then get ready to dup the pipes:


	if(c) {
		/* parent */
		dup2(p2c[1], 1);
		dup2(c2p[0], 0);
	}
	else {
		/* child */
		dup2(c2p[1], 1);
		dup2(p2c[0], 0);
	}


consider the case of the parent:  what if c2p[0] is fd 1?
then the first dup2 call overwrites fd 1 before it can be
copied to 0.

it's really a pain if you need to dup fd 0 to 1, and fd 1 to 0,
but here's a fix I wrote that should do what you want:



/*****************************************************************************/
/* this file, "fork_cpp.c", is the source for a command which exec's a       */
/* child process, sets the child's stdin to be to_child and returns a FILE   */
/* pointer to its stdout (*from_child) and the child's process id (the       */
/* function's return val).                                                   */
/*****************************************************************************/



# include <stdio.h>
# include <signal.h>



#define	RDR	0
#define	WTR	1



int ae_fork_cpp (cmd, to_child, from_child)
/*****************************************************************************/
/* ae_popen2 exec's cmd, sets the child's stdin to *to_child, and creats a   */
/* a pipe from the child's stdout to *from_child.  ae_fork_cpp () returns    */
/* the process id of the child, or -1 for an error.   It is used by the      */
/* application executive to invoke the C preprocessor.                       */
/*****************************************************************************/
   char *cmd;
   FILE *to_child;
   FILE **from_child;
   {
      int c2p[2];
      register int child_stdin;
      register int child_stdout;
      register int parent_reader;
      register int pid;

      if (pipe (c2p) < 0) {
         return (-1);
      }
      child_stdout = c2p[WTR];
      parent_reader = c2p[RDR];
      child_stdin = fileno (to_child);

      /*************************************************/
      /* if child_stdout == 0, it will be overwritten  */
      /* when dup2'ing child_stdin to 0, so change it. */
      /* this should be done before the fork, in case  */
      /* of an error.                                  */
      /*************************************************/
      if (child_stdout == 0) {
         if ((child_stdout = dup (child_stdout)) == -1) {
            close (child_stdout);
            close (parent_reader);
            return (-1);
         }
         close (child_stdout);
      }

      /********************/
      /* perform the fork */
      /********************/
      if ((pid = fork ()) == -1) {
         close (child_stdout);
         close (parent_reader);
         return (-1);
      }
      if (pid == 0) {
         /**************************************************/
         /* we are the child process - close the parent's  */
         /* file descriptor, and copy our file descriptors */
         /* to 0 and 1, our stdin and stdout fd's.  then,  */
         /* exec the appropriate command.                  */
         /**************************************************/
            close (parent_reader);
            if (child_stdout != 0) {
               if (child_stdin != 0) {
                  dup2 (child_stdin, 0);
                  close (child_stdin);
               }
               if (child_stdout != 1) {
                  dup2 (child_stdout, 1);
                  close (child_stdout);
               }
            }
            else {
               int temp;
               temp = open ("/dev/null", "w");
               dup2 (child_stdout, temp);
               close (child_stdout);
               if (child_stdin != 0) {
                  dup2 (child_stdin, 0);
                  close (child_stdin);
               }
               dup2 (temp, 1);
               close (temp);
            }
            execl("/bin/sh", "sh", "-c", cmd, 0);
            /* it is an error to get past here */
            exit(1);
	}
      else {
         /**************************************************/
         /* we are the parent process - close the child's  */
         /* stdout file descriptor, set the from_child     */
         /* file pointer, and return the pid of the child. */
         /**************************************************/
         close (child_stdout);
         *from_child = fdopen (parent_reader, "r");
         return(pid);
      }
   }