🚀 go-pugleaf

+From: LANGER STEVEN C <sglanger@vela.acs.oakland.edu>
+Newsgroups: comp.sys.sgi.graphics
+Subject: unix pipes and X
+Date: 12 Jan 93 02:54:40 GMT
+To:       info-iris-graphics@BRL.MIL

+Despite the help of several netters, I cannot seem to master the 
+mechanics of inter-process communication via pipes. Ultimately,
+I want 2-way communication between a parent and child process.
+This is what I've tried so far;

+/*
+* parent.c	launches child and should get child's output
+* standard include files
+*/

+int pipes[2];

    ...

+	pipe (pipes);

+	if (fork() != 0)

    ...

+Everything runs, the child prints to the screen via the stderr and I get
+no error codes, but the parent callback func is never entered. Could
+anyone please tell me what the error is?

+--thanks in advance, steve
+-- 
+Steve Langer  			       sglanger@argo.acs.oakland.edu (VMS)
+Oakland University		       sglanger@vela.acs.oakland.edu (Ultrix)
+---------------------------------------------------------------------------
+Standard disclaimers apply. In addition, the author makes no guarantees,
+concerning the grammatical accuracy of his writing. Therefore, any ^B's, ^C's, 
+midword character additions/deletions and other non-sense which occurs (after 
+the work leaves the author's decade old text editor via his decade old Amiga, 
+struggles through a local 1200 baud Merit server to be further mauled via the 
+remote VAX mail servers) is someone elses problem - namely yours.
+---------------------------------------------------------------------------

Remember that pipes are:
	1. UNIdirectional
	2. have two ends each; a pipe() system call returns both ends
	3. fork() creates copies of both ends of pipes.
	4. parents and children should close immediately after the fork()
	   any unnecessary ends

	int to_child[2], from_child[2], pid;

	if(pipe(to_child)) {
		fprintf(stderr,"pipe(to_child) failed.\n");
		exit(1);
	}

	if(pipe(from_child)) {
		fprintf(stderr,"pipe(from_child) failed.\n");
		exit(1);
	}

	switch(pid = fork()) {
	case -1:
		fprintf(stderr,"fork() failed.\n");
		exit(1);
	case 0:
		close(0);	make to_child pipe stdin
		if(dup(to_child[0]) != 0) {
			fprint(stderr,"unable to make child's stdin\n");
			exit(1);
		}
		close(to_child[1]);	/* unnecessary end */
		close(1);	make to_child pipe stdout
		if(dup(from_child[1]) != 1) {
			fprint(stderr,"unable to make child's stdout\n");
			exit(1);
		}
		close(from_child[0]);	/* unnecessary end */
		exec...(child program);
		fprintf(stderr,"child program failed to run.\n");
		exit(1);
	default:
		close(to_child[0]);	/* unnecessary end */
		close(from_child[1]);	/* unnecessary end */

		...

		/* if necessary, wait on pid here */
	}

	close(to_child[0]); close(to_child[1]);		/* assure all ends */
	close(from_child[0]); close(from_child[1]);	/* are closed */

Randolph J. Herber, herber@fnalf.fnal.gov, +1 708 840 2966
(Speaking for myself and not for US, US DOE, FNAL nor URA.)
(Product, trade, or service marks herein belong to their respective owners.)