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From: emmonsl@athena.ecs.csus.edu (L. Scott Emmons)
Newsgroups: comp.unix.programmer
Subject: how to use pipes (was: Re: system() question)
Message-ID: <1991Mar3.214844.15444@csusac.csus.edu>
Date: 3 Mar 91 21:48:44 GMT
References: <9321@hub.ucsb.edu> <1991Mar2.181423.13981@csusac.csus.edu> <1991Mar3.175234.11716@csusac.csus.edu>
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I thought that perhaps some people would be interested in just how to use
fork(), exec(), and pipe() to do the same thing as is possible with a
system().  I cooked up a couple of quick programs.  The first implements
redirection using fork(), exec(), and pipe().  The second does the exact
same things, except uses system() instead.

NOTE: I implemented these on a BSD system...It can probably be ported over
      to sysV reasonably easily...change SIGCHLD to SIGCLD, and that should
      be about it...

Just for kicks, I compared the output from 'time' for both versions of the
program.  Here are some average results:

using fork/exec/pipe: 0.0u 0.1s 0:00 35% 5+6k 0+0io 0pf+0w
using system:         0.2u 0.4s 0:00 70% 32+18k 0+4io 0pf+0w

While for this trivial usage the times aren't that much difference, a larger
program surely would be...

Here are the two programs, enjoy and if you have any questions, comments, etc
please feel free to email me at emmons@csus.csus.edu:

---CUT HERE start pipe.c---
/*
	This program shows how to (or one way to) implement fork() and exec()
	on a pipeline, redirecting the stdout of the exec()d program into
	the pipeline.

	This code was written by L. Scott Emmons, emmons@csus.csus.edu.
*/

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

child_changed();
int	fd[2];

main()
{
	char	ch=0;

	pipe(fd);

	if (fork()) {
		signal(SIGCHLD,child_changed);
		while(read(fd[0],&ch,1))
			putchar(ch);
		close(fd[0]);
	} else {
		dup2(fd[1],1);
		execl("/usr/ucb/last","last","emmonsl",(char *)0);
	}
}

child_changed()
{
	close(fd[1]);
}
---CUT HERE end pipe.c---

---CUT HERE start system.c---
/*
	This program shows how to (or one way to) use system() to do what
	fork(), exec(), and pipe() can do...  It redirects the output of
	a program to a file, then reads the file back in and prints it.

	This code was written by L. Scott Emmons, emmons@csus.csus.edu.
*/

#include <stdio.h>

main()
{
	FILE *fp;
	char ch;

	system("/usr/ucb/last emmonsl >filename");
	fp=fopen("filename","r");
	while (fread(&ch,1,1,fp))
		putchar(ch);
	fclose(fp);
	unlink("filename");
}
---CUT HERE end system.c---

			L. Scott Emmons
			---------------
	emmons@csus.csus.edu  <or>  ...[ucbvax]!ucdavis!csus!emmons
		Packet: kc6nfp@kg6xx.#nocal.ca.usa.na