Date: Tue, 22 Dec 1998 01:21:36 PST From: Michael Ryan <mike@NetworX.ie> To: FreeBSD Support <questions@FreeBSD.ORG> Subject: Mystery with pipe(), fork() and dup() Message-ID: <ECS9812220136A@NetworX.ie>
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Hi all,
This one has me completely stumped. I wanted to write a program
which has the same effect as
grep pattern | more
so I wrote the program listed below. As it stands, it works
perfectly every time.
However, if I change the line which reads
if (pid == 0)
to read
if (pid)
it never works! The stdio for 'more' seems to get screwed up.
Can anybody explain why it doesn't work?
Also, to my amazement, if I leave the line which reads
if (pid == 0)
alone, but delete the line which reads
close(fd[1]);
that is the 3rd last line of main(), it also fails to work!
Deleting the other "close(fd[n])" calls don't seem to make
any difference.
I'd be extremely grateful to anybody who can shed light on this.
The program:
----------------------------------------------------------------
#include <stdio.h>
#include <unistd.h>
#define GREP "grep"
#define MORE "more"
void main(int, char **);
static void error(char *);
static void error (char *msg) {
fprintf(stderr, "Error: %s\n", msg);
exit(1);
}
void main (int argc, char **argv) {
int fd[2];
pid_t pid;
if (pipe(fd)) error("pipe() failed");
if ((pid=fork()) < 0) error("fork() failed");
if (pid == 0) {
close(1);
if (dup(fd[1]) != 1) error("dup(1) failed");
close(fd[0]);
close(fd[1]);
argv[0] = GREP;
execvp(GREP, argv);
error("execvp(" GREP ") failed");
}
close(0);
if (dup(fd[0]) != 0) error("dup(0) failed");
close(fd[0]);
close(fd[1]);
execlp(MORE, MORE, 0);
error("execlp(" MORE ") failed");
}
----------------------------------------------------------------
Thanks for any help.
Bye,
Mike
mike@NetworX.ie
www.NetworX.ie
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