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Full Discussion: Problems understanding pipes
Top Forums Programming Problems understanding pipes Post 302562623 by Corona688 on Friday 7th of October 2011 12:52:35 PM
Old 10-07-2011
Some of us might be in a different timezone than you, and this forum is populated by volunteers, we are not "on call".

You also agreed to not bump posts when you registered.

I can't tell if your pseudocode's wrong or not since you didn't mention fork() at all in there.

Writing up some pseudocode for you.

---------- Post updated at 10:52 AM ---------- Previous update was at 10:41 AM ----------

The read-end won't EOF until all of the write-ends are closed, and the write-end won't die with SIGPIPE until all of the read-ends are closed, which is usually why this hangs: Forgetting to close all ends of the pipe you weren't using.

Each process you fork() gets independent copies of any pipe FD's that were open when you fork()ed, any unused ones need to be closed separately.

Also, you need to close the write-end too, once you're done with it, for the reader to hit EOF. Or the writer quitting works, too.
Code:
# parent gets writing-end of pipe, child gets reading-end of pipe.
int pipefd[2];

pid_t pid;
pipe(pipefd);

pid=fork();

if(pid < 0)
{
        perror("couldn't fork");
        exit(1);
}
else if(pid == 0) // in child
{
        dup2(pipefd[0], 0); // Overwrite STDIN with read end of pipe
        // Close writing end of pipe!  ESSENTIAL!
        close(pipefd[1]);
        execlp("/bin/cat", NULL); // exec REPLACES the current process
        perror("Couldn't exec"); exit(1);
}

// If we reach here, we must be the parent
const char *str="the owls are not what they seem\n";
int status;
dup2(pipefd[1], 1); // overwrite STDOUT with write end of pipe
close(pipefd[0]); // close the read-end!  ESSENTIAL!
write(1, str, strlen(str));  // send data to cat
close(1); // close FD so the child will get EOF.  Also essential!  wait() would wait forever otherwise.
wait(&status);
fprintf(stderr, "Returned status %d\n", WEXITSTATUS(status));

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LEARN ABOUT HPUX

pipe

pipe(2) 							System Calls Manual							   pipe(2)

NAME

       pipe() - create an interprocess channel

SYNOPSIS

DESCRIPTION

       creates	an  I/O  mechanism  called  a pipe and returns two file descriptors, fildes[0] and fildes[1].  fildes[0] is opened for reading and
       fildes[1] is opened for writing.

       A read-only file descriptor fildes[0] accesses the data written to fildes[1] on a first-in-first-out (FIFO) basis.  For details of the  I/O
       behavior of pipes see read(2) and write(2).

       By  default,  HP-UX  pipes are not STREAMS-based.  It is possible to generate the kernel so that all pipes created on a system are STREAMS-
       based.  This can only be done for HP-UX releases 10.0 and later.  STREAMS-based FIFOs (created by or are not supported on HP-UX.

       To generate a kernel that supports STREAMS-based pipes:

	 o  STREAMS/UX must be installed.

	 o  The module and the driver must be included in the file.  (When STREAMS/UX is installed, and are  automatically  added  to  the  system
	    file.)

	 o  The  tunable  parameter  (see  streampipes(5))  must  be  set  to  1  in the file.	(This is not automatically done when STREAMS/UX is
	    installed.)

	 o  The kernel must be generated and the system rebooted.  Once this is done, all pipes created by will be STREAMS-based.

       For more information, see

EXAMPLES

       The following example uses to implement the command string

RETURN VALUE

       returns one of the following values:

	      Successful completion.

	      Failure.
		     is set to indicate the error.

ERRORS

       sets to one of the following error values if the corresponding condition is true.

       or more file descriptors are currently open.

       The system file table is full.

       The file system lacks sufficient space to create the pipe.

       Could not allocate resources for both Stream heads
		      (STREAMS-based pipes only).

SEE ALSO

       sh(1), read(2), write(2), popen(3S), privileges(5), streampipes(5), streamio(7).

STANDARDS CONFORMANCE

																	   pipe(2)
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