Problems understanding pipes Post: 302562623

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Top Forums Programming Problems understanding pipes Post 302562623 by Corona688 on Friday 7th of October 2011 12:52:35 PM

10-07-2011

Registered User

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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pclose

POPEN(3)						     Library Functions Manual							  POPEN(3)

NAME

       popen, pclose - initiate I/O to/from a process

SYNOPSIS

       #include <stdio.h>

       FILE *popen(command, type)
       char *command, *type;

       pclose(stream)
       FILE *stream;

DESCRIPTION

       The arguments to popen are pointers to null-terminated strings containing respectively a shell command line and an I/O mode, either "r" for
       reading or "w" for writing.  It creates a pipe between the calling process and the command to be executed.  The value returned is a  stream
       pointer that can be used (as appropriate) to write to the standard input of the command or read from its standard output.

       A stream opened by popen should be closed by pclose, which waits for the associated process to terminate and returns the exit status of the
       command.

       Because open files are shared, a type "r" command may be used as an input filter, and a type "w" as an output filter.

SEE ALSO

       pipe(2), fopen(3S), fclose(3S), system(3), wait(2), sh(1)

DIAGNOSTICS

       Popen returns a null pointer if files or processes cannot be created, or the shell cannot be accessed.

       Pclose returns -1 if stream is not associated with a `popened' command.

BUGS

       Buffered reading before opening an input filter may leave the standard input of that filter mispositioned.  Similar problems with an output
       filter may be forestalled by careful buffer flushing, for instance, with fflush, see fclose(3S).

       Popen always calls sh, never calls csh.

7th Edition							   May 15, 1985 							  POPEN(3)

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