FIFO problem[solved] Post: 302587376

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Top Forums Programming FIFO problem[solved] Post 302587376 by Ultrix on Wednesday 4th of January 2012 06:11:53 PM

01-04-2012

Registered User

FIFO problem[solved]

I wrote a simple program to have cat pipe its output into less through a FIFO. The write works fine, but the child won't exit after writing to the FIFO. Another program needs to read from the FIFO for it to exit, but the parent (less) must wait for the data to become available to read the FIFO. So essentially, I have a deadlock.

Here's the code:

Code:

#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/wait.h>

int main( int argc, char **argv ){
        //char path[1024];
        //sprintf( path, "~/Desktop/doc/%s.nr", argv[1] );
        pid_t pid;
        char *file = argv[1];
        mkfifo( "named_pipe", 0777 );
        if( (pid = fork()) < 0 ){
                printf( "Fork error.\n" );
        }   
        else if( pid == 0 ){
                int fifo = open( "named_pipe", O_WRONLY );
                dup2( fifo, 1 ); // Attach stdout to the FIFO
                execlp( "cat", "cat", file );
                // Run cat and have it write to the FIFO
        }   
        else{
                wait( NULL ); // Wait for child to write to the FIFO
                int fifo = open( "named_pipe", O_RDONLY );
                dup2( fifo, 0 ); // Attach stdin to the FIFO
                execlp( "less", "less" );
                // Run less and have it read from the FIFO
        }   
        return 0;
}

I just want to know how I can have one program write to a FIFO through its standard output and the other program read from the FIFO through its standard input.

Also, I can't use anonymous pipes, due to losing control over I/O at the exec.

---------- Post updated at 06:11 PM ---------- Previous update was at 06:06 PM ----------

Problem solved. I changed the parent to call wait() after opening the FIFO on standard input.

Code:

int fifo = open( "named_pipe", O_RDONLY );
dup2( fifo, 0 ); // Attach stdin to the FIFO
wait( NULL ); // Wait for child to write to the FIFO
execlp( "less", "less" );
 // Run less and have it read from the FIFO

Ultrix

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mkfifo

mkfifo(3C)																mkfifo(3C)

NAME

       mkfifo() - make a FIFO file

SYNOPSIS

DESCRIPTION

       creates a new FIFO (first-in-first-out) file, at the path name to which path points.  The file permission bits of the new file are initial-
       ized from the mode argument, as modified by the process's file creation mask: for each bit set in the process's file  mode  creation  mask,
       the corresponding bit in the new file's mode is cleared (see umask(2)).	Bits in mode other than the file permission bits are ignored.

       The  FIFO owner ID is set to the process's effective-user-ID.  The FIFO group ID is set to the group ID of the parent directory if the set-
       group-ID bit is set on that directory.  Otherwise the FIFO group ID is set to the process's effective group ID.

       For details of the I/O behavior of pipes see read(2) and write(2).

       The following symbolic constants are defined in the header, and should be used to construct the value of  the  mode  argument.	The  value
       passed should be the bitwise inclusive OR of the desired permissions:

	      Read by owner.
	      Write by owner.
	      Read by group.
	      Write by group.
	      Read by other users.
	      Write by other users.

RETURN VALUE

       returns 0 upon successful completion.  Otherwise, it returns -1, no FIFO is created, and is set to indicate the error.

ERRORS

       fails and the new file is not created if any of the following conditions are encountered:

	      [EACCES]	     A component of the path prefix denies search permission.

	      [EEXIST]	     The named file already exists.

	      [EFAULT]	     The  path	argument  points  outside  the process's allocated address space.  The reliable detection of this error is
			     implementation dependent.

	      [ELOOP]	     Too many symbolic links encountered in translating the path name.

	      [ENAMETOOLONG] The length of the specified path name exceeds bytes, or the length of a component of  the	path  name  exceeds  bytes
			     while is in effect.

	      [ENOENT]	     A component of the path prefix does not exist.

	      [ENOENT]	     The path argument is null.

	      [ENOSPC]	     Not enough space on the file system.

	      [ENOTDIR]      A component of the path prefix is not a directory.

	      [EROFS]	     The directory in which the file is being created is located in a read-only file system.

AUTHOR

       was developed by HP.

SEE ALSO

       chmod(2), mknod(2), pipe(2), stat(2), umask(2), mknod(5), stat(5), thread_safety(5), types(5).

STANDARDS CONFORMANCE

																	mkfifo(3C)

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