Login or Register to Ask a Question and Join Our Community

Sponsored Content
Full Discussion: C++ How to use pipe() & fork() with stdin and stdout to another program
Top Forums Programming C++ How to use pipe() & fork() with stdin and stdout to another program Post 302195143 by vvaidyan on Wednesday 14th of May 2008 11:34:19 AM
Old 05-14-2008
Found the solution.

Between two binaries, one pipe can serve only one way communication. YOU need two pipes for to and fro communication. You need one pipe to write data to STDIN of another program (B) and need another pipe to read the STDOUT from that another program (B).

Two best links on this:

Network Programming (CSE 533) | Unix Pipes
http://www.ecst.csuchico.edu/~hilzer...f/Chapter7.pdf


How to write to STDIN of PROGRAM B from PROGAM A:

Code:
In PROGRAM A:

int StartPipe(iosockinet &s, char PROGRAM_B_INPUT[])
{
    int fd1[2];
    int fd2[2];
    pid_t pid;
    char line[MAXLINE];

    if ( (pipe(fd1) < 0) || (pipe(fd2) < 0) )
    {
        cerr << "PIPE ERROR" << endl;
        return -2;
    }
    if ( (pid = fork()) < 0 )
    {
        cerr << "FORK ERROR" << endl;
        return -3;
    }
    else  if (pid == 0)     // CHILD PROCESS
    {
        close(fd1[1]);
        close(fd2[0]);

        if (fd1[0] != STDIN_FILENO)
        {
            if (dup2(fd1[0], STDIN_FILENO) != STDIN_FILENO)
            {
                cerr << "dup2 error to stdin" << endl;
            }
            close(fd1[0]);
        }

        if (fd2[1] != STDOUT_FILENO)
        {
            if (dup2(fd2[1], STDOUT_FILENO) != STDOUT_FILENO)
            {
                cerr << "dup2 error to stdout" << endl;
            }
            close(fd2[1]);
        }

        if ( execl("path/PROGRAM B", "PROGRAM B", (char *)0) < 0 )
        {
            cerr << "system error" << endl;
            return -4;
        }

        return 0;
    }
    else        // PARENT PROCESS
    {
        int rv;
        close(fd1[0]);
        close(fd2[1]);

        if ( write(fd1[1], PROGRAM_B_INPUT, strlen(PROGRAM_B_INPUT) ) != strlen(PROGRAM_B_INPUT) )
        {
            cerr << "READ ERROR FROM PIPE" << endl;
        }

        if ( (rv = read(fd2[0], line, MAXLINE)) < 0 )
        {
            cerr << "READ ERROR FROM PIPE" << endl;
        }
        else if (rv == 0)
        {
            cerr << "Child Closed Pipe" << endl;
            return 0;
        }

        cout << "OUTPUT of PROGRAM B is: " << line;

        return 0;
    }
    return 0;
}


* should I use a different pipe?
- Yes


* how to I read stdin in PROGRAM B? using cin?

In PROGRAM B:
Code:
char line[1000];
read(STDIN_FILENO, line, MAXLEN);

 

10 More Discussions You Might Find Interesting

1. Programming

How to write to stdin of another program (program A -> [stdin]program B)

Hi, Program A: uses pipe() I am able to read the stdout of PROGAM B (stdout got through system() command) into PROGRAM A using: * child -> dup2(fd, STDOUT_FILENO); -> execl("/path/PROGRAM B", "PROGRAM B", NULL); * parent -> char line; -> read(fd, line, 100); Question: ---------... (1 Reply)
Discussion started by: vvaidyan
1 Replies

2. Programming

How to clear the content of a pipe (STDIN) after it is written to another program?

PROGRAM A <-> PROGRAM B PROGRAM A sends data as STDIN ro PROGRAM B and when PROGRAM B is executed from PROGRAM A, it sends output back to PROGRAM A. This is implemented using 2 pipes (fd1 & fd2). The above process happens in a loop and during the second run, the previous data that had been... (10 Replies)
Discussion started by: vvaidyan
10 Replies

3. UNIX for Dummies Questions & Answers

How to write to stdin of another program (program A -> [stdin]program B)

Hi, Program A: uses pipe() I am able to read the stdout of PROGAM B (stdout got through system() command) into PROGRAM A using: * child -> dup2(fd, STDOUT_FILENO); -> execl("/path/PROGRAM B", "PROGRAM B", NULL); * parent -> char line; -> read(fd, line, 100); Question: ---------... (3 Replies)
Discussion started by: vvaidyan
3 Replies

4. Programming

stdout/stdin + flushing buffers

Hi all I've run into a snag in a program of mine where part of what I entered in at the start of run-time, instead of the current value within printf() is being printed out. After failing with fflush() and setbuf(), I tried the following approach void BufferFlusher() { int in=0;... (9 Replies)
Discussion started by: JamesGoh
9 Replies

5. UNIX for Dummies Questions & Answers

fork and stdin

When a process fork(), the child share the same file descriptors as his father. Thus, they share the same stdin. Quick and dirty exemple below (sorry for the ugly gets() call) : #include <stdio.h> #include <unistd.h> int main() { char buf; if (fork()) { /*parent */ ... (1 Reply)
Discussion started by: milouz
1 Replies

6. Programming

fork&pipe "interpretting" shell - problem

hello everybode.Got some sort of "problems" with this stuff; well this is a program int main() { int Pipe; int origStdin, origStdout; int childPID; origStdin = dup(0); origStdout = dup(1); pipe(Pipe); if( (childPID = fork()) < 0 ) { perror(... (2 Replies)
Discussion started by: IdleProc
2 Replies

7. Shell Programming and Scripting

can't close stdin/stdout in shell

#!/bin/sh exec 0</dev/null exec 1>/dev/null ls -l /proc/self/fd >&2 produces total 0 lr-x------ 1 tyler users 64 Feb 18 10:38 0 -> /proc/7886/fd lrwx------ 1 tyler users 64 Feb 18 10:38 1 -> /dev/pts/4 lrwx------ 1 tyler users 64 Feb 18 10:38 2 -> /dev/pts/4 I've verified the shell is... (10 Replies)
Discussion started by: Corona688
10 Replies

8. Programming

read and write stdin/stdout in unix

Hi, i am using the below program to read from the standard input or to write to standard out put. i know that using highlevel functions this can be done better than what i have done here. i just want to know is there any other method by which i find the exact number of characters ( this... (3 Replies)
Discussion started by: MrUser
3 Replies

9. UNIX for Dummies Questions & Answers

STDIN and STDOUT

Hallo, i have a script like: if ;then echo "OK" else echo "ERROR $2 is missing" fi; if ;then touch $2 fi; if ;then cat $1 | grep xy > $2 (1 Reply)
Discussion started by: eightball
1 Replies

10. Shell Programming and Scripting

[stdin / stdout] Strategies for redirecting outputs

Well.. let's say i need to write a pretty simple script. In my script i have 2 variables which can have value of 0 or 1. $VERBOSE $LOG I need to implement these cases: ($VERBOSE = 0 && $LOG = 0) => ONLY ERROR output (STDERR to console && STDOUT to /dev/null) ($VERBOSE = 1... (5 Replies)
Discussion started by: Marmz
5 Replies

LEARN ABOUT MINIX

pipe

PIPE(2) 							System Calls Manual							   PIPE(2)

NAME

       pipe - create an interprocess communication channel

SYNOPSIS

       #include <unistd.h>

       int pipe(int fildes[2])

DESCRIPTION

       The pipe system call creates an I/O mechanism called a pipe.  The file descriptors returned can be used in read and write operations.  When
       the pipe is written using the descriptor fildes[1] up to PIPE_MAX bytes of data are buffered before the writing process	is  suspended.	 A
       read using the descriptor fildes[0] will pick up the data.

       PIPE_MAX equals 7168 under Minix, but note that most systems use 4096.

       It  is  assumed	that after the pipe has been set up, two (or more) cooperating processes (created by subsequent fork calls) will pass data
       through the pipe with read and write calls.

       The shell has a syntax to set up a linear array of processes connected by pipes.

       Read calls on an empty pipe (no buffered data) with only one end (all write file descriptors closed) returns an end-of-file.

       The signal SIGPIPE is generated if a write on a pipe with only one end is attempted.

RETURN VALUE

       The function value zero is returned if the pipe was created; -1 if an error occurred.

ERRORS

       The pipe call will fail if:

       [EMFILE]       Too many descriptors are active.

       [ENFILE]       The system file table is full.

       [ENOSPC]       The pipe file system (usually the root file system) has no free inodes.

       [EFAULT]       The fildes buffer is in an invalid area of the process's address space.

SEE ALSO

       sh(1), read(2), write(2), fork(2).

NOTES

       Writes may return ENOSPC errors if no pipe data can be buffered, because the pipe file system is full.

BUGS

       Should more than PIPE_MAX bytes be necessary in any pipe among a loop of processes, deadlock will occur.

4th Berkeley Distribution					  August 26, 1985							   PIPE(2)
All times are GMT -4. The time now is 12:57 PM.
Unix & Linux Forums Content Copyright 1993-2022. All Rights Reserved.
Privacy Policy