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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);

 

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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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