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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 FREEBSD

pipe

PIPE(2) 						      BSD System Calls Manual							   PIPE(2)

NAME

     pipe, pipe2 -- create descriptor pair for interprocess communication

LIBRARY

     Standard C Library (libc, -lc)

SYNOPSIS

     #include <unistd.h>

     int
     pipe(int fildes[2]);

     int
     pipe2(int fildes[2], int flags);

DESCRIPTION

     The pipe() system call creates a pipe, which is an object allowing bidirectional data flow, and allocates a pair of file descriptors.

     The pipe2() system call allows control over the attributes of the file descriptors via the flags argument.  Values for flags are constructed
     by a bitwise-inclusive OR of flags from the following list, defined in <fcntl.h>:

     O_CLOEXEC	 Set the close-on-exec flag for the new file descriptors.

     O_NONBLOCK  Set the non-blocking flag for the ends of the pipe.

     If the flags argument is 0, the behavior is identical to a call to pipe().

     By convention, the first descriptor is normally used as the read end of the pipe, and the second is normally the write end, so that data
     written to fildes[1] appears on (i.e., can be read from) fildes[0].  This allows the output of one program to be sent to another program: the
     source's standard output is set up to be the write end of the pipe, and the sink's standard input is set up to be the read end of the pipe.
     The pipe itself persists until all its associated descriptors are closed.

     A pipe that has had an end closed is considered widowed.  Writing on such a pipe causes the writing process to receive a SIGPIPE signal.
     Widowing a pipe is the only way to deliver end-of-file to a reader: after the reader consumes any buffered data, reading a widowed pipe
     returns a zero count.

     The bidirectional nature of this implementation of pipes is not portable to older systems, so it is recommended to use the convention for
     using the endpoints in the traditional manner when using a pipe in one direction.

RETURN VALUES

     The pipe() function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate
     the error.

ERRORS

     The pipe() and pipe2() system calls will fail if:

     [EMFILE]		Too many descriptors are active.

     [ENFILE]		The system file table is full.

     [ENOMEM]		Not enough kernel memory to establish a pipe.

     The pipe2() system call will also fail if:

     [EINVAL]		The flags argument is invalid.

SEE ALSO

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

HISTORY

     The pipe() function appeared in Version 3 AT&T UNIX.

     Bidirectional pipes were first used on AT&T System V Release 4 UNIX.

     The pipe2() function appeared in FreeBSD 10.0.

BSD
								    May 1, 2013 							       BSD
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