Implementing 2 pipes between a parent and child process Post: 84432

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Top Forums Programming Implementing 2 pipes between a parent and child process Post 84432 by bwgoudey on Saturday 24th of September 2005 10:07:38 PM

09-24-2005

Registered User

Implementing 2 pipes between a parent and child process

Hi all,
I'm trying to write a program that has some data it wants to send through a filter program(in this case tr), and then recieve the output from that filter program. The way I'm trying to do it is by setting up two pipes between the programs and piping the data in through one pipe and back out through the other one. However all it does at the minute is hang. I think waiting for input from stdin but I can't figure out why. I'm guessing that the child process must not be getting sent any data but I don't how know how to fix it.

Here is the code (sorry if its a little long)

Code:

int main (void)
{
    pid_t pid;
    int pipe_in[2];
    int pipe_out[2];
    char buffer[800];

    /* Create the pipe.  */
    pipe (pipe_in);
    pipe (pipe_out);

    /* Create the child process.  */
    pid = vfork ();
    if (pid == (pid_t) 0) /* child 1 */
    {
		/*close unneeded*/
                close(pipe_in[1]);
                close(pipe_out[0]);
		
		/*Make pipes the stdin and stdout*/
                dup2(pipe_in[0], 0);
                close(pipe_in[0]);
                dup2(pipe_out[1], 1);
                close(pipe_out[1]);

		/*change a to b*/ 
                execl("/bin/tr", "tr", "a", "b" , 0);
                perror("exec prog1");
                exit(1);
    }
    else /*parent*/
    {
                /*close unneeded*/
                close(pipe_in[0]);
                close(pipe_out[1]);
                /*write to and then read from child*/
                write(pipe_in[1], "This is the data\n", 14);
                read(pipe_out[0], buffer, sizeof(buffer));

                /*send EOF to child*/
                close(pipe_in[1]);
                /*wait for end then close other end of pipe*/
                waitpid(pid, NULL, 0);
                close(pipe_out[0]);
                return EXIT_SUCCESS;
    }
}

Any help is greatly appreciated
Ben Goudey

bwgoudey

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LEARN ABOUT MINIX

fork

FORK(2) 							System Calls Manual							   FORK(2)

NAME

       fork - create a new process

SYNOPSIS

       #include <sys/types.h>
       #include <unistd.h>

       pid_t fork(void)

DESCRIPTION

       Fork causes creation of a new process.  The new process (child process) is an exact copy of the calling process except for the following:

	      The child process has a unique process ID.

	      The child process has a different parent process ID (i.e., the process ID of the parent process).

	      The  child  process has its own copy of the parent's descriptors.  These descriptors reference the same underlying objects, so that,
	      for instance, file pointers in file objects are shared between the child and the parent, so that an lseek(2) on a descriptor in  the
	      child  process  can affect a subsequent read or write by the parent.  This descriptor copying is also used by the shell to establish
	      standard input and output for newly created processes as well as to set up pipes.

	      The child starts with no pending signals and an inactive alarm timer.

RETURN VALUE

       Upon successful completion, fork returns a value of 0 to the child process and returns the process ID of the child process  to  the  parent
       process.   Otherwise, a value of -1 is returned to the parent process, no child process is created, and the global variable errno is set to
       indicate the error.

ERRORS

       Fork will fail and no child process will be created if one or more of the following are true:

       [EAGAIN]       The system-imposed limit on the total number of processes under execution would be exceeded.  This limit	is  configuration-
		      dependent.  (The kernel variable NR_PROCS in <minix/config.h> (Minix), or <minix/const.h> (Minix-vmd).)

       [ENOMEM]       There is insufficient (virtual) memory for the new process.

SEE ALSO

       execve(2), wait(2).

3rd Berkeley Distribution					   May 22, 1986 							   FORK(2)

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