Need help in IPC through Pipes

Question

I am Working On a lab. A father process will create two son processes A and B. Son A will send some string to son B through pipe.son B will Invert the String case of the String Got from Son A and will send back the Inverted string to son A.after receiving the inverted string son A will print it to the screen.

here is the code.

#include <stdio.h>   
#include <unistd.h>   
#include <stdlib.h>
#include <ctype.h>   

void process_A(int input_pipe[], int output_pipe[])
{
    int c;    
    char ch; 
    int rc;  


    close(input_pipe[1]); 
    close(output_pipe[0]); 

    while ((c = getchar()) > 0) {
        ch = (char)c;
        rc = write(output_pipe[1], &ch, 1);
    if (rc == -1) { 
        perror("A_TO_B: write");
        close(input_pipe[0]);
        close(output_pipe[1]);
        exit(1);
        }

    rc = read(input_pipe[0], &ch, 1);
    c = (int)ch;
    if (rc <= 0) { 
        perror("A_TO_B: read");
        close(input_pipe[0]);
        close(output_pipe[1]);
        exit(1);
        }
    putchar(c);
    }
    close(input_pipe[0]);
    close(output_pipe[1]);
    exit(0);
}

void process_B(int input_pipe[], int output_pipe[])
{
    int c;   
    char ch; 
    int rc;   
    close(input_pipe[1]); 
    close(output_pipe[0]); 
    while (read(input_pipe[0], &ch, 1) > 0) {
        c = (int)ch;
        if (isascii(c) && isupper(c))
            c = tolower(c);
          else if (isascii(c) && islower(c))
            c = toupper(c);
        ch = (char)c;
        rc = write(output_pipe[1], &ch, 1);
        if (rc == -1) {
            perror("B_TO_A: write");
            close(input_pipe[0]);
            close(output_pipe[1]);
            exit(1);
        }
    }

    close(input_pipe[0]);
    close(output_pipe[1]);
    exit(0);
}


int main(int argc, char* argv[])
{
    /* 2 arrays to contain file descriptors, for two pipes. */
    int A_TO_B[2];
    int B_TO_A[2];
    int pid;       
    int rc,i,State;       

    /* first, create one pipe. */
    rc = pipe(A_TO_B);
    if (rc == -1) {
    perror("main: pipe A_TO_B");
    exit(1);
    }
    /* create another pipe. */
    rc = pipe(B_TO_A);
    if (rc == -1) {
    perror("main: pipe B_TO_A");
    exit(1);
    }

    for(i=0;i<2;i++)
    {
        if((pid=fork()) <0){perror("fork failed\n");};
        if((i==0) && (pid ==0))
        {
            process_A(A_TO_B, B_TO_A); 
        }
        else if((i==1)&&(pid==0))
        {
            process_B(B_TO_A, A_TO_B); 
        }
        else if(pid>0)
        {
           wait( &State );          
        }   
    }

    return 0;   
}

the problem is When i run the program the Son B gets Block. I need u guys help. Thanks in advance.

Answer 1

OK, diagram:

initially: parent process: has
  B_TO_A[0] and [1] open,
  has A_TO_B[0] and [1] open
fork (makes copy)
parent:                                child (pid==0):
B_TO_A both open, A_TO_B both open     call process_A: close unwanted pipe ends, loop

call wait(), wait for one child        loop reads stdin, writes one pipe, reads other pipe

if we ever get here:

fork (makes copy)
parent:                                child (pid==0):
B_TO_A both open, A_TO_B both open     call process_B: close unwanted pipe ends, loop

parent: both ends of both pipes open
call wait(), wait for one child        loop reads one pipe, writes other pipe

First, you will usually not get to "if we ever get here" because the child running process_A() runs in a loop until either EOF on stdin (if that occurs first) or one of the pipe read/write calls fails (e.g., due to EOF on input_pipe[0]). Since the parent is still waiting in a wait() call, and has both ends of both pipes open, there's no EOF on the pipe (EOF on a pipe occurs after you read all the data written by all writers, and all dups of the write end have been closed). So the only way to get there is to hit EOF on stdin, so that the while loop does not run.

Second, if you do get around to forking again and doing process_B(), that child will also wait forever, because one write end of the pipe it's reading from is still open... in the parent! The parent won't close it, because the parent will be waiting forever in wait.

In general, what you need to do here is:

• create two pipes (like you do now)
• fork once, and run process_A() in the child
• fork again (in the parent), and run process_B() in the (new) child
• close both ends of both pipes (in the parent)
• wait for both children now, after both have gotten started

The error handling gets a bit messy since you have to do something (such as kill() the first child) if you can't start the second child. So you need to know how far along you have gotten. You can still loop to fork twice but you can't wait inside the loop, and with just two trips around the loop, each of which do rather different steps, you might as well just write it all out without a loop.