One more question about forks/pipes

Thanks again to all who helped me out yesterday. I tried to finish up this program (takes 2 chars from user [using forking/pipes] and outputs them). This is what I have so far and it isn't working:

Code:

---

#include <stdio.h>

int main( int argc, char * argv[] ){

        int i, j, buff = 0;
        char p[2];
        char string[256];

        if( pipe(p)==-1 ){
                printf("pipe failed");
                exit(-1);
        }

        printf("Enter two chars to compare: \n");
        for(i; i<2; i++) {
                switch( fork() ){
                case 0:
                        buff = getchar();
                        printf("PID of child &#37;d is %d\n", i+1, getpid() );
                        write( p[1], &buff, sizeof(int) );
                        return 0;
                        break;
                case -1:
                        printf("A forking error has occurred\n");
                        exit(-1);
                        break;
                default:
                        break;
                }
        }

        printf( "PID of parent is %d\n", getpid() );

        while( i-- ){
                read( p[0], &buff, sizeof(int) );
                printf("this came out: %c\n", buff);
        }
}

---

when I compile and run I get the following:

Code:

---

bash-2.03$ gcc -oq2 q2.c
bash-2.03$ q2
Enter two chars to compare:
PID of parent is 9624
t
this came out:
g
PID of child 2 is 9626

PID of child 1 is 9625
c
this came out:
bash-2.03$

---

Can anybody tell me whats wrong?

Reading through your code multiple times, it appears that besides any other problems you might have, you're never read a value from the user.

Code:

---

#include <stdio.h>

---

First of all, you are missing the header files for fork() and pipe(). What this means is that you functions have implicit definitions.

Code:

---

char p[2];
char string[256];

---

Here you have 'magic numbers'.

Now, here is what I suspect what is going on. I think you want to do something like

parent<------------------------pipe--------------------------------------filter program
stdout---------------------------------------------------------------------------stdin
|-----------------------------------------------------------------------------------^
| -----------------------------------------------------------------------------------|
|--------------- --------------------------------------------------------------------|
prompt------>terminal------------------------------------------------------->|


Right?

The problem is that you are confusing buffering on the terminal with buffering over a pipe(). Buffering on a terminal is "line buffered." Buffering over a pipe() isn't. You might want to consider getting the book "Advanced Programming in the Unix Enviroment" by Stevens and Rago. I think it's Chapter 15 that talks about IPC's.

> char p[2];
Read the pipe() manual page to find out the correct type.

> return 0;
Read about _exit() to decide whether it is the more appropriate way of exiting from a child process.