One more question about forks/pipes

[suprropmp]

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:

#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:

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?

[MacGyver]

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.

[Overworked_PhD]

#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.

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.

[Salem]

> 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.