Thread: help with pipeing stdin and stdout from child procces

Howdy,...

I trying to create a pipe between two applications (gui and console) for that i need to understand the pipeing world ...

A console application uses stdin and stdout and i wish to pipe it to the gui application so stdin of console application will be stdout of gui and stdout of console will be stdin of gui application.

i read the man file of dup2() , popen() , pipe() and abit stuck.

i wish to achieve the next :
parent send command to child.
child do some thing and answer to parent
parent print the answer.

Using a tutorial :

Code:

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

int main(){

	pid_t pid;
	int rv;
	int	commpipe[2];		/* This holds the fd for the input & output of the pipe */
	char str[] = "Whats up dock?";
	/* Setup communication pipeline first */
	if(pipe(commpipe)){
		fprintf(stderr,"Pipe error!\n");
		exit(1);
	}

	/* Attempt to fork and check for errors */
	if( (pid=fork()) == -1){
		fprintf(stderr,"Fork error. Exiting.\n");  /* something went wrong */
		exit(1);        
	}

	if(pid){
		/* A positive (non-negative) PID indicates the parent process */
		dup2(commpipe[1],1);	/* Replace stdout with out side of the pipe */
		close(commpipe[0]);		/* Close unused side of pipe (in side) */
		setvbuf(stdout,(char*)NULL,_IONBF,0);	/* Set non-buffered output on stdout */
		scanf("&#37;s",str);
		printf("%s\n",str);
		fprintf(stderr,"Gui  sent %s \n",str); //Just to be sure.
		wait(&rv);				/* Wait for child process to end */
		fprintf(stderr,"Child exited with a %d value\n",rv);
	}
	else{
		/* A zero PID indicates that this is the child process */
		dup2(commpipe[0],0);	/* Replace stdin with the in side of the pipe */
		close(commpipe[1]);		/* Close unused side of pipe (out side) */
		/* Replace the child fork with a new process */
		if(execl("child","child",NULL) == -1){
			fprintf(stderr,"execl Error!");
			exit(1);
		}
	}
	return 0;
}

Code:

/**
 *  The idea is to alter stdin and stdout  :
 *
 *  this part should use parent procces (parent.c) as its keyboard and screen
 * i wish to run this application as a demon.
 */
#include <stdio.h>

int main(){
	char string[100];
	
	printf("External Process\n");
	do{
		printf("Enter Command: ");
		fflush(stdout);				/* Must flush to see command prompt */
		fgets(string,100,stdin);
		printf("(child got) %s\n",string);		/* No flush necessary because new line flushes */
	}while(!strstr(string,"exit"));
	
	return 0;
}