C Communicate childrens with parent using pipes [closed] - c

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My program create as many children as the user put in argument. I have a problem with sending the message to the parent. I wish every child sent a message to the parent using a pipe. I do not know how to do it. I know how it works for one child: http://tldp.org/LDP/lpg/node11.html but do not know how to do it for many.
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
int main(int argc, char **argv)
{
pid_t pid;
if(argc < 3)
{
printf("Not enought arguments");
exit(0);
}
int number = atoi(argv[2]); // number of childern
pid_t pids[number],pid_s;
int i,n=number,status;
int pipes[number*2];
char buff[512];
int r=0,w=0,rr=0,ww=0;
for(i=0; i<n; i++)
{
if(pipe(pipes+(i*2)) < 0)
{
perror("failed to allocate pipes");
}
}
if(strcmp("-p", argv[1]) == 0)
{
//
}
if(strcmp("-f", argv[1]) == 0)
{
//
}
switch (pid = fork()) {
case -1:
perror("Error in fork");
exit(0);
break;
case 0:
for(i=0; i < n; i++)
{
if((pids[i] = fork()) < 0)
{
perror("error fork");
}
else if(pids[i] == 0)
{
close(pipes[0+r]);
char *reply = "message";
int val = getpid();
write(pipes[1+w], &val, strlen(reply)+1);
r+=2;
w+=2;
printf("Stworzylem dziecko z numerem: %d \n", getpid());
execvp(argv[0],NULL);
}
}
while(n > 0)
{
pid_s = wait(&status);
--n;
}
break;
default:
for(i=0; i<n; i++)
{
close(pipes[1+rr]);
int n;
read(pipes[0+ww],&n,sizeof(n));
printf("Wiadomosc: %d \n", n);
rr+=2;
ww+=2;
}
if(wait(0) == -1)
{
}
break;
}
return 0;
}
I want the program to create N children.
Each child has to send a message to the parent. I used the exec task because in my task it was written that every child should operate as a separate program. I created a sample stream, but it is not working.
For now program created N childs process. Communication to parent proces from child not work.
EDIT:
It works for me to send and receive data always gets through the same thing. I send the PID of the process, but pick always the same.

Answer 1

I can not follow what you are trying to do with the double-fork business.
And I'm not inclined to write the whole thing for you, but here's an outline of how I would write the code:
int * pids;
int * fds;
int pipefds[2];
char * message = "message";
int alive = 0;
pids = malloc(n * sizeof(int *));
fds = malloc(n * sizeof(int *));
for (i = 0; i < n; ++i) {
if (pipe(pipefds) == -1) exit(1);
switch (pid = fork()) {
case -1: /* error */
exit(2);
case 0: /* child */
close(pipefds[0]);
write(pipefds[1], message, strlen(message)+1);
_exit(0);
default: /* parent */
closefds[1];
pids[i] = pid;
fds[i] = pipefds[0];
++alive;
}
}
while (alive > 0) {
/* select on the fds[] of all alive (fds[i] != -1) children */
/* if EINTR, do waitpid(-1, &status, WNOHANG); */
/* to see if a child has exited */
/* if so, find it in pids, say at index 'x', and: */
/* close(fds[x]); fds[x] = -1; --alive; */
/* if select said I/O ready for a pipe */
/* then read that pipe */
}

Answer 2

Pipes are an extremely poorly suited tool for this issue as they are a fundamentally one to one communication method. What you need is a many to one communication method. For this I would suggest using a message queue. This will permit your parent to subscribe to the message queue and for an unlimited number of children to write to it and their messages all to be multiplexed to the parent.