How to pass values between pipes - c

Question

So I create 10 processes and in each process add a bunch of random numbers together so that each process holds a partial sum. How do I pass the partial sum back to the parent process? I am having a little difficulty and any help would be much appreciated. Thank-you
#include <stdio.h>
#include <time.h>
#include <unistd.h>
#include <sys/wait.h>
int MAX_PROC = 10;
pid_t pids[10];
void childProcess(void);
void parentProcess(void);
int startChildren(void);
main() {
srand(time(NULL));
clock_t begin,end;
double timeSpent;
begin = clock();
startChildren();
end = clock();
timeSpent = (double)(end-begin);
//printf("\n Time taken: %f \n", timeSpent);
return 0;
}
int startChildren(void) {
int i = 0;
for (i = 0; i < MAX_PROC; ++i) {
pids[i] = fork();
if (pids[i] == 0) {
childProcess();
return(0);
}
}
int status;
pid_t pid;
while (MAX_PROC > 0) {
pid = wait(&status);
MAX_PROC--;
}
parentProcess();
return(0);
}
void childProcess(void)
{
int i = 0;
int partial = 0;
for(i=0; i<10000; i++) {
int r = rand() % 101;
partial += r;
}
}
void parentProcess(void)
{
printf("*** Parent is done ***\n");
}

Answer 1

Here's an example of how to use socketpair() to pass data between parent/child processes (in UNIX-like OSes):
#include <sys/types.h>
#include <sys/socket.h>
#include <stdio.h>
#define DATA1 "In Xanadu, did Kublai Khan . . ."
#define DATA2 "A stately pleasure dome decree . . ."
/*
* This program creates a pair of connected sockets, then forks and
* communicates over them. This is very similar to communication with pipes;
* however, socketpairs are two-way communications objects. Therefore,
* this program can send messages in both directions.
*/
main()
{
int sockets[2], child;
char buf[1024];
if (socketpair(AF_UNIX, SOCK_STREAM, 0, sockets) < 0) {
perror("opening stream socket pair");
exit(1);
}
if ((child = fork()) == -1)
perror("fork");
else if (child) { /* This is the parent. */
close(sockets[0]);
if (read(sockets[1], buf, 1024, 0) < 0)
perror("reading stream message");
printf("-->%s\n", buf);
if (write(sockets[1], DATA2, sizeof(DATA2)) < 0)
perror("writing stream message");
close(sockets[1]);
} else { /* This is the child. */
close(sockets[1]);
if (write(sockets[0], DATA1, sizeof(DATA1)) < 0)
perror("writing stream message");
if (read(sockets[0], buf, 1024, 0) < 0)
perror("reading stream message");
printf("-->%s\n", buf);
close(sockets[0]);
}
}