C++ Execute one child process iteration after another process - c++

Question

I want to create two child processes with some iterations in them so that iteration l in process X always executes after iteration l+1 in process Y.
#include <stdio.h>
#include <unistd.h>
#include <bits/stdc++.h>
#include <sys/wait.h>
#include <semaphore.h>
using namespace std;
sem_t mut;
int x = 2;
int main()
{
int cpX = fork();
sem_init(&mut, 1, 1);
if (cpX == 0)
{
//child A code
for (int i = 0; i < 10; i++)
{
sem_wait(&mut);
cout << "Inside X:" << getpid() << ", " << i << '\n';
sleep(rand() % 5);
}
exit(0);
}
else
{
int cpY = fork();
if (cpY == 0)
{
//child B code
for (int i = 0; i < 10; i++)
{
cout << "Inside Y:" << getpid() << ", " << i << '\n';
sleep(rand() % 5);
sem_post(&mut);
}
//sem_wait(&mut);
exit(0);
}
else
{
//sleep(50);
//wait(NULL);
//wait(NULL);
exit(0);
// wait(NULL);
}
}
}
However here, X executes once, and then Y starts executing and X never executes again. Why is this happening?

Answer 1

The parent/child during a fork is doing a copy so they are not referring to the same semaphore object. Hence, mut is not shared as what you assumed. Your snippet is near correct you just need to do minor changes to work as you expect.
Since you are using an unnamed semaphore, you need to instantiate the semaphore object in a shared area:
#include <sys/mman.h>
sem_t *mut = (sem_t*)mmap(NULL, sizeof(*mut), PROT_READ |PROT_WRITE,MAP_SHARED|MAP_ANONYMOUS, -1, 0);
sem_init(mut, 1, 1);
With this minor adjustments, your snippet above now works as expected as they are now referring to the same semaphore object.
Also, some tips: sem_post is an unblocking call, whereas sem_wait is a blocking call. You need to be also aware that a signal sent by sem_post could get lost when nobody is waiting on the semaphore during that moment.
The best reference of this kinds of things is Unix Network Programming: Interprocess Communication by Richard Stevens