Shared mutex between process - process

Question

I am learning mutex usage between process. I have a question w.r.t this.
Since each process has its own address space, the mutex defined by one process cannot be seen by another. On googling, I came across usage of pthread_mutex_setpshared() to get this done. I am attaching my code below. But looks like even now, the mutex is not shared between processes. How is this to be modified.
I know this can be achieved using named semaphore. But want to know the usage using mutex.
#include <stdio.h>
#include <pthread.h>
#include <sys/types.h>
#include <semaphore.h>
#include <fcntl.h>
int main()
{
pid_t pid;
int ch = 0;
//sem_t *s1;
//s1 = sem_open("/sem", O_CREAT, 0666, 0);
pthread_mutex_t m1;
pthread_mutexattr_t attr;
pthread_mutexattr_init(&attr);
pthread_mutexattr_setpshared(&attr, PTHREAD_PROCESS_SHARED);
pthread_mutex_init(&m1, &attr);
pid = fork();
if(pid == 0)
{
pthread_mutex_lock(&m1);
printf("\nChild process");
printf("\nPress 1 to unlock from child:");
scanf("%d", &ch);
if(ch == 1)
{
pthread_mutex_unlock(&m1);
}
//sem_post(s1);
}
else if (pid > 0)
{
pthread_mutex_lock(&m1);
//sem_wait(s1);
printf("\nParent process");
printf("\nPress 2 to unlock from parent:");
scanf("%d", &ch);
if(ch == 2)
{
pthread_mutex_unlock(&m1);
}
}
else
{
printf("\nError");
}
}

Answer 1

As well as setting the PTHREAD_PROCESS_SHARED attribute, you need to arrange to have the mutex itself (your m1 variable) stored in shared memory, accessible by both processes. A duplicate, as created by your fork(), does not work.
If you use mmap() to create an anonymous mapping with the MAP_SHARED flag, and allocate the mutex within that, then it will be shared between the processes after the fork() and will work.