How to use POSIX semaphores on forked processes in C?

Question

I want to fork multiple processes and then use a semaphore on them. Here is what I tried:

sem_init(&sem, 1, 1);   /* semaphore*, pshared, value */
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Answer 1

Linux minimal anonymous sem_init + mmap MAP_ANONYMOUS example

I like this setup as it does not pollute any global namespace as sem_open does.

The only downside is that MAP_ANONYMOUS is not POSIX, and I don't know any replacement: Anonymous shared memory? shm_open for example takes a global identifier just like sem_open.

main.c:

#define _GNU_SOURCE
#include 
#include 
#include 
#include 
#include 
#include 
#include 

int main(int argc, char **argv) {
    pid_t pid;
    typedef struct {
        sem_t sem;
        int i;
    } Semint;

    Semint *semint;
    size_t size = sizeof(Semint);
    semint = (Semint *)mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_ANONYMOUS | MAP_SHARED, 0, 0);
    assert(semint != MAP_FAILED);
    /* 1: shared across processes
     * 0: initial value, wait locked until one post happens (making it > 0)
     */
    sem_init(&semint->sem, 1, 0);
    semint->i = 0;
    pid = fork();
    assert(pid != -1);
    if (pid == 0) {
        sleep(1);
        semint->i = 1;
        msync(&semint->sem, size, MS_SYNC);
        sem_post(&semint->sem);
        exit(EXIT_SUCCESS);
    }
    if (argc == 1) {
        sem_wait(&semint->sem);
    }
    /* Was modified on the other process. */
    assert(semint->i == 1);
    wait(NULL);
    sem_destroy(&semint->sem);
    assert(munmap(semint, size) != -1);
    return EXIT_SUCCESS;
}

Compile:

gcc -g -std=c99 -Wall -Wextra -o main main.c -lpthread

Run with sem_wait:

./main

Run without sem_wait:

./main 1

Without this synchronization, the assert is very likely to fail, since the child sleeps for one whole second:

main: main.c:39: main: Assertion `semint->i == 1' failed.

Tested on Ubuntu 18.04. GitHub upstream.