Shared memory demo using mmap

mmap_demo_main.c

// Note: to compile, you must link with -lrt.
// cc mmap_demo_main.c -o main.out -lrt

#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/wait.h>

static const unsigned int SLEEP_TIME = 5;

int main(void) {
    const size_t MAPPING_SIZE = sizeof(int);

    int* i_ptr = (int*)malloc(MAPPING_SIZE);

    int fd = -1;
    void* mapping = mmap(i_ptr, MAPPING_SIZE, PROT_READ | PROT_WRITE, MAP_SHARED | MAP_ANONYMOUS, fd, 0);
    if (mapping == (void*)-1) {
        free(i_ptr);
        return EXIT_FAILURE;
    }

    int* m_ptr = (int*)mapping;
    *m_ptr = 1;

    pid_t pid = fork();
    if (pid < 0) {
        perror("fork");
        return EXIT_FAILURE;
    }
    else if (pid == 0) {
        // child waits for a while, then write 0 to shared memory
        sleep(SLEEP_TIME);
        *m_ptr = 0;
    }
    else {
        // parent waits until child writes 0 to shared memory
        while (*m_ptr) {
            continue;
        }

        pid_t wret = 0;
        int wstatus = 0;
        while ((wret = waitpid(-1, &wstatus, 0)) != -1) {
            printf("Terminated pid %d\n", wret);
        }

    }

    free(i_ptr);
    munmap(mapping, MAPPING_SIZE);

    return EXIT_SUCCESS;
}