The most simple working example
#define _XOPEN_SOURCE 700
#include <stdio.h>
#include <sys/mman.h>
#include <unistd.h>
#include <fcntl.h>
int main(){
int fd; // shared memory working segment
int * sp; // segment pointer
fd = shm_open("myshm", O_RDWR | O_CREAT, 0600);
ftruncate(fd, sizeof(int));
sp = mmap(NULL, sizeof(int), PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
*sp = 10;
munmap(sp, sizeof(int));
shm_unlink("myshm");
}