sharedmem1.c

//producer consumer using concurrent processes
// gcc <program.c> -lrt
// remember to put -lrt at the end

#include <stdio.h>             
#include <string.h>
#include <ctype.h>
#include <sys/mman.h>
#include <sys/fcntl.h>
#include <assert.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/wait.h>

// shared memory
struct permission {
	int user, group, other;
} *perm;

int main() {
	int shmfd = shm_open ("MyFile_SHM", O_CREAT | O_EXCL | O_RDWR, S_IRUSR | S_IWUSR);
	assert (shmfd != -1);

	/* Resize the region to store 1 struct instance */
	assert (ftruncate (shmfd, sizeof (struct permission)) != -1);

	/* Map the object into memory so file operations aren't needed */
	struct permission *perm = mmap (NULL, sizeof (struct permission),
	                                PROT_READ | PROT_WRITE, MAP_SHARED, shmfd, 0);     // addr==NULL allows the kernel to choose the addr for the mapping
	assert (perm != MAP_FAILED);

	/* Create a child process and write to the mapped/shared region */
	pid_t child_pid = fork();
	if (child_pid == 0) {  // child process
	    perm->user = 6;
	    perm->group = 4;
	    perm->other = 0;

	    /* Unmap and close the child's shared memory access */
	    munmap (perm, sizeof (struct permission));
	    close (shmfd);
	    return 0;
	  }

	/* Make the parent wait until the child has exited */
	wait (NULL);

	/* Read from the mapped/shared memory region */
	printf ("Permission bit-mask: 0%d%d%d\n", perm->user, perm->group, perm->other);

	/* Unmap, close, and delete the shared memory object */
	munmap (perm, sizeof (struct permission));
	close (shmfd);
	shm_unlink ("MyFile_SHM");
}