share-memory

get.c

#include <stdio.h>
#include <sys/mman.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>

#define STORAGE_ID    "/SHM_TEST"
#define STORAGE_SIZE  sizeof(int)

int main(int argc, char *argv[])
{
	int res, fd;
	pid_t pid;
	void *addr;

	pid = getpid();

	// get shared memory file descriptor (NOT a file)
	fd = shm_open(STORAGE_ID, O_RDONLY, S_IRUSR | S_IWUSR);
	if (fd == -1) {
		return -1;
	}

	// map shared memory to process address space
	addr = mmap(NULL, STORAGE_SIZE, PROT_READ, MAP_SHARED, fd, 0);
	if (addr == MAP_FAILED) {
		perror("mmap");
		return -1;
	}
	// place data into memory
    
	printf("./GET pid %d: Read from shared memory: %d\n", pid, *(int*)addr);
	return 0;
}

run.sh

#!/bin/bash


SET="set"
GET="get"
CFLAGS="-lrt"

gcc -o ${SET} ${SET}.c ${CFLAGS}
gcc -o ${GET} ${GET}.c ${CFLAGS}

./${SET} &
./${SET} &
./${SET} &
./${SET} &
./${SET} &
./${SET} &
./${SET} &
sleep 1
./${GET}
sleep 1

rm ${SET} ${GET}

set.c

#include <stdio.h>
#include <sys/mman.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>

#define STORAGE_ID    "/SHM_TEST"
#define STORAGE_SIZE  sizeof(int)

int main(int argc, char *argv[])
{
	int res, fd, len;
	pid_t pid;
	void *addr;
	// get shared memory file descriptor (NOT a file)
	fd = shm_open(STORAGE_ID, O_RDWR | O_CREAT, S_IRUSR | S_IWUSR);
	if (fd == -1) {
		return -1;
	}

	// extend shared memory object as by default it's initialized with size 0
	res = ftruncate(fd, sizeof(int));
	if (res == -1) {
		return -1;
	}

	// map shared memory to process address space
	addr = mmap(NULL, STORAGE_SIZE, PROT_WRITE, MAP_SHARED, fd, 0);
	if (addr == MAP_FAILED) {
		return -1;
	}

    for (int i = 0; i < 10000; i++) {
        *(int *)addr = *(int *)addr + 1;
    }

	// wait for someone to read it
	sleep(2);

	// mmap cleanup
	res = munmap(addr, STORAGE_SIZE);
	if (res == -1) {
		return -1;
	}

	// shm_open cleanup
	fd = shm_unlink(STORAGE_ID);
	if (fd == -1) {
		return -1;
	}
	return 0;
}