POSIX shared memory IPC example (shm_open, mmap), working on Linux and macOS

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 32

int main(int argc, char *argv[])
{
	int res;
	int fd;
	char data[STORAGE_SIZE];
	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)
	{
		perror("open");
		return 10;
	}

	// 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 30;
	}

	// place data into memory
	memcpy(data, addr, STORAGE_SIZE);

	printf("PID %d: Read from shared memory: \"%s\"\n", pid, data);

	return 0;
}

run.sh

#!/bin/bash

function is_linux
{
	if [[ "$(uname)" == "Linux" ]]; then
		echo 1
		return
	fi

	echo 0
}

SET="set"
GET="get"
CFLAGS=""
if [[ "$(is_linux)" == "1" ]]; then
	CFLAGS="-lrt"
fi

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

./${SET} &
sleep 1
./${GET}
if [[ "$(is_linux)" == "1" ]]; then
	echo "/dev/shm:"
	ls -l /dev/shm
fi
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 32
#define DATA "Hello, World! From PID %d"

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

	pid = getpid();
	sprintf(data, DATA, pid);

	// get shared memory file descriptor (NOT a file)
	fd = shm_open(STORAGE_ID, O_RDWR | O_CREAT, S_IRUSR | S_IWUSR);
	if (fd == -1)
	{
		perror("open");
		return 10;
	}

	// extend shared memory object as by default it's initialized with size 0
	res = ftruncate(fd, STORAGE_SIZE);
	if (res == -1)
	{
		perror("ftruncate");
		return 20;
	}

	// map shared memory to process address space
	addr = mmap(NULL, STORAGE_SIZE, PROT_WRITE, MAP_SHARED, fd, 0);
	if (addr == MAP_FAILED)
	{
		perror("mmap");
		return 30;
	}

	// place data into memory
	len = strlen(data) + 1;
	memcpy(addr, data, len);

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

	// mmap cleanup
	res = munmap(addr, STORAGE_SIZE);
	if (res == -1)
	{
		perror("munmap");
		return 40;
	}

	// shm_open cleanup
	fd = shm_unlink(STORAGE_ID);
	if (fd == -1)
	{
		perror("unlink");
		return 100;
	}

	return 0;
}

what is the different betweent shm_open+mmap and shmget? I suggest is the same

what is the different betweent shm_open+mmap and shmget? I suggest is the same

一个是posix一个是systemv

I think get.c is missing an munmap call at the end, just like set.c. And both files are missing a close(fd) at the end as well.

And generally it's best to close the file descriptor right after the memory is mapped with mmap.