POSIX Shared Memory Example

.gitignore

*.o
send_string
receive_string

Makefile

CXXFLAGS = -g -std=c++17 -Wall -Wextra -Wpedantic
TARGETS = send_string receive_string
LDLIBS = -lrt

MESSAGE = "Testing 1...2...3..."

.PHONY: test clean

all: $(TARGETS)

send_string: send_string.o shm.o
	$(CXX) $(CXXFLAGS) -o $@ $^ $(LDLIBS)

receive_string: receive_string.o shm.o
	$(CXX) $(CXXFLAGS) -o $@ $^ $(LDLIBS)

SHELL = /bin/bash
test: $(TARGETS)
	./send_string $(MESSAGE)
	diff <(echo $(MESSAGE)) <(./receive_string)

clean:
	rm -f *.o $(TARGETS)

receive_string.cpp

#include <cstdlib>
#include <iostream>

#include "shm.h"

using std::cout;
using std::endl;

const auto REGION_SIZE = 4096;
const auto REGION_NAME = "MESSAGE";
const auto REGION_PERMS = S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH;

int main()
{
    SharedMemory region(
            REGION_NAME,
            O_RDONLY,
            REGION_PERMS,
            REGION_SIZE,
            PROT_READ
    );

    char *buffer = reinterpret_cast<char *>(region.ptr);
    cout << buffer << endl;

    region.unlink();

    return EXIT_SUCCESS;
}

send_string.cpp

#include <cstdlib>
#include <cstring>
#include <iostream>

#include "shm.h"

using std::cerr;
using std::endl;

const auto REGION_SIZE = 4096;
const auto REGION_NAME = "MESSAGE";
const auto REGION_PERMS = S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH ;

int main(int argc, char *argv[])
{
    if (argc != 2) {
        cerr << "Usage: " << argv[0] << " MESSAGE" << endl;
        return EXIT_FAILURE;
    }
    auto message = argv[1];

    auto len = strlen(message);
    if (len > REGION_SIZE - 1) {
        cerr << "Message too long." << endl;
        return EXIT_FAILURE;
    }

    SharedMemory region(
        REGION_NAME,
        O_CREAT | O_RDWR,
        REGION_PERMS,
        REGION_SIZE,
        PROT_READ | PROT_WRITE);

    char *buffer = reinterpret_cast<char *>(region.ptr);

    snprintf(buffer, REGION_SIZE, "%s", message);

    return EXIT_SUCCESS;
}

shm.cpp

#include <cstdlib>

#include <sys/types.h>
#include <unistd.h>

#include "shm.h"

SharedMemory::SharedMemory(std::string name, int oflag, mode_t mode, size_t length, int prot)
{
    this->name = name;

    fd = shm_open(name.c_str(), oflag, mode);
    if (fd < 0) {
        perror("shm_open");
        exit(EXIT_FAILURE);
    }

    if ((oflag & O_CREAT) == O_CREAT) {
        auto ok = ftruncate(fd, length);
        if (ok < 0) {
            perror("ftruncate");
            exit(EXIT_FAILURE);
        }
    }

    ptr = mmap(NULL, length, prot, MAP_SHARED, fd, 0);
    if (ptr == MAP_FAILED) {
        perror("mmap");
        exit(EXIT_FAILURE);
    }
}

void SharedMemory::unlink()
{
    auto ok = shm_unlink(name.c_str());
    if (ok < 0) {
        perror("shm_unlink");
        exit(EXIT_FAILURE);
    }
}

shm.h

#ifndef SHM_H
#define SHM_H

#include <string>

#include <sys/mman.h>
#include <sys/stat.h>
#include <fcntl.h>

class SharedMemory {
    public:
        SharedMemory(std::string, int, mode_t, size_t, int);
        void unlink();
        void *ptr;
    private:
        int fd;
        std::string name;
};

#endif