Jepessen Jepessen - 1 month ago 32
C# Question

Shared memory synchronization between c++ and c#

I've written two little programs. A C++ program that writes a string in a shared memory, and a C# program that reads the string from shared memory and writes it on the console.

Programs work well, but at the moment I write the string in a timed loop, and I read it from another timed loop, with same frequency.

What I want to do is to create some sort of locking. I want to use a semaphore/mutex/whatever, so from C# I cannot read shm while C++ program is writing it, and viceversa.

I've read something like named mutex and semamphore. I can create them in both programs, but I really don't know how to use them properly.

C++ program:
MemoryWriter.h

#ifndef MEMORYWRITER_H_
#define MEMORYWRITER_H_

#include <windows.h>
#include <string>
#include <random>

class MemoryWriter {

public:

MemoryWriter(const std::wstring& name, size_t size);
std::string createRandomData() const;
void write(const std::string& data);

private:

char getRandomCharacter() const;
void createSharedMemory();

private:

std::wstring m_memoryName;
size_t m_memorySize = 0;
HANDLE m_shmHandler = 0;
};
#endif // !MEMORYWRITER_H_


MemoryWriter.cpp

#include "MemoryWriter.h"
#include <random>
#include <iostream>

///////////////////////////////////////////////////////////////////////////////
// USING SECTION //
///////////////////////////////////////////////////////////////////////////////

using std::string;

///////////////////////////////////////////////////////////////////////////////
// CONSTANTS SECTION //
///////////////////////////////////////////////////////////////////////////////

const char MinCharacter{ 'A' };
const char MaxCharacter{ 'z' };

///////////////////////////////////////////////////////////////////////////////
// PUBLIC SECTION //
///////////////////////////////////////////////////////////////////////////////

MemoryWriter::MemoryWriter(const std::wstring& name, size_t size) :
m_memoryName(name),
m_memorySize(size) {
createSharedMemory();
}

string MemoryWriter::createRandomData() const {
string data;
for (size_t i = 0; i < m_memorySize; i++) {
data += getRandomCharacter();
}
return data;
}

void MemoryWriter::write(const string& data) {
if (!m_shmHandler) {
return;
}
auto buffer = MapViewOfFile(m_shmHandler, FILE_MAP_ALL_ACCESS, 0, 0, m_memorySize);
if (NULL == buffer) {
std::cerr << "Cannot use MapViewOfFile: null buffer." << std::endl;
return;
}
CopyMemory(buffer, data.c_str(), data.size());
}

//////////////////////////////////////////////////////////////////////////////
// PRIVATE SECTION //
//////////////////////////////////////////////////////////////////////////////

char MemoryWriter::getRandomCharacter() const {
return MinCharacter + rand() % 24;
}

void MemoryWriter::createSharedMemory() {
m_shmHandler = OpenFileMapping(FILE_MAP_ALL_ACCESS, FALSE, m_memoryName.c_str());

if (!m_shmHandler) {
m_shmHandler = CreateFileMapping(
INVALID_HANDLE_VALUE,
NULL,
PAGE_READWRITE,
0,
m_memorySize,
m_memoryName.c_str());
}
}


main.cpp

#include "MemoryWriter.h"
#include <iostream>
#include <string>
#include <thread>

int main(int argc, char* argv[])
{
std::wstring memoryName{ L"shm_1" };
size_t memorySize{ 80 };
MemoryWriter writer(memoryName, memorySize);

while (true) {
std::string data;
data = writer.createRandomData();
writer.write(data);
std::cout << "C++: Written in shm - " << data << std::endl;
std::this_thread::sleep_for(std::chrono::milliseconds(100));
}

return 0;
}


C# program

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;

namespace SharedMemoryReader
{
class Program
{
static void Main(string[] args)
{
string shmName = "shm_1";
int shmSize = 80;
var shm = System.IO.MemoryMappedFiles.MemoryMappedFile.CreateOrOpen(shmName, shmSize);
while (true)
{
var view = shm.CreateViewStream();
if (view == null)
{
continue;
}
byte[] data = new byte[shmSize];
view.Read(data, 0, shmSize);
string text = System.Text.Encoding.Default.GetString(data);
System.Console.WriteLine("C#: Read from shm - " + text);
System.Threading.Thread.Sleep(100);
}
}
}
}

Answer

You will need a named synchronization object. Maybe a mutex.

In C++ you can call CreateMutex while in C# there is the Mutex class that handles it for you.

Both links come with examples, maybe you give it a try and come back if you encounter a specific problem.