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I have two C application that can communicate via win32 IPC APIs (CreateFileMapping() etc.)

I have to replace client application with a Python application.

I have tried the following libraries on Python side.

import win32file, win32api

But this libraries does not have CreateFileMapping() functions.

I have tried also mmap.mmap() function but I could not observe any communication. 

import mmap

sharedMemory = mmap.mmap(0, 512, "Local\\SharedBuffer")

sharedMemory.write("AB")

I have also tried "Global\SharedBuffer" and "SharedBuffer" as shared Memory names both two side. 

#define SHARED_BUFFER_NAME          ((LPCSTR)L"Local\\SharedBuffer")

HANDLE bufferHandle = CreateFileMapping(INVALID_HANDLE_VALUE, NULL, PAGE_READWRITE, 0, 512, SHARED_BUFFER_NAME);

// Create a map for accessing Shared Buffer
sharedBuffer = (char*)MapViewOfFile(bufferHandle, FILE_MAP_ALL_ACCESS, 0, 0, SHARED_BUFFER_SIZE);

memset(sharedBuffer, 0, SHARED_BUFFER_SIZE);

while (sharedBuffer[0] == 0);

while (1);

win32 APIs are not mandatory for me. I need only simple shared buffer between C and python application on Windows Machine.

Thanks

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6
  • 2
    why are you casting a const wchar_t* to a const char*? Commented Sep 30, 2014 at 7:50
  • Where is this casting? MapViewOfFile() function returns 'void*' so I am casting it to char array to access bytes. Commented Sep 30, 2014 at 7:53
  • 1
    You are casting here: #define SHARED_BUFFER_NAME ((LPCSTR)L"Local\\SharedBuffer") It should be LPCWSTR but you casted it to a const char* aka LPCSTR.. Commented Oct 2, 2014 at 23:24
  • 1
    Also, are you sure that python's mmap works with windows memory mapped objects? It seems to work like in Java (memory-mapped FILES).. meaning that you memory map a file on disk instead of pages. But I'm not sure if that's true or not. You'll have to test it. Commented Oct 2, 2014 at 23:30
  • 1
    @Brandon: Windows uses memory-mapped "file sections" for shared memory. Commented May 19, 2023 at 20:06

1 Answer 1

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17

I tested this.. it works.. Run the C++ code first. It will create a memory map. Then run the python code. It will write to the map. The C++ code will read the map and print what was written to it..

I know this code is BAD because I don't serialise the data properly (aka writing the size to the file first then the data, etc..) but meh.. It's JUST a BASIC working example.. Nothing more.

Python:

import mmap

shm = mmap.mmap(0, 512, "Local\\Test") #You should "open" the memory map file instead of attempting to create it..
if shm:
    shm.write(bytes("5", 'UTF-8'));
    shm.write(bytes("Hello", 'UTF-8'))
    print("GOOD")

C++:

#include <windows.h>
#include <cstring>
#include <cstdbool>
#include <iostream>

typedef struct
{
    void* hFileMap;
    void* pData;
    char MapName[256];
    size_t Size;
} SharedMemory;

bool CreateMemoryMap(SharedMemory* shm)
{
    if ((shm->hFileMap = CreateFileMapping(INVALID_HANDLE_VALUE, NULL, PAGE_READWRITE, 0, shm->Size, shm->MapName)) == NULL)
    {
        return false;
    }

    if ((shm->pData = MapViewOfFile(shm->hFileMap, FILE_MAP_ALL_ACCESS, 0, 0, shm->Size)) == NULL)
    {
        CloseHandle(shm->hFileMap);
        return false;
    }
    return true;
}

bool FreeMemoryMap(SharedMemory* shm)
{
    if (shm && shm->hFileMap)
    {
        if (shm->pData)
        {
            UnmapViewOfFile(shm->pData);
        }

        if (shm->hFileMap)
        {
            CloseHandle(shm->hFileMap);
        }
        return true;
    }
    return false;
}

int main()
{
    SharedMemory shm = {0};
    shm.Size = 512;
    sprintf(shm.MapName, "Local\\Test");

    if (CreateMemoryMap(&shm))
    {
        char* ptr = (char*)shm.pData;
        memset(ptr, 0, shm.Size);

        while (ptr && (*ptr == 0))
        {
            Sleep(100);
        }

        int size = (int)*ptr;
        ptr += sizeof(char);

        int i = 0;
        for (; i < size; ++i)
        {
            std::cout<<ptr[i];
        }
        FreeMemoryMap(&shm);
    }
}
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1 Comment

Btw.. this is C code.. not really C++ (other than the use of std::cout I took this C structure and its accompanying functions from my github.

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