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I wrote a simple inter-process communication with a memory-mapped file. The code works relatively well, but I have a problem with the buffer that I’ll explain shortly. Here is the code (C++, Windows):

#define UNICODE
#define _UNICODE

#include <iostream>
#include <tchar.h>
#include <Windows.h>

int wmain(int argc, wchar_t** argv)
{
    if (argc != 2)
    {
        std::cout << "Usage: `win32mmap w` for writing, or `win32mmap r` for reading.n";
        return -1;
    }

    HANDLE hMapFile;
    HANDLE hEvent;
    HANDLE isOpened = CreateEvent(NULL, true, false, L"IsOpened"); // To check if a `win32mmap w` runs

    if (wcscmp(argv[1], L"w") == 0)
    {
        SetEvent(isOpened);

        hMapFile = CreateFileMapping(INVALID_HANDLE_VALUE, NULL, PAGE_READWRITE, 0, 1024, L"mmapFile");
        if (hMapFile == NULL)
        {
            std::cout << "CreateFileMapping() Error: " << GetLastError() << "n";
            return GetLastError();
        }

        hEvent = CreateEvent(NULL, true, false, L"mmapEvent");
        if (hEvent == INVALID_HANDLE_VALUE || hEvent == NULL)
        {
            std::cout << "CreateEvent() Error: " << GetLastError() << "n";
            return GetLastError();
        }

        char* buff = (char*)MapViewOfFile(hMapFile, FILE_MAP_WRITE, 0, 0, 0);
        if (!buff)
        {
            std::cout << "MapViewOfFile() Error: " << GetLastError() << "n";
            return GetLastError();
        }

        while (buff[0] != L'.')
        {
            std::cin >> buff;
            SetEvent(hEvent);
        }

        UnmapViewOfFile(buff);
    }

    else if (wcscmp(argv[1], L"r") == 0)
    {
        if (WaitForSingleObject(isOpened, 0) == WAIT_TIMEOUT)
        {
            std::cout << "Waiting for `win32mmap w`...";
            WaitForSingleObject(isOpened, INFINITE);
            std::cout << "n";
        }

        hMapFile = OpenFileMapping(FILE_MAP_ALL_ACCESS, false, L"mmapFile");
        if (hMapFile == NULL)
        {
            std::cout << "CreateFileMapping() Error: " << GetLastError() << "n";
            return GetLastError();
        }

        hEvent = OpenEvent(EVENT_ALL_ACCESS, false, L"mmapEvent");
        if (hEvent == INVALID_HANDLE_VALUE || hEvent == NULL)
        {
            std::cout << "CreateFile() Error: " << GetLastError() << "n";
            return GetLastError();
        }

        char* buff = (char*)MapViewOfFile(hMapFile, FILE_MAP_READ, 0, 0, 0);
        if (!buff)
        {
            std::cout << "MapViewOfFile() Error: " << GetLastError() << "n";
            return GetLastError();
        }

        if (!buff)
        {
            std::cout << "MapViewOfFile() Error: " << GetLastError() << "n";
            return GetLastError();
        }

        while (true)
        {
            WaitForSingleObject(hEvent, INFINITE);
            ResetEvent(hEvent);

            if (buff[0] == '.')
            {
                break;
            }

            std::cout << buff << "n";
        }

        UnmapViewOfFile(buff);
    }

    else
    {
        std::cout << "Usage: `win32mmap w` for writing, or `win32mmap r` for reading.n";
        return -1;
    }

    CloseHandle(hMapFile);

    return 0;
}

The program is a simple inter-process communication "chat" that relies on memory-mapped files. To use the program, you need to make two executable instance of the program: 

win32mmap w
and 
win32mmap r
. The first instance is used to type text that is displayed in the second instance. When you type 
.
in the first instance, both of them are terminated.

My problem is when I run the 2 instances of the program, and I type the world 

Hello
in the first instance ( 
win32mmap w
), the second instance shows 
Hello
as expected. But when I type 
Hello World
in the first instance, the second instance shows only the word 
World
instead of 
Hello World
. How can I fix the code that the buffer will get the whole text?

Anonymous Changed status to publish May 14, 2021