Send data between processes via QSharedMemory

[MR.Smith]

Hello,

I'm need to communicate with another process in OS.
First process I would like to write on Qt (5.6.1). Second process on Visual Studio 2013 on WinAPI. I think that QSharedMemory is a good way for this.

I used Shared Memory Example. It's correctly work in single process.

Already I used Creating Named Shared Memory example from MSDN. It's also correctly work.

But I can't transfer data from Qt application to WinAPI application. Qt application is correctly write data and I can read it in Qt application, but in WinAPI application is not possible to connect to shared memory and read it. Error: Could not open file mapping object.

May be there is more detailed example for QSharedMemory?
What is a problem here?

My source code is bellow.
Creating shared memory:

Dialog::Dialog(QWidget *parent) :
    QDialog(parent),
    ui(new Ui::Dialog),
    sharedMemory("Global\\SharedMemoryExample")
{
    ui->setupUi(this);
}

For write data to shared memory:

void Dialog::on_writeButton_clicked()
{
    QString data = ui->bufferLineEdit->text();
    if(sharedMemory.isAttached())
        sharedMemory.detach();

    QBuffer buffer;
    buffer.open(QBuffer::ReadWrite);
    QDataStream out(&buffer);
    out << data;
    int size = buffer.size();

    if(!sharedMemory.create(size))
    {
        ui->bufferLineEdit->setText("ERROR: Shared Memory not created");
        return;
    }
    sharedMemory.lock();
    char *to = (char*)sharedMemory.data();
    const char *from = buffer.data().data();
    memcpy(to, from, qMin(sharedMemory.size(), size));
    sharedMemory.unlock();
}

For read shared memory:

void Dialog::on_readButton_clicked()
{
    ui->bufferLineEdit->clear();
    if (!sharedMemory.isAttached())
    {
        ui->bufferLineEdit->setText("ERROR: Shared Memory not attached");
        return;
    }
    QBuffer buffer;
    QDataStream in(&buffer);
    QString data;
    data.clear();

    sharedMemory.lock();
    buffer.setData((char*)sharedMemory.constData(), sharedMemory.size());
    buffer.open(QBuffer::ReadOnly);
    in >> data;
    sharedMemory.unlock();

    sharedMemory.detach();
    ui->bufferLineEdit->setText(data);
}

And second WinApi application:

#include <windows.h>
#include <stdio.h>
#include <conio.h>
#include <tchar.h>
#pragma comment(lib, "user32.lib")

#define BUF_SIZE 256
TCHAR szName[]=TEXT("Global//SharedMemoryExample");

int _tmain()
{
   HANDLE hMapFile;
   LPCTSTR pBuf;

   hMapFile = OpenFileMapping(
                   FILE_MAP_ALL_ACCESS,   // read/write access
                   FALSE,                 // do not inherit the name
                   szName);               // name of mapping object

   if (hMapFile == NULL)
   {
      _tprintf(TEXT("Could not open file mapping object (%d).\n"),
             GetLastError());
      return 1;
   }

   pBuf = (LPTSTR) MapViewOfFile(hMapFile, // handle to map object
               FILE_MAP_ALL_ACCESS,  // read/write permission
               0,
               0,
               BUF_SIZE);

   if (pBuf == NULL)
   {
      _tprintf(TEXT("Could not map view of file (%d).\n"),
             GetLastError());

      CloseHandle(hMapFile);

      return 1;
   }

   MessageBox(NULL, pBuf, TEXT("Process2"), MB_OK);

   UnmapViewOfFile(pBuf);

   CloseHandle(hMapFile);

   return 0;
}

[SGaist]

Hi and welcome to devnet,

Did you see that you are not using the same key for both applications ?

For the Qt application you are using backslashes and for the WinAPI application your are using forward slashes.

[MR.Smith]

@SGaist Thank you! I'm fix it. But it's doesn't fix a problem. I'm already testing both variants. In my post it is only typo.

[SGaist]

I don't remember if there's a limitation in the characters you can use as key, but i'd start with only ASCII stuff without any special signs.

[MR.Smith]

I'm already try to run two copies of Qt application. And it can't transfet data from one thread to another.

[SGaist]

What exact error are you getting ?

[MR.Smith]

@SGaist I'm changed value of key to simple ASCII: "example".
Not effect (((
May be you have an example? I searching it in Google but only one example in qt.io was found.
No any other(((

[VRonin]

Probably off topic, but did you consider using QLocalSocket and WINAPI Named Pipe instead

[MR.Smith]

@VRonin Good idea. It's will be my fallback. I need to high speed data transfer (up to 10 MBit/s) and shared memory is a faster method of IPC. It's most suitable for my application.

[VRonin]

Disclaimer

what follows is just speculation, not advice.

---

The problem with shared memory, as I understand it, is that is no way to signal the reader application that new data is available to consume.

10 MBit/s is not a lot, named pipe should be able to manage it without a problem, in fact, a TCP socket connected to localhost should also be able to handle that transfer rate without a sweat. Both their typical transfer rates are in the multiple Gbit/sec

[SGaist]

Check the detailed documentation of QSharedMemory and the setNativeKey. They explain how to achieve communication with non Qt application.

[MR.Smith]

@SGaist When I run two copies of Ot application and write some data to shared memory from one of them and the next trying read data from another I have problem: sharedMemory.isAttached() == FALSE for shared memory with same keys.

[jsulm]

@MR.Smith Didn't you say before that it is working when using two Qt apps and doesn't work if one app is not a Qt app?

[MR.Smith]

@VRonin Thank you for recommendation. I will try to use named pipe in my application. It's also good solution for my application.

[MR.Smith]

@jsulm Yes, it's true. I can transfer data from one Qt application to another copy of this Qt application, but I can't transfer data from Qt application to WinAPI application.

[jsulm]

@MR.Smith @SGaist pointed out what you should try

[MR.Smith]

I got successfully transfer data via named pipes. It's good solution and I will use it, not shared memory.
Thank you every one for your help and recommendations.