Sharing Data with Memory Mapped Files

• Download MMFAPP1.zip - 22.7 KB

Introduction

This article shows how two processes can share data with each other with the help of memory mapped files. The example was written using VC++ 6.0

Background

I wanted a flexible way to share data in between two different processes. Memory mapped files have very important role in Windows, as well as in some other operating systems.

Using the code

In this sample application, one process will write some data to a memory mapped file and other processes will then read that data. Here I am using two different instances of the same application to both read from and write data to a memory mapped file.

Creating the memory mapped file and mapping it to memory

m_hFileMMF = CreateFileMapping(INVALID_HANDLE_VALUE,NULL,PAGE_READWRITE,0,4*1024,"MyMMF");
DWORD dwError = GetLastError();
if ( ! m_hFileMMF )
 { MessageBox(_T("Creation of file mapping failed"));
 }
 else
 { m_pViewMMFFile = MapViewOfFile(m_hFileMMF,FILE_MAP_ALL_ACCESS,0,0,0); // map all file
    if(! m_pViewMMFFile )
    { MessageBox(_T("MapViewOfFile function failed"));
    }
 }

Writing data to memory mapped file

  
  UpdateData(TRUE);
  g_mutex.Lock();
  if(m_pViewMMFFile )
	lstrcpy( (LPTSTR) m_pViewMMFFile, m_Text);
  g_mutex.Unlock();

Reading data from memory mapped file

  g_mutex.Lock();
  if(m_pViewMMFFile)
  {
    lstrcpy(sReadText, (LPCTSTR) m_pViewMMFFile);
    m_ReadText = sReadText;
    GetDlgItem(IDC_EDIT2)->SetWindowText(m_ReadText);
  }
  g_mutex.Unlock();

Points of Interest

I think memory mapped files can solve many problems very easily but people don't use them very often.