HOWTO: Sharing Data Between Processes Using Memory-Mapped Files

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SUMMARY

While using shared tables is generally the simplest and most efficient way to share data between two processes, it is possible to use memory-mapped files to accomplish the same task. The following example uses the Win32 functions CreateFileMapping, MapViewOfFile and UnmapViewOfFile to use the Windows NT pagefile as a memory-mapped file and pass a string between two copies of Visual FoxPro.

NOTE: This example is intended for use only under Windows NT, and was written and tested using Windows NT 4.0.

MORE INFORMATION

  1. Start one instance of Visual FoxPro.
  2. Create and run a program containing the following code:
          DECLARE INTEGER CreateFileMapping IN kernel32.DLL INTEGER hFile, ;
             INTEGER lpFileMappingAttributes, INTEGER flProtect, ;
             INTEGER dwMaximumSizeHigh, INTEGER dwMaximumSizeLow, ;
             STRING lpName
    
          DECLARE INTEGER MapViewOfFile IN kernel32.DLL ;
             INTEGER hFileMappingObject, INTEGER dwDesiredAccess, ;
             INTEGER dwFileOffsetHigh, INTEGER dwFileOffsetLow, ;
             INTEGER dwNumberofBytesToMap
    
          DECLARE INTEGER GetLastError IN kernel32.DLL
    
          DECLARE INTEGER lstrcpy IN kernel32.DLL ;
             INTEGER lpString1, STRING @lpString2
    
          DECLARE INTEGER UnmapViewOfFile IN kernel32.DLL ;
             INTEGER lpBaseAddress
    
          DECLARE INTEGER CloseHandle IN kernel32.DLL INTEGER hObject
    
          #DEFINE page_writeread 4
          #DEFINE page_readwrite 4
          #DEFINE file_map_read 4
          #DEFINE file_map_write 2
    
          * FFFFFFFF (2^32) means: Use the Windows NT pagefile as the memory-
          * mapped file.
          * Otherwise, you could substitute a handle that you have created.
          #DEFINE usepagefile 2^32
    
          * Arbitrary name for the file mapping.
          szename = "hello"
    
          * This says: Create a file-mapping object from the pagefile,
          * using a default security descriptor, giving Read/Write access
          * to the committed region, with a maximum size of 4096 bytes,
          * named szeName, and return the handle to this file-mapping
          * object in variable 'handle'.
          handle = createfilemapping(usepagefile, 0, page_readwrite, 0, ;
             4096, szename)
          IF handle = 0
             WAIT WINDOW "CreateFileMapping failed - LastError: " ;
                + LTRIM(STR(getlasterror()))
             RETURN
          ENDIF
    
          * This says: Given the handle obtained above, give full access
          * to the file-mapping object, starting at offset 0, and map the
          * entire file into my process' address space.
          addhandle = mapviewoffile(handle, file_map_write, 0, 0, 0)
          IF handle = 0
             WAIT WINDOW "MapViewOfFile failed - LastError: " ;
                + LTRIM(STR(getlasterror()))
             RETURN
          ENDIF
    
          * Now, the file can be treated just like a memory address.
          * Use lstrcpy to copy the FoxPro string to addhandle, the
          * starting address of the mapped view.
    
          * The text you want to write to the shared memory-mapped
          * file goes here.
          stxt1 = "Hello world, I'm writing to a shared page file."
          ret = lstrcpy(addhandle, @stxt1)
    
          * When the message box appears, run the corresponding program
          * to read the memory-mapped file in a second instance of FoxPro.
          * Unmapping the file view and closing the handle removes the text
          * from the file.
          = MESSAGEBOX("Now switch to the other application")
    
          * When we've finished with it, unmap the file-mapping object
          * from our address space and release the handle.
          =UnmapViewOfFile (addhandle)
          =CloseHandle(handle)
    								
  3. When the message box containing the text "Now switch to the other application" appears, run a second instance of Visual FoxPro, create and run a program containing the following code:
          DECLARE INTEGER CreateFileMapping IN kernel32.DLL INTEGER hFile, ;
             INTEGER lpFileMappingAttributes, INTEGER flProtect, ;
             INTEGER dwMaximumSizeHigh, INTEGER dwMaximumSizeLow, ;
             STRING lpName
    
          DECLARE INTEGER MapViewOfFile IN kernel32.DLL ;
             INTEGER hFileMappingObject, INTEGER dwDesiredAccess, ;
             INTEGER dwFileOffsetHigh, INTEGER dwFileOffsetLow, ;
             INTEGER dwNumberofBytesToMap
    
          DECLARE INTEGER GetLastError IN kernel32.DLL
    
          DECLARE INTEGER lstrcpy IN kernel32.DLL ;
             STRING @lpString1, INTEGER lpString2
    
          DECLARE INTEGER UnmapViewOfFile IN kernel32.DLL ;
             INTEGER lpBaseAddress
    
          DECLARE INTEGER CloseHandle IN kernel32.DLL INTEGER hObject
    
          #DEFINE page_writeread 4
          #DEFINE page_readwrite 4
          #DEFINE file_map_read 4
          #DEFINE file_map_write 2
    
          * FFFFFFFF (2^ 32) means: Use the Windows NT pagefile as the memory-
          * mapped file. Otherwise, you could substitute a handle that you
          * have created.
          #DEFINE usepagefile 2^32
          #DEFINE maxbuffersize 254
    
          * Arbitrary name for the file mapping.
          szename = "hello"
    
          * This says: Create a file-mapping object from the pagefile,
          * using a default security descriptor, giving Read/Write access
          * to the committed region, with a maximum size of 4096 bytes,
          * named szeName, and return the handle to this file-mapping
          * object in variable handle.
          handle = createfilemapping(usepagefile, 0, page_readwrite, ;
             0, 4096, szename)
          IF handle = 0
             WAIT WINDOW "CreateFileMapping failed - LastError: " ;
                + LTRIM(STR(getlasterror()))
             RETURN
          ENDIF
    
          * This says: Given the handle obtained above, give full
          * access to the file-mapping object, starting at offset 0,
          * and map the entire file into my process' address space.
          addhandle = mapviewoffile(handle, file_map_write, 0, 0, 0)
          IF handle = 0
             WAIT WINDOW "MapViewOfFile failed - LastError: " ;
                + LTRIM(STR(getlasterror()))
             RETURN
          ENDIF
    
          * Now, the file can be treated just like a memory address.
          * Use lstrcpy to copy the Foxpro string to the Foxpro
          * string stxt2 from the starting address of the mapped view.
    
          * Buffer to receive passed string.
          stxt2 = SPACE(254)
          ret = lstrcpy(@stxt2, addhandle)
    
          =MESSAGEBOX ("String I read was: " + stxt2)
    
          * When we've finished with it, unmap the file-mapping object
          * from our address space and release the handle.
    
          =UnmapViewOfFile(addhandle)
          =CloseHandle (handle)
    								
  4. The second instance of Visual FoxPro should display the string written to the pagefile by the first instance. To unmap the pagefile and release all the handles, press OK on the message boxes in both instances of Visual FoxPro.
The sample code presented here is complex. Answers to questions regarding this example may require that Microsoft Product Support Engineers spend some time familiarizing themselves with the code.

REFERENCES

Win32 SDK Help; topic: "CreateFileMapping"; "MapViewOfFile"; "UnmapViewOfFile"

For further background on the uses of memory-mapped files:

"Advanced Windows: The Developer's Guide to the Win32 API for Windows NT 3.5 and Windows 95," Jeffry Richter, Chapter 7, Microsoft Press

Properties

Article ID: 188535 - Last Review: February 23, 2014 - Revision: 2.0
APPLIES TO
  • Microsoft Visual FoxPro 5.0 Standard Edition
  • Microsoft Visual FoxPro 5.0a
Keywords: 
kbnosurvey kbarchive kbhowto KB188535

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