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Program Communication

Started by Running_Wolf Apr 14, 2005 at 1:37 PM 9 replies 1.3k views

Original Post

Apr 14, 2005 at 1:37 PM

Hi, I was just wondering what the best way to share information between two simultaneously running programs is. Using a temporary, saved file on the hard disk seems a little crude. Are there any other ways? Thanks in advance.

mmelson

Apr 14, 2005 at 1:44 PM

There are a few ways. You should take a look at shared memory, pipes, and sockets.

Personally, whenever I have to share stuff between two programs that are running on the same machine, I tend to default to the shared memory approach.

Trap

Apr 14, 2005 at 1:45 PM

The most generic method is probably using sockets.

mmelson

Apr 14, 2005 at 1:49 PM

Yeah, sockets do have the benefit of being easily extended to work between two machines, but I like being able to use the shared memory as I would any other memory.

But, as usual, the best solution depends on what the programs are doing.

nmi

Apr 14, 2005 at 1:57 PM

Sockets are slow, your data has to go through the network stack.
Use shared memory instead.

Call CreateFileMapping with hFile set to (HANDLE)0xFFFFFFFF to create a shared memory region in the pagefile (also set dwMaximumSizeHigh and dwMaximumSizeLow in this case), or the handle to an open file to use this instead.

HANDLE hMapFile;hMapFile = CreateFileMapping(hFile,    // Current file handle.     NULL,                              // Default security.     PAGE_READWRITE,                    // Read/write permission.     0,                                 // Max. object size.     0,                                 // Size of hFile.     "MyFileMappingObject");            // Name of mapping object.  if (hMapFile == NULL) {     ErrorHandler("Could not create file-mapping object."); }

Then call MapViewOfFile:

LPVOID lpMapAddress;lpMapAddress = MapViewOfFile(hMapFile, // Handle to mapping object.     FILE_MAP_ALL_ACCESS,               // Read/write permission     0,                                 // Max. object size.     0,                                 // Size of hFile.     0);                                // Map entire file.  if (lpMapAddress == NULL) {     ErrorHandler("Could not map view of file."); }

For the second process, call OpenFileMapping:

HANDLE hMapFile;LPVOID lpMapAddress;hMapFile = OpenFileMapping(FILE_MAP_ALL_ACCESS, // Read/write permission.     FALSE,                             // Do not inherit the name    "MyFileMappingObject");            // of the mapping object.  if (hMapFile == NULL) {     ErrorHandler("Could not open file-mapping object."); }  lpMapAddress = MapViewOfFile(hMapFile, // Handle to mapping object.     FILE_MAP_ALL_ACCESS,               // Read/write permission.     0,                                 // Max. object size.     0,                                 // Size of hFile.     0);                                // Map entire file.  if (lpMapAddress == NULL) {     ErrorHandler("Could not map view of file."); }

Running_Wolf

Author

Running_Wolf

Apr 14, 2005 at 2:02 PM

Okay, thanks for the input thus far. One more thing, (and I am looking on MSDN for more info but meanwhile...), after setting up the shared memory object as the example gave. How do I set which variables and whatnot are shared?

Vanke

Apr 14, 2005 at 2:04 PM

Don't forget semaphores with you're shared memory or you could end up in lots of trouble. Or is that only a linux thing?

nmi

Apr 14, 2005 at 2:07 PM

Once you called MapViewOfFile, you get a pointer to the memory mapped region of the shared file (the pagefile or a normal one).

Every access to this memory is visible in both processes. So you can just set a pointer to point into this region, and the object it points to will be shared:

struct MyData {  int i;};MyData* myPointer = (MyData*)lpMapAddress;myPointer->i = 3;

This will set i to 3 for the other process too.

nmi

Apr 14, 2005 at 2:10 PM

Quote:
Original post by Vanke
Don't forget semaphores with you're shared memory or you could end up in lots of trouble. Or is that only a linux thing?

No, Windows has semaphores too ;-)

Call CreateSemaphore with lpName set to an uniqe string, so that both processes use the same semaphore.

Running_Wolf

Author

Running_Wolf

Apr 14, 2005 at 4:19 PM

Okay, thanks for all the input. I will start experimenting with it.

iMalc

182

iMalc

Apr 15, 2005 at 1:07 AM

Don't overlook WM_COPY_DATA it is very useful, even if just to kick off the other method of communication mentioned.

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