Create a shared memory segment

Question

Create a shared memory segment

Flaviu_ 1,031

Hello. How can I create a shared memory segment, in Windows ? I ask this because I have found several examples for Linux, but I need it in Windows.

There, I need to write and read char* on a given position ... but I don't know how to accomplish that. Can you help me ?

2 answers

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Answer 1

Castorix31 90,521

You can see MSDN docs, like Sharing Files and Memory

Flaviu_ 1,031 Reputation points

2020-10-09T08:28:00.42+00:00

I saw that document, "share a file or memory between" is alike sharing a memory segment ?
Castorix31 90,521 Reputation points

2020-10-09T08:42:13.467+00:00

Yes.

"To share memory that is not associated with a file, a process must use the CreateFileMapping function and specify INVALID_HANDLE_VALUE as the hFile parameter instead of an existing file handle."
Drake Wu - MSFT 996 Reputation points

2020-10-09T09:14:10.443+00:00

Creating Named Shared Memory

Answer 2

RLWA32 49,461

There, I need to write and read char* on a given position

Shared memory using CreateFileMapping and MapViewOfFile is made accessible to a process at a virtual address that is valid in that process. A pointer (e.g., char*) that is stored in shared memory will not be valid when that shared memory is mapped into a different process. And shared memory can be mapped to different virtual addresses in the various process that share it.

Storing data in shared memory doesn't present a problem. However, storing pointers is another thing. A mechanism for using pointers in shared memory is to use Based Pointers which work with offsets.

Flaviu_ 1,031 Reputation points

2020-10-09T11:35:20.433+00:00
Kindly thank you for your post, it means a lot for me.
I have an request, to create a function that has the following signature:

synhandle CreateSharedMemory(const char *name, int size);

and another to write in this shared memory:

int WriteSharedMemory(synhandle handle, int pos, char *value, int size);

So, I suspect that cannot use CreateFileMapping ... but then what ?
RLWA32 49,461 Reputation points

2020-10-09T11:54:41.593+00:00

Please clarify if you intend to store data or pointers in the shared memory.
We don't know what the WriteSharedMemory function does.

If your intent is to write an array of char starting at a specific offset from the base address of the shared memory (returned by MapViewOfFile) then you shouldn't have a problem.

Writing a C Style string ( array of char ) to shared memory is fine. You have only written data.
But storing a pointer in shared memory is problematic.
Flaviu_ 1,031 Reputation points

2020-10-09T12:24:04.51+00:00
I don't have the body of these functions, I need to write myself knowing only parameters:

synhandle CreateSharedMemory(const char *name, int size); int WriteSharedMemory(synhandle handle, int pos, char *value, int size); synhandle OpenSharedMemory(const char *name, int size); int ReadSharedMemory(synhandle handle, int pos, char *value, int size);

where synhandle is void*

I only need a little guidance, that's all. Sorry if puzzled something here ...
RLWA32 49,461 Reputation points

2020-10-09T12:36:44.537+00:00

I'll assume that your intent is to copy data to/from shared memory.

In your example, synhandle would be the base address of shared memory returned from MapViewOfFile, and pos would be the offset from the base address.

In WriteSharedMemory value would be a C style string containing size characters (including null terminator). This can be copied into shared memory.

In ReadSharedMemory value would be a C style string of size characters that is to receive the data copied out of shared memory.
RLWA32 49,461 Reputation points

2020-10-09T14:11:59.623+00:00

Another way to look at it would be that synhandle is the Windows HANDLE returned from CreateFileMapping/OpenFileMapping and the WriteSharedMemory and ReadSharedMemory functions each call MapViewOfFile to map the shared memory and obtain a pointer to the memory's base address;

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Create a shared memory segment

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