CODEWITHSUNDEEP
c++winapi
Jochen
Jochen

Reputation: 1776

CreateFileMapping() returns null

I am trying to follow this tutorial on MSDN: Creating Named Shared Memory.

Unfortunately, I get NULL from CreateFileMapping(). The file shmfile.txt exists on my file system, so I thought no problems should occur, the mapping would be created, and the file would take its role as my shared memory object.

What am I doing wrong?

This is my code:

...
#define BUF_SIZE 256
TCHAR szName[]=TEXT("C:\\Users\\joe\\shmfolder\\shmfile.txt"); 
int main(){
    HANDLE hMapFile; // filehandle
LPCTSTR pBuf; 
hMapFile = CreateFileMapping(
    INVALID_HANDLE_VALUE,    // use paging file
    NULL,                    // default security
    PAGE_READWRITE,          // read/write access
    0,                       // maximum object size (high-order DWORD)
    BUF_SIZE,                // maximum object size (low-order DWORD)
    szName);                 // name of mapping object

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

Upvotes: 0

Views: 5003

Answers (1)

Jerry Coffin
Jerry Coffin

Reputation: 490118

Right now, you're telling CreateFileMapping to create the mapping in the page file, then use the name of your existing file as the name of the file mapping.

The name you give for a file mapping object can have local\ or global\ as a prefix, but can't contain any other back-slashes.

If you want to map your pre-existing file as a shared memory region, you'd do something on this general order:

TCHAR szName[]=TEXT("C:\\Users\\joe\\shmfolder\\shmfile.txt"); 
TCHAR szMapName[]=TEXT("SharedFile"); 

HANDLE file = CreateFile(szName, ...);
HANDLE mapping = CreateFileMapping(file, ..., szMapName);

Upvotes: 5

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