Question

I have a problem understanding how to use the API function CreateFileMapping(...).

I’ve been trying to map a small file and then reverse it’s content. It has only about 1 Kb, but I keep getting the “Not enough memory” error.

I have opened the file by calling CreateFile and got it’s size with GetFileSize.

Then I call:

CreateFileMapping(fileHandle, 
                  NULL, 
                  PAGE_READWRITE | SEC_RESERVE, 
                  fileSize + 1, 
                  fileSize + 1, 
                  NULL);

I suspect that the problem is with passing the fileSize + 1 as dwFileOffsetHighand dwFileOffsetLow, but I have a hard time understanding what should I pass to it instead.

Any hints are greatly appreciated!

Answer 1

dwFileOffsetHigh and dwFileOffsetLow are two 32 bit values that are combined to form a single 64 bit value. This function was implemented this way because it pre-dates widespread compiler support for 64 bit values.

I think your misunderstanding is in believing that the high and low mean upper and lower limits.

In your case your value (assuming fileSize is around 1KB) is nowhere near requiring 64 bits so you should pass fileSize+1 for dwFileOffsetLow and 0 for dwFileOffsetHigh.

However, if you are attempting to map the entire file you can simply pass 0 for both parameters.

From the documentation:

dwMaximumSizeHigh [in]

The high-order DWORD of the maximum size of the
file mapping object.

dwMaximumSizeLow [in]

The low-order DWORD of the maximum size of the
file mapping object.

If this parameter and dwMaximumSizeHigh are 0 (zero), the maximum size
of the file mapping object is equal to the current size of the file
that hFile identifies.

An attempt to map a file with a length of 0 (zero) fails with an error
code of ERROR_FILE_INVALID. Applications should test for files with a
length of 0 (zero) and reject those files.