Difference between OPEN_ALWAYS and CREATE_ALWAYS in CreateFile() of Windows API

Question

Can anyone explain what the difference is between the creation dispositions OPEN_ALWAYS and CREATE_ALWAYS of the CreateFile() function of the windows API?

To me it seems that they both simply 'create the file if it does not already exist'.

Answer 1

CREATE_ALWAYS also truncates the contents if the file already exists. On the other hand, OPEN_ALWAYS will not clobber an already existing file.

Here's how the different values work in tabular form:

                         |                    When the file...
This argument:           |             Exists            Does not exist
-------------------------+------------------------------------------------------
CREATE_ALWAYS            |            Truncates             Creates
CREATE_NEW         +-----------+        Fails               Creates
OPEN_ALWAYS     ===| does this |===>    Opens               Creates
OPEN_EXISTING      +-----------+        Opens                Fails
TRUNCATE_EXISTING        |            Truncates              Fails