Nasty gotcha: STGM_READ | STGM_WRITE does not grant read/write access

Raymond Chen

You might think that if you want to get read/write access, you could pass STGM_READ | STGM_WRITE. You would be wrong. You have to pass STGM_READ­WRITE. The three flags STGM_READ, STGM_WRITE, and STGM_READ­WRITE are mutually exclusive. If you try to combine them, you get a weird mess. In particular, since the numerical value of STGM_READ is zero, passing STGM_READ | STGM_WRITE is numerically equivalent to passing STGM_WRITE, which grants write-only access. The documentation for the STGM_* constants specifically says “It is not valid to use more than one element from a single group,” and STGM_READ and STGM_WRITE belong to the Access group (as does STGM_READ­WRITE). These values date back to the days of MS-DOS, where function 3Dh (Open File) passed an access mode in the AL register.

The bottom three bits specified the requested access (0 = read-only, 1 = write-only, 2 = read/write), and the remaining bits were reserved. Later, when networking support was added in approximately MS-DOS 3.5, three more bits were pressed into service:

Sharing modes were 0 = compatibility mode, 1 = deny all, 2 = deny write, 3 = deny read, 4 = deny none.

These values were carried forward into Windows as flags to the Open­File function:

These flags were then carried forward into the STGM constants with the same numerical values.

Raymond Chen

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