STGC

The STGC enumeration constants specify the conditions for performing the commit operation in the IStorage::Commit and IStream::Commit methods.

Defined in the IOLETypes pseudo-interface (oletyp.idl).

typedef enum tagSTGC
{
STGC_DEFAULT = 0,
STGC_OVERWRITE = 1,
STGC_ONLYIFCURRENT = 2,
STGC_DANGEROUSLYCOMMITMERELYTODISKCACHE = 4
} STGC;

Elements
STGC_DEFAULT

Indicates that none of the other values apply. You can specify this condition or some combination of the other three. You would use this value mainly to make your code more readable.

STGC_OVERWRITE

Indicates that the commit operation can overwrite existing data to reduce overall space requirements. This value is not recommended for typical usage because it is not as robust as the default case. In this case, it is possible for the commit to fail after the old data is overwritten but before the new data is completely committed. Then, neither the old version nor the new version of the storage object will be intact.

You can use this value in cases where:

the user has indicated a willingness to risk losing the data
the low memory save sequence will be used to safely save the storage object to a smaller file
a previous commit returned STG_E_MEDIUMFULL but overwriting the existing data would provide enough space to commit changes to the storage object

Note that the commit operation checks for adequate space before any overwriting occurs. Thus, even with this value specified, if the commit operation fails due to space requirements, the old data will remain safe. The case where data loss can occur is when the commit operation fails due to some reason other than lack of space and the STGC_OVERWRITE value was specified.

STGC_ONLYIFCURRENT

Prevents multiple users of a storage object from overwriting one another's changes. The commit operation occurs only if there have been no changes to the saved storage object since the user most recently opened the storage object. Thus, the saved version of the storage object is the same version that the user has been editing. If other users have changed the storage object, the commit operation fails and returns the STG_E_NOTCURRENT value. You can override this behavior by calling the Commit method again using the STGC_DEFAULT value.

STGC_DANGEROUSLYCOMMITMERELYTODISKCACHE

Commits the changes to a write-behind disk cache, but does not save the cache to the disk. In a write-behind disk cache, the operation that writes to disk actually writes to a disk cache thus increasing performance. The cache is eventually written to the disk but usually not until after the write operation has already returned. The performance increase comes at the expense of an increased risk of losing data if a problem occurs before the cache is saved and the data in the cache is lost.

If you do not specify this value, then committing changes to root level storage objects is robust even if a disk cache is used. The two-phase commit process ensures that data is stored on the disk and not just to the disk cache.

Comments
You can specify STGC_DEFAULT or some combination of the other three values. Typically, you would use STGC_ONLYIFCURRENT to protect the storage object in cases where more than one user can edit the object simultaneously.