enumeration constants specify the conditions for performing the commit
operation in the IStorage::Commit
Defined in the IOLETypes
typedef enum tagSTGC
STGC_DEFAULT = 0,
STGC_OVERWRITE = 1,
STGC_ONLYIFCURRENT = 2,
STGC_DANGEROUSLYCOMMITMERELYTODISKCACHE = 4
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.
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.
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.
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.
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.
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