I have a binary file, when I opend it, I used ::StgOpenStorage with STGM_READWRITE | STGM_SHARE_DENY_WRITE | STGM_TRANSACTED mode to get a root storage named rootStorage. And then, I used rootStorage.OpenStream with STGM_READWRITE | STGM_SHARE_EXCLUSIVE mode to get a substream named subStream.
Next, I wrote some data with subStream.Wirte(...), and called subStream.Commit(STGC_DEFAULT), but it just couldn't write the data in the file.
And I tried rootStorage.Commit(STGC_DEFAULT) also, the data can be written.
But when I used UltraCompare Professional - Binary Compare to compare the original file with the file I opend, a lot of extra data had been written at the end of the file. The extra data seems to be from the beginning of the file.
I just want to write a little data into the file while opening it. What should I do?
I had found out why there are extra data on my file.
1. Why should I use IStorage.Commit()
I used STGM_READWRITE mode to create a storage. It's called transacted mode. In transacted mode, changes are accumulated and are not reflected in the storage object until an explicit commit operation is done. So I need to call rootStorage.Commit().
2. Why there are extra data after calling IStorage.Commit(STGC_DEFAULT)
According to this website:
The OLE-provided compound files use a two phase commit process unless STGC_OVERWRITE is specified in the grfCommitFlags parameter. This two-phase process ensures the robustness of data in case the commit operation fails. First, all new data is written to unused space in the underlying file. If necessary, new space is allocated to the file. Once this step has been successfully completed, a table in the file is updated using a single sector write to indicate that the new data is to be used in place of the old. The old data becomes free space to be used at the next commit. Thus, the old data is available and can be restored in case an error occurs when committing changes. If STGC_OVERWRITE is specified, a single phase commit operation is used.