.PPT File Format – User edits and the persist directory

PowerPoint 97–2003 presentation

PowerPoint can save by appending to the stream. An edit adds new top-level records, a persist directory and a user edit atom, and leaves the old records where they are. So the stream holds both current and out-of-date records, and a reader finds the current ones by following references, not by reading from the start.

UserEditAtom

Record type 0x0FF5, recVer 0, recInstance 0, recLen 0x1C or 0x20.

OffsetSizeFieldMeaning
84lastSlideIdRefThe last slide viewed, for the newest edit; otherwise ignored.
122versionA build number; should be 0 and is ignored.
141minorVersion0.
151majorVersion3.
164offsetLastEditThe offset of the previous UserEditAtom, less than this record’s. Zero means there is none.
204offsetPersistDirectoryThe offset of this edit’s PersistDirectoryAtom.
244docPersistIdRefThe persist id of the DocumentContainer. Always 1.
284persistIdSeedGreater than or equal to every persist id in the file.
322lastViewThe view last used: 1 slide, 2 slide master, 3 notes, 7 outline, 8 slide sorter, 11 slide show, and others.
342unusedIgnored.
364encryptSessionPersistIdRefOptional: the persist id of the CryptSession10Container of an encrypted file.

PersistDirectoryAtom

Record type 0x1772. Its body is a list of PersistDirectoryEntry structures:

  1. 0persistId the first id, the low 20 bits of a 32-bit value20 bits
  2. 0cPersist how many offsets follow, the high 12 bits12 bits
  3. 4rgPersistOffset one 32-bit stream offset for each id, persistId, persistId + 1, …4 bytes each
A PersistDirectoryEntry.

A persist id is a number that stands for a top-level record: slides, the document container and so on refer to each other by it, not by offset, so an edit can move a record and only change the directory.

Finding the live records

  1. Read the CurrentUserAtom and go to offsetToCurrentEdit in the PowerPoint Document stream.
  2. Read the UserEditAtom there. Go to its offsetPersistDirectory and read that PersistDirectoryAtom.
  3. Go to offsetLastEdit and repeat, until it is zero.
  4. Build the directory from the oldest edit forward, so that a newer edit’s offset for an id replaces an older one.
  5. Look up docPersistIdRef in the directory: that is the DocumentContainer.
  6. Every record the presentation refers to by persist id is found through the directory the same way.

Records that nothing reaches this way are dead: leftovers from earlier saves. Their contents are not specified and must be ignored.