Abandon all hope ye who try to parse this file format.
—Tim Sweeney, Unreal Packages
Of the few resources out there, I couldn't find any which actually go through an existing package step by step, which I think would have helped me a lot when starting out. As a result, I've decided to write this brief guide in the hope that it may help someone.
As stated, there are a few existing guides, some parts of which I have copied/paraphrased here; the rest I have written myself.
The package used in this example is CTF-Face, aka "Facing Worlds".
The UT package format has the following structure:
|Package Header||Contains information about the package, such as the size and offset of tables within the package.|
|Name Table||Contains a list of human-readable Unreal names (which correspond to the UnrealScript "Name" data type).|
|Import Table||Contains a list of objects in other packages which this package refers to.|
|Export Table||Contains a list of objects contained in (aka "exported by") this package.|
|Data||The actual package data itself (textures, sounds, brushes, scripts, etc).|
Long int/DWORD data types are signed, 4 bytes long, and stored in little-endian format.
The package header always begins at offset 0. If the package version is less than 68, "Heritage" count/offset values may also be present instead of the GUID.
|4||0x04||4||File Version||Version of the engine which saved the file.|
|8||0x08||4||Package Flags||Bitflags describing the package (see Package Flags section).|
|12||0x0C||4||Name Count||Number of names stored in the name table. Always >= 0.|
|16||0x10||4||Name Offset||Offset into the file of the name table, in bytes.|
|20||0x14||4||Export Count||Number of exported objects in the export table. Always >= 0.|
|24||0x18||4||Export Offset||Offset into the file of the export table, in bytes.|
|28||0x1C||4||Import Count||Number of imported objects in the import table. Always >= 0.|
|32||0x20||4||Import Offset||Offset into the file of the import table, in bytes.|
|36||0x24||16||GUID||Globally unique identifier; a unique number used for caching and avoiding package naming/version conflicts.|
|-||-||4||Generation n Export Count||-|
|-||-||4||Generation n Name Count||-|
The package header for CTF-Face is shown below. Hover over the table rows to highlight the corresponding bytes within the raw header data.
|File Version||0x44000000 → 68|
|Package Flags||0x01000000 → PKG_AllowDownload|
|Name Count||0x43050000 → 1,347|
|Name Offset||0x40000000 → 64|
|Export Count||0x31040000 → 1,073|
|Export Offset||0x2AAD0E00 → 961,834|
|Import Count||0x8F000000 → 143|
|Import Offset||0xDBA70E00 → 960,475|
|Generation Count||0x01000000 → 1|
|Export Count||0x31040000 → 1,073|
|Name Count||0x43050000 → 1,347|
*When downloading a package in-game, files are saved using the GUID as their filename. With each of the four long int values read in little-endian format, CTF-Face would be saved as "68722E6411D38195E0006BA2D2321081.uxx".
The name table contains all unique object names within the package. The structure is as follows:
|Object Name Length||1||String length of the null-terminated object name (including the null byte), in bytes.|
|Object Name||n||The name of the object. Objects may share names (e.g. "Texture").|
|Object Flags||4||Bitflags describing the object (see Object Flags section).|
As indicated in the package header, the name table for CTF-Face begins at byte 64. The first five values are shown below:
|#||Offset||Object Name Length||Object Name||Object Flags|
The import table contains n entries, where n is the import count specified in byte 28 of the header. Each table entry is structured as follows:
|Class Package||Compact Index*||Name table index for the package which this object's class object resides in.|
|Class Name||Compact Index||Name table index for this object's class.|
|Package Index**||Long Int||The index of the package this object resides in. This can be an index into the import table itself, or NULL.|
|Object Name||Compact Index||Name table index for this object.|
*See the Compact Index section to understand how values are ascertained from this data type.
**See Object References.
The first five entries of CTF-Face's import table are as follows:
|#||Class Package||Class Name||Package Index||Object Name|
|1||0x610A||673 → "Core"||0x6B06||427 → "Package"||0x00000000||NULL||0x7602||182 → "Engine"|
|2||0x610A||673 → "Core"||0x6B06||427 → "Package"||0x00000000||NULL||0x440E||900 → "SkyCity"|
|3||0x610A||673 → "Core"||0x6B06||427 → "Package"||0x00000000||NULL||0x4112||1,153 → "ShaneChurch"|
|4||0x610A||673 → "Core"||0x700A||688 → "Class"||0xFFFFFFFF||-1 → 0 → "Engine"||0x0C||12 → "Brush"|
|5||0x610A||673 → "Core"||0x6B06||427 → "Package"||0xFEFFFFFF||-2 → 1 → "SkyCity"||0x3B||59 → "Base"|
As with the import table, the export table contains n entries, where n is the export count specified in byte 20 of the header. Each table entry is structured as follows:
|Class Index||Compact Index||Points to the class object describing the class of this object.|
|Super Index||Compact Index||If this is a field (a struct, class, property, or another field subclass), points to the superfield object of the field.|
|Package Index||Long Int||Points to the package object describing the package this object resides in.|
|Object Name||Compact Index||This object's name.|
|Object Flags||Long Int||Bitflags describing the object.|
|Serial Size||Compact Index||Size of the object's serialised data, in bytes.|
|Serial Offset||Compact Index||If >= 0, offset into this file of the start of the object's serialised data.|
The first three entries of CTF-Face's export table are as follows:
|#||Class Index||Super Index||Package Index||Object Name||Object Flags||Serial Size||Serial Offset|
|1||0xF801||-120 → "LevelInfo"||0x00||NULL||0x00000000||NULL||0x6F02||175 → "LevelInfo0"||0x01000702||
||0x4E04||270 bytes||0x6BB302||19,691 or 0x4CEB|
|2||0x8E||-14 → "Light"||0x00||NULL||0x00000000||NULL||0x7903||249 → "Light0"||0x01000702||
||0x5001||80 bytes||0x79B702||19,961 or 0x4DF9|
|3||0x8E||-14 → "Light"||0x00||NULL||0x00000000||NULL||0x7C03||252 → "Light5"||0x01000702||
||0x5001||80 bytes||0x49B902||20,041 or 0x4E49|
Packages may have several "flags" or properties, as detailed in the table below. The package flags value for CTF-Face is 0x01 which corresponds to the PKG_AllowDownload flag.
Allow downloading package.
[This should be] disabled for game publisher distribution (so that people aren't able to download retail files freely).
|PKG_ClientOptional||0x0002||Purely optional for clients.|
Only needed on the server side.
For example, the IpDrv.u package is tagged as ServerSideOnly, because it contains classes like the server-uplink which only need to run on the server, and will be frequently updated.
|PKG_BrokenLinks||0x0008||Loaded from linker with broken import links.|
|PKG_Need||0x8000||Client needs to download this package.|
A package's flags can be read by performing a bitwise AND on its flag value with each of the pre-defined flag values. For example, if a package had a package flag of 11 (or 0x0B in hex):
0x0B & 0x01 // 1 - PKG_AllowDownload flag present 0x0B & 0x02 // 2 - PKG_ClientOptional flag present 0x0B & 0x04 // 0 - PKG_ServerSideOnly flag NOT present 0x0B & 0x08 // 1 - PKG_BrokenLinks flag present
The PKG_Unsecure and PKG_Need flags aren't present as their values are greater than 0x0B.
As with the package file itself, objects (i.e. the data contained within the package) can have several flags as well:
|RF_Transactional||0x00000001||Object is transactional (i.e. supports editor undo/redo).|
|RF_Unreachable||0x00000002||Object is not reachable on the object graph.|
|RF_Public||0x00000004||Object is visible outside its package (i.e. can be referenced by external package files).|
|RF_TagImp||0x00000008||Temporary import tag in load/save.|
|RF_TagExp||0x00000010||Temporary export tag in load/save.|
|RF_SourceModified||0x00000020||Modified relative to source files.|
|RF_TagGarbage||0x00000040||Check during garbage collection.|
|RF_NeedLoad||0x00000200||During load, indicates object needs loading.|
|RF_HighlightedName||0x00000400||A hardcoded name which should be syntax-highlighted.|
|RF_InSingularFunc||0x00000800||In a singular function.|
|RF_Suppress||0x00001000||Suppressed log name.|
|RF_InEndState||0x00002000||Within an EndState call.|
|RF_Transient||0x00004000||Can't be saved or loaded.|
|RF_PreLoading||0x00008000||Data is being preloaded from file.|
|RF_LoadForClient||0x00010000||In-file load for client.|
|RF_LoadForServer||0x00020000||In-file load for client.|
|RF_LoadForEdit||0x00040000||In-file load for client.|
|RF_Standalone||0x00080000||Keep object around for editing even if unreferenced.|
|RF_NotForClient||0x00100000||Don't load this object for the game client.|
|RF_NotForServer||0x00200000||Don't load this object for the game server.|
|RF_NotForEdit||0x00400000||Don't load this object for the editor.|
|RF_Destroyed||0x00800000||Object Destroy has already been called.|
|RF_NeedPostLoad||0x01000000||Object needs to be postloaded.|
|RF_HasStack||0x02000000||Has execution stack.|
|RF_Native||0x04000000||Native (UClass only).|
|RF_Marked||0x08000000||Marked (for debugging).|
|RF_DebugPostLoad||0x20000000||For debugging Serialize calls.|
|RF_DebugSerialize||0x40000000||For debugging Serialize calls.|
|RF_DebugDestroy||0x80000000||For debugging Destroy calls.|
Objects within a package may reference other objects. To differentiate between an object in the export table and the import table, a positive value is given for export entries, and negative for the import. To "convert" a negative value to a usable index, remove the negative sign and subtract 1 (e.g. -5 → 5 → 4). This can be done easily using the bitwise NOT operator.
Part of the decoration in the red base uses a rune texture:
The name of this texture is "runeSgn2", part of the "Deco" group within the texture package SkyCity. This object's package index value is -72, meaning it's index #71 in the import table. This is the Deco object, which in turn has a package index of -2, i.e. the SkyCity object. As SkyCity is its own package (SkyCity.utx), this object has a package index of 0 or NULL, i.e. it has no "parent" package.
Tim Sweeney, the format's creator, states
Compact indices exist so that small numbers can be stored efficiently.
Rather than storing small numbers in a fixed 4-byte "long int"/DWORD, the compact index format is variable in size, ranging from 1-5 bytes. A comparison highlighting the difference in size is shown below:
|Value||Long Int/DWORD||Compact Index||Difference|
Converting a value to a compact index is achieved by using 1-2 bits of each byte to store information about the value, and the remaining bits to store the value itself.
The first byte of a compact index is structured as follows:
Bits 6 and 7 are essentially used as flags:
All other bytes, if any, are structured as follows:
The maximum value that can be stored is 0x7FFFFFFF (2,147,483,647) as defined in the Object class.
This example will show how to represent the number -100 as a compact index.
The absolute value, 100, is represented in binary as 01100100. As bits 6 and 7 are required to indicate continuation and the negative sign, the last two bits (0 and 1) have to be moved along to the next byte:
Put back together, the binary value is now 0000000111100100 or 0xE401 in hex, saving two bytes compared to a regular long int value.
Enter a number below to see the value as represented by the compact index format.
Page last updated: 8th March 2020