VPK File Format

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The VPK file format is a new package format used by Left 4 Dead to store game related content.

Conception

Prior to Left 4 Dead, typical Source engine games stored their content in GCF files. Executable files, modifiable files (e.g. configuration files) and custom content were copied and stored locally on the user's hard drive. Possibly brought on by poor performance, the NCF file format was introduced and all game content was copied entirely to the hard drive. This, however, introduced a new problem. Source engine materials and models are stored in thousands of small files and it would be expensive to continuously open and close these files. The solution was the conception of the VPK file format which is used to store Left 4 Dead materials, models and particles in a handful of files which can be quickly accessed.

Design Decisions

Preload Data

In order to efficiently access small or critical files, the beginning of each file can optionally be stored in the VPK directory. In practice, this seems to be limited to the first 1000 bytes of Source engine materials (VMT files) which are typically only a few hundred bytes in size.

Multiple Archives

Previous Source engine games that had been distributed by the more advanced GCF file format had the luxury of internally fragmenting new and updated files. This meant that new and updated files could be efficiently downloaded and saved with minimal bandwidth and disk IO. Because the new VPK files are independent of distribution (Left 4 Dead is distributed by NCF files and Steam knows nothing of the VPK file format), their content is split up over multiple archives that seem to be limited to about 32 MB in size. Because of this, when a file in a specific file is updated, only the VPK archive that contains the file needs to be updated. Additionally, new files can be downloaded to their own individual archives. This is why most of the newer archives are small in size; their contents are limited the the files added in a single update.

File Format

A VPK package is actually spread out over multiple files sharing the same extension. The directory is stored in a specific file called <name>_dir.vpk and the content is spread over several additional archive files called <name>_*.vpk (where * is the zero based archive index). Consequentially, there are two file formats:

Directory File Format

Header

Originally, the VPK file had no header or identifier. This changed when the June 25, 2009 Left 4 Dead update was released adding support for third party campaigns. VPK directory files created after this date have the following header:

struct VPKHeader
{
	unsigned int Signature;
	unsinged int Version;
	unsigned int DirectoryLength;
};
Variable Description
Signature Identifying signature. Always 0x55aa1234.
Version Version number. Always 1.
DirectoryLength The length of the directory. If the file data is not stored in archives, its offset is the offset of the start of the directory plus this number.

One can check if a VPK file was created before the update by checking if the first four bytes do not match the above signature.

Directory

The format of the directory is a little unorthodox. It consists of a tree three levels deep that seems to be structured for file size. The first level of the tree consists of file extensions (e.g. vmt, vtf and mdl), the second level consists of directory paths (e.g. materials/brick, materials/decals/asphalt and models/infected), and the third level consists of file names, file information and preload data. Each tree node begins with a null terminated ASCII string and empty strings are used to signify the end of a parent node. Pseudo-code to read the directory might look something like:

ReadString(file)
	string = ""
	while true
		char = ReadChar(file)
		if char = null
			return string
		string = string + char
ReadDirectory(file)
	while true
		extension = ReadString(file)
		if extension = ""
			break
		while true
			path = ReadString(file)
			if path = ""
				break
			while true
				file = ReadString(file)
				if file = ""
					break
				ReadFileInformationAndPreloadData(file)

Immediately following the null terminator for the file name is the following structure:

struct VPKDirectoryEntry
{
	unsigned int CRC;
	unsigned short PreloadBytes;
	unsigned short ArchiveIndex;
	unsigned int EntryOffset;
	unsigned int EntryLength;
	unsigned short Terminator;
};
Variable Description
CRC A 32bit CRC of the file's data.
PreloadBytes The number of preload bytes.
ArchiveIndex The zero based index of the archive this file's data is contained in. If 0x7fff, the data follows the directory.
EntryOffset If ArchiveIndex is 0x7fff, the offset of the file data relative to the end of the directory (see the header for more details). Otherwise, the offset of the data from the start of the specified archive.
EntryLength If zero, the entire file is stored in the preload data, otherwise, the number of bytes stored starting at EntryOffset.
Terminator Always 0xffff.

If a file contains preload data, the preload data immediately follows the above structure. The entire size of a file is PreloadBytes + EntryLength.

Archive File Format

VPK Archives store raw file data. They have no identifying header and know nothing of their contents. Though not necessary, the raw file data is typically tightly packed.

See also

External Links