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APT Cache File Format - Chapter 2
Structures
This is the first item in the file.
struct Header
{
// Signature information
unsigned long Signature;
short MajorVersion;
short MinorVersion;
bool Dirty;
// Size of structure values
unsigned short HeaderSz;
unsigned short PackageSz;
unsigned short PackageFileSz;
unsigned short VersionSz;
unsigned short DependencySz;
unsigned short ProvidesSz;
unsigned short VerFileSz;
// Structure counts
unsigned long PackageCount;
unsigned long VersionCount;
unsigned long DependsCount;
unsigned long PackageFileCount;
unsigned long MaxVerFileSize;
// Offsets
unsigned long FileList; // PackageFile
unsigned long StringList; // StringItem
// Allocation pools
struct
{
unsigned long ItemSize;
unsigned long Start;
unsigned long Count;
} Pools[7];
// Package name lookup
unsigned long HashTable[512]; // Package
};
- Signature
- This must contain the hex value 0x98FE76DC which is designed to verify
that the system loading the image has the same byte order and byte size as
the system saving the image
- MajorVersion
- MinorVersion
- These contain the version of the cache file, currently 0.2.
- Dirty
- Dirty is true if the cache file was opened for reading, the client expects
to have written things to it and have not fully synced it. The file should
be erased and rebuilt if it is true.
- HeaderSz
- PackageSz
- PackageFileSz
- VersionSz
- DependencySz
- VerFileSz
- ProvidesSz
- *Sz contains the sizeof() that particular structure. It is used as an
extra consistancy check on the structure of the file.
If any of the size values do not exactly match what the client expects then
the client should refuse the load the file.
- PackageCount
- VersionCount
- DependsCount
- PackageFileCount
- These indicate the number of each structure contianed in the cache.
PackageCount is especially usefull for generating user state structures.
See Package::Id for more info.
- MaxVerFileSize
- The maximum size of a raw entry from the original Package file
(ie VerFile::Size) is stored here.
- FileList
- This contains the index of the first PackageFile structure. The PackageFile
structures are singely linked lists that represent all package files that
have been merged into the cache.
- StringList
- This contains a list of all the unique strings (string item type strings) in
the cache. The parser reads this list into memory so it can match strings
against it.
- Pools
- The Pool structures manage the allocation pools that the generator uses.
Start indicates the first byte of the pool, Count is the number of objects
remaining in the pool and ItemSize is the structure size (alignment factor)
of the pool. An ItemSize of 0 indicates the pool is empty. There should be
the same number of pools as there are structure types. The generator
stores this information so future additions can make use of any unused pool
blocks.
- HashTable
- HashTable is a hash table that provides indexing for all of the packages.
Each package name is inserted into the hash table using the following has
function:
unsigned long Hash(string Str)
{
unsigned long Hash = 0;
for (const char *I = Str.begin(); I != Str.end(); I++)
Hash += *I * ((Str.end() - I + 1));
return Hash % _count(Head.HashTable);
}
By iterating over each entry in the hash table it is possible to iterate over
the entire list of packages. Hash Collisions are handled with a singely linked
list of packages based at the hash item. The linked list contains only
packages that macth the hashing function.
This contians information for a single unique package. There can be any
number of versions of a given package. Package exists in a singly
linked list of package records starting at the hash index of the name in
the Header->HashTable.
struct Pacakge
{
// Pointers
unsigned long Name; // Stringtable
unsigned long VersionList; // Version
unsigned long TargetVer; // Version
unsigned long CurrentVer; // Version
unsigned long TargetDist; // StringTable (StringItem)
unsigned long Section; // StringTable (StringItem)
// Linked lists
unsigned long NextPackage; // Package
unsigned long RevDepends; // Dependency
unsigned long ProvidesList; // Provides
// Install/Remove/Purge etc
unsigned char SelectedState; // What
unsigned char InstState; // Flags
unsigned char CurrentState; // State
// Unique ID for this pkg
unsigned short ID;
unsigned long Flags;
};
- Name
- Name of the package.
- VersionList
- Base of a singely linked list of version structures. Each structure
represents a unique version of the package. The version structures
contain links into PackageFile and the original text file as well as
detailed infromation about the size and dependencies of the specific
package. In this way multiple versions of a package can be cleanly handled
by the system. Furthermore, this linked list is guarenteed to be sorted
from Highest version to lowest version with no duplicate entries.
- TargetVer
- CurrentVer
- This is an index (pointer) to the sub version that is being targeted for
upgrading. CurrentVer is an index to the installed version, either can be
0.
- TargetDist
- This indicates the target distribution. Automatic upgrades should not go
outside of the specified dist. If it is 0 then the global target dist should
be used. The string should be contained in the StringItem list.
- Section
- This indicates the deduced section. It should be "Unknown" or the section
of the last parsed item.
- NextPackage
- Next link in this hash item. This linked list is based at Header.HashTable
and contains only packages with the same hash value.
- RevDepends
- Reverse Depends is a linked list of all dependencies linked to this package.
- ProvidesList
- This is a linked list of all provides for this package name.
- SelectedState
- InstState
- CurrentState
- These corrispond to the 3 items in the Status field found in the status
file. See the section on defines for the possible values.
SelectedState is the state that the user wishes the package to be
in.
InstState is the installation state of the package. This normally
should be Ok, but if the installation had an accident it may be otherwise.
CurrentState indicates if the package is installed, partially installed or
not installed.
- ID
- ID is a value from 0 to Header->PackageCount. It is a unique value assigned
by the generator. This allows clients to create an array of size PackageCount
and use it to store state information for the package map. For instance the
status file emitter uses this to track which packages have been emitted
already.
- Flags
- Flags are some usefull indicators of the package's state.
This contians information for a single package file. Package files are
referenced by Version structures. This is a singly linked list based from
Header.FileList
struct PackageFile
{
// Names
unsigned long FileName; // Stringtable
unsigned long Archive; // Stringtable
unsigned long Component; // Stringtable
unsigned long Version; // Stringtable
unsigned long Origin; // Stringtable
unsigned long Label; // Stringtable
unsigned long Architecture; // Stringtable
unsigned long Size;
// Linked list
unsigned long NextFile; // PackageFile
unsigned short ID;
unsigned long Flags;
time_t mtime; // Modification time
};
- FileName
- Refers the the physical disk file that this PacakgeFile represents.
- Archive
- Component
- Version
- Origin
- Label
- Architecture
- NotAutomatic
- This is the release information. Please see the files document for a
description of what the release information means.
- Size
- Size is provided as a simple check to ensure that the package file has not
been altered.
- ID
- See Package::ID.
- Flags
- Provides some flags for the PackageFile, see the section on defines.
- mtime
- Modification time for the file at time of cache generation.
This contians the information for a single version of a package. This is a
singley linked list based from Package.Versionlist.
The version list is always sorted from highest version to lowest version by
the generator. Also there may not be any duplicate entries in the list (same
VerStr).
struct Version
{
unsigned long VerStr; // Stringtable
unsigned long Section; // StringTable (StringItem)
unsigned long Arch; // StringTable
// Lists
unsigned long FileList; // VerFile
unsigned long NextVer; // Version
unsigned long DependsList; // Dependency
unsigned long ParentPkg; // Package
unsigned long ProvidesList; // Provides
unsigned long Size;
unsigned long InstalledSize;
unsigned long Hash;
unsigned short ID;
unsigned char Priority;
};
- VerStr
- This is the complete version string.
- FileList
- References the all the PackageFile's that this version came out of. FileList
can be used to determine what distribution(s) the Version applies to. If
FileList is 0 then this is a blank version. The structure should also have
a 0 in all other fields excluding VerStr and Possibly NextVer.
- Section
- This string indicates which section it is part of. The string should be
contained in the StringItem list.
- Arch
- Architecture the package was compiled for.
- NextVer
- Next step in the linked list.
- DependsList
- This is the base of the dependency list.
- ParentPkg
- This links the version to the owning package, allowing reverse dependencies
to determine the package.
- ProvidesList
- Head of the linked list of Provides::NextPkgProv, forward provides.
- Size
- InstalledSize
- The archive size for this version. For debian this is the size of the .deb
file. Installed size is the uncompressed size for this version
- Hash
- This is a characteristic value representing this package. No two packages
in existance should have the same VerStr and Hash with different contents.
- ID
- See Package::ID.
- Priority
- This is the parsed priority value of the package.
Dependency contains the information for a single dependency record. The records
are split up like this to ease processing by the client. The base of list
linked list is Version.DependsList. All forms of dependencies are recorded
here including Conflicts, Suggests and Recommends.
Multiple depends on the same package must be grouped together in
the Dependency lists. Clients should assume this is always true.
struct Dependency
{
unsigned long Version; // Stringtable
unsigned long Package; // Package
unsigned long NextDepends; // Dependency
unsigned long NextRevDepends; // Reverse dependency linking
unsigned long ParentVer; // Upwards parent version link
// Specific types of depends
unsigned char Type;
unsigned char CompareOp;
unsigned short ID;
};
- Version
- The string form of the version that the dependency is applied against.
- Package
- The index of the package file this depends applies to. If the package file
does not already exist when the dependency is inserted a blank one (no
version records) should be created.
- NextDepends
- Linked list based off a Version structure of all the dependencies in that
version.
- NextRevDepends
- Reverse dependency linking, based off a Package structure. This linked list
is a list of all packages that have a depends line for a given package.
- ParentVer
- Parent version linking, allows the reverse dependency list to link
back to the version and package that the dependency are for.
- Type
- Describes weather it is depends, predepends, recommends, suggests, etc.
- CompareOp
- Describes the comparison operator specified on the depends line. If the high
bit is set then it is a logical or with the previous record.
- ID
- See Package::ID.
Provides handles virtual packages. When a Provides: line is encountered
a new provides record is added associating the package with a virtual
package name. The provides structures are linked off the package structures.
This simplifies the analysis of dependencies and other aspects A provides
refers to a specific version of a specific package, not all versions need to
provide that provides.
There is a linked list of provided package names started from each
version that provides packages. This is the forwards provides mechanism.
struct Provides
{
unsigned long ParentPkg; // Package
unsigned long Version; // Version
unsigned long ProvideVersion; // Stringtable
unsigned long NextProvides; // Provides
unsigned long NextPkgProv; // Provides
};
- ParentPkg
- The index of the package that head of this linked list is in. ParentPkg->Name
is the name of the provides.
- Version
- The index of the version this provide line applies to.
- ProvideVersion
- Each provides can specify a version in the provides line. This version allows
dependencies to depend on specific versions of a Provides, as well as allowing
Provides to override existing packages. This is experimental.
- NextProvides
- Next link in the singly linked list of provides (based off package)
- NextPkgProv
- Next link in the singly linked list of provides for 'Version'.
VerFile associates a version with a PackageFile, this allows a full
description of all Versions in all files (and hence all sources) under
consideration.
struct pkgCache::VerFile
{
unsigned long File; // PackageFile
unsigned long NextFile; // PkgVerFile
unsigned long Offset;
unsigned short Size;
}
- File
- The index of the package file that this version was found in.
- NextFile
- The next step in the linked list.
- Offset
- Size
- These describe the exact position in the package file for the section from
this version.
StringItem is used for generating single instances of strings. Some things
like Section Name are are usefull to have as unique tags. It is part of
a linked list based at Header::StringList.
struct StringItem
{
unsigned long String; // Stringtable
unsigned long NextItem; // StringItem
};
- String
- The string this refers to.
- NextItem
- Next link in the chain.
All strings are simply inlined any place in the file that is natural for the
writer. The client should make no assumptions about the positioning of
strings. All stringtable values point to a byte offset from the start of the
file that a null terminated string will begin.
Several structures use variables to indicate things. Here is a list of all
of them.
#define pkgDEP_Depends 1
#define pkgDEP_PreDepends 2
#define pkgDEP_Suggests 3
#define pkgDEP_Recommends 4
#define pkgDEP_Conflicts 5
#define pkgDEP_Replaces 6
#define pkgOP_OR 0x10
#define pkgOP_LESSEQ 0x1
#define pkgOP_GREATEREQ 0x2
#define pkgOP_LESS 0x3
#define pkgOP_GREATER 0x4
#define pkgOP_EQUALS 0x5
The lower 4 bits are used to indicate what operator is being specified and
the upper 4 bits are flags. pkgOP_OR indicates that the next package is
or'd with the current package.
#define pkgSTATE_Unkown 0
#define pkgSTATE_Install 1
#define pkgSTATE_Hold 2
#define pkgSTATE_DeInstall 3
#define pkgSTATE_Purge 4
#define pkgSTATE_Ok 0
#define pkgSTATE_ReInstReq 1
#define pkgSTATE_Hold 2
#define pkgSTATE_HoldReInstReq 3
#define pkgSTATE_NotInstalled 0
#define pkgSTATE_UnPacked 1
#define pkgSTATE_HalfConfigured 2
#define pkgSTATE_UnInstalled 3
#define pkgSTATE_HalfInstalled 4
#define pkgSTATE_ConfigFiles 5
#define pkgSTATE_Installed 6
#define pkgFLAG_Auto (1 << 0)
#define pkgFLAG_New (1 << 1)
#define pkgFLAG_Obsolete (1 << 2)
#define pkgFLAG_Essential (1 << 3)
#define pkgFLAG_ImmediateConf (1 << 4)
Zero is used for unparsable or absent Priority fields.
#define pkgPRIO_Important 1
#define pkgPRIO_Required 2
#define pkgPRIO_Standard 3
#define pkgPRIO_Optional 4
#define pkgPRIO_Extra 5
#define pkgFLAG_NotSource (1 << 0)
#define pkgFLAG_NotAutomatic (1 << 1)
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APT Cache File Format
$Id: cache.sgml,v 1.7 1999/05/23 22:55:55 jgg Exp $
Jason Gunthorpe jgg@debian.org