NBT

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Revision as of 01:21, 6 October 2012 by Dav1d (talk | contribs) (OpenNBT → PyNBT)
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The Named Binary Tag (NBT) file format is an extremely simple, albiet annoying (did we really need yet another format?) structured binary format used by the Minecraft game for a variety of things. Due to this, several third-party utilities now also utilize the format.

Current Uses

The NBT format is currently used in several places, chiefly:

  • In the Protocol as part of Slot Data
  • Multiplayer saved server list (servers.dat).
  • Player data (both single player and multiplayer, one file per player). This includes such things as inventory and location.
  • Saved worlds (both single player and multiplayer).
    • World index file (level.dat) that contains general information (spawn point, time of day, etc...)
    • Chunk data (see Region Files)

Unfortunately, the NBT files you can encounter as a developer will be stored in three different ways, just to make things interesting.

  • Uncompressed,
  • gzip'd,
  • zlib'd (aka DEFLATE with a few bytes extra)

Libraries

There are many, many libraries for manipulating NBT, written in several languages, and often several per language. For example,

Unless you have specific goals or licence requirements, it is extremely recommended to go with one of the existing libraries.

Utilities

Almost every 3rd-party Minecraft application uses NBT on some level. There also exist several dedicated NBT editors, which will likely be useful to you if you are developing an NBT library of your own. These include:

  • NBTEdit (C#, Mono-capable), one of the very first NBT editors.
  • NEINedit (Obj-C), an OS X specific editor.

Specification

The NBT file format is extremely simple, and writing a library capable of reading/writing it is a simple affair, usually taking no more than an hour, depending on your skills and language of choice. There are 11 datatypes supported by this format, and one type used to close compound tags. It is strongly advised to read this entire section (no skimming!) or you will run into issues.


Type ID Type Name Payload Size (Bytes) Description
0 TAG_End 0 This tag serves no purpose but to signify the end of an open TAG_Compound. In most libraries, this type is abstracted away and never seen. TAG_End is not named.
1 TAG_Byte 1 A single signed byte
2 TAG_Short 2 A single signed short
3 TAG_Int 4 A single signed integer
4 TAG_Long 8 A single signed long (typically long long in C/C++)
5 TAG_Float 4 A single IEEE-754 single-precision floating point number
6 TAG_Double 8 A single IEEE-754 double-precision floating point number
7 TAG_Byte_Array ... A length-prefixed array of signed bytes. The prefix is a signed integer (thus 4 bytes)
8 TAG_String ... A length-prefixed UTF-8 string. The prefix is an unsigned short (thus 2 bytes)
9 TAG_List ... A list of nameless tags, all of the same type. The list is prefixed with the Type ID of the items it contains (thus 1 byte), and the length of the list as a signed integer (a further 4 bytes).
10 TAG_Compound ... Effectively a list of a named tags. Order is not guaranteed.
11 TAG_Int_Array ... A length-prefixed array of signed integers. The prefix is a signed integer (thus 4 bytes) and indicates the number of 4 byte integers.


There are five simple things to remember:

  • Everything is in big-endian. Unless you're using Java, you will most likely have to swap it to little-endian. See the Wikipedia article on Endianness.
  • Every NBT file will always begin with a TAG_Compound. No exceptions.
  • Every tag begins with a single byte which is is the Type ID of the following tag.
  • Every tag, except TAG_End, begins with a TAG_String, minus the Type ID. This is the name given to whatever is stored in the tag.
  • The preceding two rules do not apply to the first level of tags within a TAG_List.

For example, here's the example layout of a TAG_Short on disk:

Type ID Length of Name Name Payload
Decoded 2 9 shortTest 32767
On Disk (in hex) 02 00 09 73 68 6F 72 74 54 65 73 74 7F FF

If this TAG_Short had been in a TAG_List, it would have been nothing more than the payload, since the type is implied and tags within the first level of a list are nameless.

Examples

There are two defacto example files used for testing your implementation (test.nbt & bigtest.nbt), originally provided by Markus. The example output provided below was generated using PyNBT's debug-nbt tool.

test.nbt

This first example is an uncompressed "Hello World" NBT example. Should you parse it correctly, you will get a structure similar to the following: 

 TAG_Compound('hello world'): 1 entries
 {
   TAG_String('name'): 'Bananrama'
 }

bigtest.nbt

This second example is an uncompressed test of every available tag. If your program can successfully parse this file, then you've done well. Note that the tags under TAG_List do not have a name, as mentioned above. 

 TAG_Compound('Level'): 11 entries
 {
   TAG_Compound('nested compound test'): 2 entries
   {
     TAG_Compound('egg'): 2 entries
     {
       TAG_String('name'): 'Eggbert'
       TAG_Float('value'): 0.5
     }
     TAG_Compound('ham'): 2 entries
     {
       TAG_String('name'): 'Hampus'
       TAG_Float('value'): 0.75
     }
   }
   TAG_Int('intTest'): 2147483647
   TAG_Byte('byteTest'): 127
   TAG_String('stringTest'): 'HELLO WORLD THIS IS A TEST STRING \xc3\x85\xc3\x84\xc3\x96!'
   TAG_List('listTest (long)'): 5 entires
   {
     TAG_Long(None): 11
     TAG_Long(None): 12
     TAG_Long(None): 13
     TAG_Long(None): 14
     TAG_Long(None): 15
   }
   TAG_Double('doubleTest'): 0.49312871321823148
   TAG_Float('floatTest'): 0.49823147058486938
   TAG_Long('longTest'): 9223372036854775807L
   TAG_List('listTest (compound)'): 2 entires
   {
     TAG_Compound(None): 2 entries
     {
       TAG_Long('created-on'): 1264099775885L
       TAG_String('name'): 'Compound tag #0'
     }
     TAG_Compound(None): 2 entries
     {
       TAG_Long('created-on'): 1264099775885L
       TAG_String('name'): 'Compound tag #1'
     }
   }
   TAG_Byte_Array('byteArrayTest (the first 1000 values of (n*n*255+n*7)%100, starting with n=0 (0, 62, 34, 16, 8, ...))'): [1000 bytes]
   TAG_Short('shortTest'): 32767
 }

servers.dat

The servers.dat file contains a list of multiplayer servers you've added to the game. To mix things up a bit, this file will always be uncompressed. Below is an example of the structure seen in servers.dat. 

 TAG_Compound(''): 1 entries
 {
   TAG_List('servers'): 2 entires
   {
     TAG_Compound(None): 2 entries
     {
       TAG_String('ip'): '199.167.132.229:25620'
       TAG_String('name'): 'Dainz1 - Creative'
     }
     TAG_Compound(None): 2 entries
     {
       TAG_String('ip'): '76.127.122.65:25565'
       TAG_String('name'): 'minstarmin4'
     }
   }
 }

level.dat

This final example is of a single player level.dat, which is compressed using gzip. Notice the player's inventory and general world details such as spawn position, world name, and the game seed. 

 TAG_Compound(''): 1 entries
 {
   TAG_Compound('Data'): 17 entries
   {
     TAG_Byte('raining'): 0
     TAG_Long('RandomSeed'): 3142388825013346304L
     TAG_Int('SpawnX'): 0
     TAG_Int('SpawnZ'): 0
     TAG_Long('LastPlayed'): 1323133681772L
     TAG_Int('GameType'): 1
     TAG_Int('SpawnY'): 63
     TAG_Byte('MapFeatures'): 1
     TAG_Compound('Player'): 24 entries
     {
       TAG_Int('XpTotal'): 0
       TAG_Compound('abilities'): 4 entries
       {
         TAG_Byte('instabuild'): 1
         TAG_Byte('flying'): 1
         TAG_Byte('mayfly'): 1
         TAG_Byte('invulnerable'): 1
       }
       TAG_Int('XpLevel'): 0
       TAG_Int('Score'): 0
       TAG_Short('Health'): 20
       TAG_List('Inventory'): 13 entires
       {
         TAG_Compound(None): 4 entries
         {
           TAG_Byte('Count'): 1
           TAG_Byte('Slot'): 0
           TAG_Short('id'): 24
           TAG_Short('Damage'): 0
         }
         TAG_Compound(None): 4 entries
         {
           TAG_Byte('Count'): 1
           TAG_Byte('Slot'): 1
           TAG_Short('id'): 25
           TAG_Short('Damage'): 0
         }
         TAG_Compound(None): 4 entries
         {
           TAG_Byte('Count'): 1
           TAG_Byte('Slot'): 2
           TAG_Short('id'): 326
           TAG_Short('Damage'): 0
         }
         TAG_Compound(None): 4 entries
         {
           TAG_Byte('Count'): 1
           TAG_Byte('Slot'): 3
           TAG_Short('id'): 29
           TAG_Short('Damage'): 0
         }
         TAG_Compound(None): 4 entries
         {
           TAG_Byte('Count'): 10
           TAG_Byte('Slot'): 4
           TAG_Short('id'): 69
           TAG_Short('Damage'): 0
         }
         TAG_Compound(None): 4 entries
         {
           TAG_Byte('Count'): 3
           TAG_Byte('Slot'): 5
           TAG_Short('id'): 33
           TAG_Short('Damage'): 0
         }
         TAG_Compound(None): 4 entries
         {
           TAG_Byte('Count'): 43
           TAG_Byte('Slot'): 6
           TAG_Short('id'): 356
           TAG_Short('Damage'): 0
         }
         TAG_Compound(None): 4 entries
         {
           TAG_Byte('Count'): 64
           TAG_Byte('Slot'): 7
           TAG_Short('id'): 331
           TAG_Short('Damage'): 0
         }
         TAG_Compound(None): 4 entries
         {
           TAG_Byte('Count'): 20
           TAG_Byte('Slot'): 8
           TAG_Short('id'): 76
           TAG_Short('Damage'): 0
         }
         TAG_Compound(None): 4 entries
         {
           TAG_Byte('Count'): 64
           TAG_Byte('Slot'): 9
           TAG_Short('id'): 331
           TAG_Short('Damage'): 0
         }
         TAG_Compound(None): 4 entries
         {
           TAG_Byte('Count'): 1
           TAG_Byte('Slot'): 10
           TAG_Short('id'): 323
           TAG_Short('Damage'): 0
         }
         TAG_Compound(None): 4 entries
         {
           TAG_Byte('Count'): 16
           TAG_Byte('Slot'): 11
           TAG_Short('id'): 331
           TAG_Short('Damage'): 0
         }
         TAG_Compound(None): 4 entries
         {
           TAG_Byte('Count'): 1
           TAG_Byte('Slot'): 12
           TAG_Short('id'): 110
           TAG_Short('Damage'): 0
         }
       }
       TAG_Short('HurtTime'): 0
       TAG_Short('Fire'): -20
       TAG_Float('foodExhaustionLevel'): 0.0
       TAG_Float('foodSaturationLevel'): 5.0
       TAG_Int('foodTickTimer'): 0
       TAG_Short('SleepTimer'): 0
       TAG_Short('DeathTime'): 0
       TAG_List('Rotation'): 2 entires
       {
         TAG_Float(None): 1151.9342041015625
         TAG_Float(None): 32.249679565429688
       }
       TAG_Float('XpP'): 0.0
       TAG_Float('FallDistance'): 0.0
       TAG_Short('Air'): 300
       TAG_List('Motion'): 3 entires
       {
         TAG_Double(None): -2.9778325794951344e-11
         TAG_Double(None): -0.078400001525878907
         TAG_Double(None): 1.1763942772801152e-11
       }
       TAG_Int('Dimension'): 0
       TAG_Byte('OnGround'): 1
       TAG_List('Pos'): 3 entires
       {
         TAG_Double(None): 256.87499499518492
         TAG_Double(None): 112.62000000476837
         TAG_Double(None): -34.578128612797634
       }
       TAG_Byte('Sleeping'): 0
       TAG_Short('AttackTime'): 0
       TAG_Int('foodLevel'): 20
     }
     TAG_Int('thunderTime'): 2724
     TAG_Int('version'): 19132
     TAG_Int('rainTime'): 5476
     TAG_Long('Time'): 128763
     TAG_Byte('thundering'): 1
     TAG_Byte('hardcore'): 0
     TAG_Long('SizeOnDisk'): 0
     TAG_String('LevelName'): 'Sandstone Test World'
   }
 }

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