Structure Packing

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• The Lua string library, including [string.byte] and [string.char], along with BitwiseOperators, provide very rudimentary functions for doing this.
• [lpack] (5.0/5.1) - a library for packing and unpacking binary data, written in C. (LuizHenriqueDeFigueiredo)
• [struct] (5.0) - another library for packing and unpacking binary data, written in C. (RobertoIerusalimschy)
  • [2] - extensions by Flemming Madsen
  • [Alien] "uses a slightly modified version of Roberto Ierusalimschy’s struct library that can unpack binary blobs (userdata) instead of just strings". notes: LuaList:2009-10/msg00489.html
• [Lunary] (5.1) - another library for packing and unpacking binary data, written in C. ([differences from other libraries]) (Jérôme Vuarand)
• [vstruct] (5.1) - a library for packing and unpacking binary data, written entirely in Lua, using the string library. (Ben Kelly)

Notes/Examples:

• Comparison of above structure packing libraries - LuaList:2010-03/msg00292.html (Sam Roberts)
• ReadWriteFormat - simple pack/unpack implemented with Lua string library
• PortableNetworkGraphicsParser / ModuleCompressDeflateLua - examples of unpacking using Lua string library
• the [LuaJIT] FFI allows defining structs in standard C syntax. They can be accessed just like table elements [1].
• struct was at least at one time considered for LuaFiveTwo (LuaList:2008-02/msg00720.html).

Other programming languages cross reference:

• Perl [unpack][pack], Python [struct library]
• [Wikipedia:Data structure alignment]