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Version: 5.4

format-strings

The first argument to string.pack, string.packsize, and string.unpack is a format string, which describes the layout of the structure being created or read.

A format string is a sequence of conversion options. The conversion options are as follows:

  • <: sets little endian
  • >: sets big endian
  • =: sets native endian
  • ![n]: sets maximum alignment to n (default is native alignment)
  • b: a signed byte (char)
  • B: an unsigned byte (char)
  • h: a signed short (native size)
  • H: an unsigned short (native size)
  • l: a signed long (native size)
  • L: an unsigned long (native size)
  • j: a lua_Integer
  • J: a lua_Unsigned
  • T: a size_t (native size)
  • i[n]: a signed int with n bytes (default is native size)
  • I[n]: an unsigned int with n bytes (default is native size)
  • f: a float (native size)
  • d: a double (native size)
  • n: a lua_Number
  • cn: a fixed-sized string with n bytes
  • z: a zero-terminated string
  • s[n]: a string preceded by its length coded as an unsigned integer with n bytes (default is a size_t)
  • x: one byte of padding
  • Xop: an empty item that aligns according to option op (which is otherwise ignored)
  • ' ': (space) ignored

(A "[n]" means an optional integral numeral.) Except for padding, spaces, and configurations (options "xX <=>!"), each option corresponds to an argument in string.pack or a result in string.unpack.

For options "!n", "sn", "in", and "In", n can be any integer between 1 and 16. All integral options check overflows; string.pack checks whether the given value fits in the given size; string.unpack checks whether the read value fits in a Lua integer. For the unsigned options, Lua integers are treated as unsigned values too.

Any format string starts as if prefixed by "!1=", that is, with maximum alignment of 1 (no alignment) and native endianness.

Native endianness assumes that the whole system is either big or little endian. The packing functions will not emulate correctly the behavior of mixed-endian formats.

Alignment works as follows: For each option, the format gets extra padding until the data starts at an offset that is a multiple of the minimum between the option size and the maximum alignment; this minimum must be a power of 2. Options "c" and "z" are not aligned; option "s" follows the alignment of its starting integer.

All padding is filled with zeros by string.pack and ignored by string.unpack.