eLua reference manual - pack

This module allows for arbitrary packing of data into Lua strings and unpacking data from Lua strings. In this way, a string can be used to store data in a platform-indepdendent manner. It is based on the lpack module from Luiz Henrique de Figueiredo (with some minor tweaks).

Both methods of this module (pack and unpack) use a format string to describe how to pack/unpack the data. The format string contains one or more data specifiers, each data specifier is applied to a single variable that must be packed/unpacked. The data specifier has the following general format:

[endianness]<format specifier>[count]

where:

• endianness is an optional endian flags that specifies how the numbers that are to be packed/unpacked are stored in memory. It can be either:
  1. '<' for little endian.
  2. '>' for big endian.
  3. '=' for native endian (the platform's endian order, default).
• format specifier describes what kind of variable will be packed/unpacked. The format specifier is case-sensitive. The possible values of this parameter are summarized in the table below:

  Format specifier	Corresponding variable type
  'z'	zero-terminated string
  'p'	string preceded by length byte
  'P'	string preceded by length word
  'a'	string preceded by length size_t
  'A'	string
  'f'	float
  'd'	double
  'n'	Lua number
  'c'	char
  'b'	byte = unsigned char
  'h'	short
  'H'	unsigned short
  'i'	int
  'I'	unsigned int
  'l'	long
  'L'	unsigned long

• count is an optional counter for the format specifier. For example, i5 instructs the code to pack/unpack 5 integer variables, as opposed to i that specifies a single integer variable.