# Struct Module | Since | Origin / Contributor | Maintainer | Source | | :----- | :-------------------- | :---------- | :------ | | 2015-02-13 | [Roberto Ierusalimschy](http://www.inf.puc-rio.br/~roberto/struct/), [Philip Gladstone](https://github.com/pjsg) | [Philip Gladstone](https://github.com/pjsg) | [struct.c](../../app/modules/struct.c)| This module offers basic facilities to convert Lua values to and from C structs. Its main functions are `struct.pack`, which packs multiple Lua values into a struct-like string; and `struct.unpack`, which unpacks multiple Lua values from a given struct-like string. The first argument to both functions is a *format string*, which describes the layout of the structure. The format string is a sequence of conversion elements, which respect the current endianness and the current alignment requirements. Initially, the current endianness is the machine's native endianness and the current alignment requirement is 1 (meaning no alignment at all). You can change these settings with appropriate directives in the format string. Note that the float and double conversions are only available with a floating point NodeMCU build. ### Format String The elements in the format string are as follows: - `" "` (empty space) ignored. - `"!n"` flag to set the current alignment requirement to *n* (necessarily a power of 2); an absent *n* means the machine's native alignment. - `">"` flag to set mode to big endian. - `"<"` flag to set mode to little endian. - `"x"` a padding zero byte with no corresponding Lua value. - `"b"` a signed `char`. - `"B"` an unsigned `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). - `"T"` a `size_t` (native size). - `"in"` a signed integer with *n* bytes. An absent *n* means the native size of an `int`. - `"In"` like `"in"` but unsigned. - `"f"` a `float` (native size). - `"d"` a `double` (native size). - `"s"` a zero-terminated string. - `"cn"` a sequence of exactly *n* chars corresponding to a single Lua string. An absent *n* means 1. When packing, the given string must have at least *n* characters (extra characters are discarded). - `"c0"` this is like `"cn"`, except that the *n* is given by other means: When packing, *n* is the length of the given string; when unpacking, *n* is the value of the previous unpacked value (which must be a number). In that case, this previous value is not returned. #### Examples To pack and unpack the structure struct Str { char b; int i[4]; }; you can use the string `"