struct

Library for Converting Data to and from C Structs for Lua 5.1/5.2

(Original document http://www.inf.puc-rio.br/~roberto/struct/)

This library offers basic facilities to convert Lua values to and from C structs. Its main functions are `struct.pack <struct.pack(fmt, d1, d2, ...)>`_, which packs multiple Lua values into a struct-like string; and `struct.unpack <struct.unpack(fmt, s, [i])>`_, 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 endianess and the current alignment requirements. Initially, the current endianess 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.

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.

  • "&gt;" flag to set mode to big endian.

  • "&lt;" 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 "i*n*" 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 "c*n*", 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.

Lua API

struct.pack(fmt, d1, d2, ...)

Returns a string containing the values d1, d2, etc. packed according to the format string fmt.

struct.unpack(fmt, s, [i])

Returns the values packed in string s according to the format string fmt. An optional i marks where in s to start reading (default is 1). After the read values, this function also returns the index in s where it stopped reading, which is also where you should start to read the rest of the string.

struct.size(fmt)

Returns the size of a string formatted according to the format string fmt. The format string should contain neither the option s nor the option c0.

Examples

  • The code print(struct.size("i")) prints the size of a machine’s native int.

  • To pack and unpack the structure

    struct Str {
char b;
int i[4];
};

    you can use the string "&lt;!4biiii".

  • If you need to code a structure with a large array, you may use string.rep to automatically generate part of the string format. For instance, for the structure

    struct Str {
double x;
int i[400];
};

    you can build the format string with the code "d"..string.rep("i", 400).

  • To pack a string with its length coded in its first byte, use the following code:

    x = struct.pack("Bc0", string.len(s), s)

  • To unpack that string, do as follows:

    s = struct.unpack("Bc0", x)

    Note that the length (read by the element "B") is not returned.

  • Suppose we have to decode a string s with an unknown number of doubles; the end is marked by a zero value. We can use the following code:

    local a = {}
local i = 1         -- index where to read
while true do
local d
d, i = struct.unpack("d", s, i)
if d == 0 then break end
a[#a + 1] = d
end

  • To pack a string in a fixed-width field of 10 characters padded with blanks, do as follows:

    x = struct.pack("c10", s .. string.rep(" ", 10))

License

This package is distributed under the MIT license. See copyright notice at the end of file struct.c.

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