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

(download)

This library 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 fist 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). We can change these settings with appropriate directives in the format string.

The elements in the format string are as follows:

"!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.
" " (empty space) ignored.
"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).
"in" a signed integer with n bytes (where n must be a power of 2). 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.

Lua API

All functions are registered inside a table struct. ul>

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. For obvious reasons, the format string cannot contain neither the option s nor the option c0.

Installing

To install, simply compile the file struct.c as a dynamic library. In Linux you can use the following command:

> gcc -Wall -O2 -shared -o struct.so struct.c

In Mac, you should define the environment variable MACOSX_DEPLOYMENT_TARGET as 10.3 and then write

> gcc -bundle -undefined dynamic_lookup -Wall -O2 -o struct.so struct.c

In Windows, you must generate a DLL exporting the single symbol luaopen_struct.

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];
};
```
in Linux/gcc/Pentium (little-endian, maximum alignment of 4), you can use the string "<!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 may 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)
```
Notice 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
  table.insert(a, d)
end

To pack a string in a fixed-width field with 10 characters padded with blanks, do as follows:
```
x = struct.pack("c10", s .. string.rep(" ", 10))
```

Tests

File teststruct contains a full test script for this package. It is also a good source of examples.

$Id: struct.html,v 1.4 2008/04/18 20:10:24 roberto Exp $