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.">"
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 signedchar
."B"
an unsignedchar
."h"
a signedshort
(native size)."H"
an unsignedshort
(native size)."l"
a signedlong
(native size)."L"
an unsignedlong
(native size)."T"
asize_t
(native size)."in"
a signed integer with n bytes. An absent n means the native size of anint
."In"
like"i*n*"
but unsigned."f"
afloat
(native size)."d"
adouble
(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 nativeint
.To pack and unpack the structure
struct Str { char b; int i[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 structurestruct 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
.
id-1087943