Format Strings
Here are the formatting codes. Initially endianness is set to native and alignment is set to
none (!).
| Character | Description |
|---|---|
> |
use big endian |
< |
use little endian |
! |
use machine's native alignment |
!n |
set the current alignment to n (a power of 2) |
(space) |
ignored |
x |
padding zero byte with no corresponding Lua value |
xn |
padding n bytes |
Xn |
padding n align (default to current or native, whichever is smaller) |
b/B |
a signed/unsigned char/byte |
h/H |
a signed/unsigned short (native size) |
l/L |
a signed/unsigned long (native size) |
i/I |
a signed/unsigned int (native size) |
in/In |
a signed/unsigned int with n bytes (a power of 2) |
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. The string supplied for packing must have at least n characters; extra characters are ignored. |
c0 |
this is like "cn", except that the n is given by other means: When packing, n is the actual length of the supplied 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. |
( |
stop capturing values |
) |
start capturing values |
= |
current offset |
To match:
struct Str {
char b;
int i[4];
}
with little-endian, max align 4, use the format: <!4biiii
To pack and unpack Pascal-style strings:
local sp = struct.pack('Bc0', string.len(txt), txt)
local txt = struct.unpack('Bc0', sp)
To pack a string in a fixed-width field with 10 characters padded with blanks:
local bin = struct.pack('c10', txt .. string.rep(' ', 10))