NAME
Raku port of Perl's pack() / unpack() built-ins
SYNOPSIS
use P5pack; # exports pack(), unpack(), activates Buf.unpack()
DESCRIPTION
This module tries to mimic the behaviour of Perl's pack and unpack built-ins as closely as possible in the Raku Programming Language.
Currently supported directives are: a A c C h H i I l L n N q Q s S U v V w x Z
Drop-in replacement for use experimental :pack.
ORIGINAL PERL 5 DOCUMENTATION
pack TEMPLATE,LIST
Takes a LIST of values and converts it into a string using the
rules given by the TEMPLATE. The resulting string is the
concatenation of the converted values. Typically, each converted
value looks like its machine-level representation. For example, on
32-bit machines an integer may be represented by a sequence of 4
bytes, which will in Perl be presented as a string that's 4
characters long.
See perlpacktut for an introduction to this function.
The TEMPLATE is a sequence of characters that give the order and
type of values, as follows:
a A string with arbitrary binary data, will be null padded.
A A text (ASCII) string, will be space padded.
Z A null-terminated (ASCIZ) string, will be null padded.
b A bit string (ascending bit order inside each byte,
like vec()).
B A bit string (descending bit order inside each byte).
h A hex string (low nybble first).
H A hex string (high nybble first).
c A signed char (8-bit) value.
C An unsigned char (octet) value.
W An unsigned char value (can be greater than 255).
s A signed short (16-bit) value.
S An unsigned short value.
l A signed long (32-bit) value.
L An unsigned long value.
q A signed quad (64-bit) value.
Q An unsigned quad value.
(Quads are available only if your system supports 64-bit
integer values _and_ if Perl has been compiled to support
those. Raises an exception otherwise.)
i A signed integer value.
I A unsigned integer value.
(This 'integer' is _at_least_ 32 bits wide. Its exact
size depends on what a local C compiler calls 'int'.)
n An unsigned short (16-bit) in "network" (big-endian) order.
N An unsigned long (32-bit) in "network" (big-endian) order.
v An unsigned short (16-bit) in "VAX" (little-endian) order.
V An unsigned long (32-bit) in "VAX" (little-endian) order.
j A Perl internal signed integer value (IV).
J A Perl internal unsigned integer value (UV).
f A single-precision float in native format.
d A double-precision float in native format.
F A Perl internal floating-point value (NV) in native format
D A float of long-double precision in native format.
(Long doubles are available only if your system supports
long double values _and_ if Perl has been compiled to
support those. Raises an exception otherwise.)
p A pointer to a null-terminated string.
P A pointer to a structure (fixed-length string).
u A uuencoded string.
U A Unicode character number. Encodes to a character in char-
acter mode and UTF-8 (or UTF-EBCDIC in EBCDIC platforms) in
byte mode.
w A BER compressed integer (not an ASN.1 BER, see perlpacktut
for details). Its bytes represent an unsigned integer in
base 128, most significant digit first, with as few digits
as possible. Bit eight (the high bit) is set on each byte
except the last.
x A null byte (a.k.a ASCII NUL, "\000", chr(0))
X Back up a byte.
@ Null-fill or truncate to absolute position, counted from the
start of the innermost ()-group.
. Null-fill or truncate to absolute position specified by
the value.
( Start of a ()-group.
One or more modifiers below may optionally follow certain letters
in the TEMPLATE (the second column lists letters for which the
modifier is valid):
! sSlLiI Forces native (short, long, int) sizes instead
of fixed (16-/32-bit) sizes.
! xX Make x and X act as alignment commands.
! nNvV Treat integers as signed instead of unsigned.
! @. Specify position as byte offset in the internal
representation of the packed string. Efficient
but dangerous.
> sSiIlLqQ Force big-endian byte-order on the type.
jJfFdDpP (The "big end" touches the construct.)
< sSiIlLqQ Force little-endian byte-order on the type.
jJfFdDpP (The "little end" touches the construct.)
The ">" and "<" modifiers can also be used on "()" groups to force
a particular byte-order on all components in that group, including
all its subgroups.
The following rules apply:
* Each letter may optionally be followed by a number indicating
the repeat count. A numeric repeat count may optionally be
enclosed in brackets, as in "pack("C[80]", @arr)". The repeat
count gobbles that many values from the LIST when used with
all format types other than "a", "A", "Z", "b", "B", "h", "H",
"@", ".", "x", "X", and "P", where it means something else,
described below. Supplying a "*" for the repeat count instead
of a number means to use however many items are left, except
for:
* "@", "x", and "X", where it is equivalent to 0.
* <.>, where it means relative to the start of the string.
* "u", where it is equivalent to 1 (or 45, which here is
equivalent).
One can replace a numeric repeat count with a template letter
enclosed in brackets to use the packed byte length of the
bracketed template for the repeat count.
For example, the template "x[L]" skips as many bytes as in a
packed long, and the template "$t X[$t] $t" unpacks twice
whatever $t (when variable-expanded) unpacks. If the template
in brackets contains alignment commands (such as "x![d]"), its
packed length is calculated as if the start of the template
had the maximal possible alignment.
When used with "Z", a "*" as the repeat count is guaranteed to
add a trailing null byte, so the resulting string is always
one byte longer than the byte length of the item itself.
When used with "@", the repeat count represents an offset from
the start of the innermost "()" group.
When used with ".", the repeat count determines the starting
position to calculate the value offset as follows:
* If the repeat count is 0, it's relative to the current
position.
* If the repeat count is "*", the offset is relative to the
start of the packed string.
* And if it's an integer n, the offset is relative to the
start of the nth innermost "( )" group, or to the start of
the string if n is bigger then the group level.
The repeat count for "u" is interpreted as the maximal number
of bytes to encode per line of output, with 0, 1 and 2
replaced by 45. The repeat count should not be more than 65.
* The "a", "A", and "Z" types gobble just one value, but pack it
as a string of length count, padding with nulls or spaces as
needed. When unpacking, "A" strips trailing whitespace and
nulls, "Z" strips everything after the first null, and "a"
returns data with no stripping at all.
If the value to pack is too long, the result is truncated. If
it's too long and an explicit count is provided, "Z" packs
only "$count-1" bytes, followed by a null byte. Thus "Z"
always packs a trailing null, except when the count is 0.
* Likewise, the "b" and "B" formats pack a string that's that
many bits long. Each such format generates 1 bit of the
result. These are typically followed by a repeat count like
"B8" or "B64".
Each result bit is based on the least-significant bit of the
corresponding input character, i.e., on "ord($char)%2". In
particular, characters "0" and "1" generate bits 0 and 1, as
do characters "\000" and "\001".
Starting from the beginning of the input string, each 8-tuple
of characters is converted to 1 character of output. With
format "b", the first character of the 8-tuple determines the
least-significant bit of a character; with format "B", it
determines the most-significant bit of a character.
If the length of the input string is not evenly divisible by
8, the remainder is packed as if the input string were padded
by null characters at the end. Similarly during unpacking,
"extra" bits are ignored.
If the input string is longer than needed, remaining
characters are ignored.
A "*" for the repeat count uses all characters of the input
field. On unpacking, bits are converted to a string of 0s and
1s.
* The "h" and "H" formats pack a string that many nybbles (4-bit
groups, representable as hexadecimal digits, "0".."9"
"a".."f") long.
For each such format, pack() generates 4 bits of result. With
non-alphabetical characters, the result is based on the 4
least-significant bits of the input character, i.e., on
"ord($char)%16". In particular, characters "0" and "1"
generate nybbles 0 and 1, as do bytes "\000" and "\001". For
characters "a".."f" and "A".."F", the result is compatible
with the usual hexadecimal digits, so that "a" and "A" both
generate the nybble "0xA==10". Use only these specific hex
characters with this format.
Starting from the beginning of the template to pack(), each
pair of characters is converted to 1 character of output. With
format "h", the first character of the pair determines the
least-significant nybble of the output character; with format
"H", it determines the most-significant nybble.
If the length of the input string is not even, it behaves as
if padded by a null character at the end. Similarly, "extra"
nybbles are ignored during unpacking.
If the input string is longer than needed, extra characters
are ignored.
A "*" for the repeat count uses all characters of the input
field. For unpack(), nybbles are converted to a string of
hexadecimal digits.
* The "p" format packs a pointer to a null-terminated string.
You are responsible for ensuring that the string is not a
temporary value, as that could potentially get deallocated
before you got around to using the packed result. The "P"
format packs a pointer to a structure of the size indicated by
the length. A null pointer is created if the corresponding
value for "p" or "P" is "undef"; similarly with unpack(),
where a null pointer unpacks into "undef".
If your system has a strange pointer size--meaning a pointer
is neither as big as an int nor as big as a long--it may not
be possible to pack or unpack pointers in big- or
little-endian byte order. Attempting to do so raises an
exception.
* The "/" template character allows packing and unpacking of a
sequence of items where the packed structure contains a packed
item count followed by the packed items themselves. This is
useful when the structure you're unpacking has encoded the
sizes or repeat counts for some of its fields within the
structure itself as separate fields.
For "pack", you write length-item"/"sequence-item, and the
length-item describes how the length value is packed. Formats
likely to be of most use are integer-packing ones like "n" for
Java strings, "w" for ASN.1 or SNMP, and "N" for Sun XDR.
For "pack", sequence-item may have a repeat count, in which
case the minimum of that and the number of available items is
used as the argument for length-item. If it has no repeat
count or uses a '*', the number of available items is used.
For "unpack", an internal stack of integer arguments unpacked
so far is used. You write "/"sequence-item and the repeat
count is obtained by popping off the last element from the
stack. The sequence-item must not have a repeat count.
If sequence-item refers to a string type ("A", "a", or "Z"),
the length-item is the string length, not the number of
strings. With an explicit repeat count for pack, the packed
string is adjusted to that length. For example:
This code: gives this result:
unpack("W/a", "\004Gurusamy") ("Guru")
unpack("a3/A A*", "007 Bond J ") (" Bond", "J")
unpack("a3 x2 /A A*", "007: Bond, J.") ("Bond, J", ".")
pack("n/a* w/a","hello,","world") "\000\006hello,\005world"
pack("a/W2", ord("a") .. ord("z")) "2ab"
The length-item is not returned explicitly from "unpack".
Supplying a count to the length-item format letter is only
useful with "A", "a", or "Z". Packing with a length-item of
"a" or "Z" may introduce "\000" characters, which Perl does
not regard as legal in numeric strings.
* The integer types "s", "S", "l", and "L" may be followed by a
"!" modifier to specify native shorts or longs. As shown in
the example above, a bare "l" means exactly 32 bits, although
the native "long" as seen by the local C compiler may be
larger. This is mainly an issue on 64-bit platforms. You can
see whether using "!" makes any difference this way:
printf "format s is %d, s! is %d\n",
length pack("s"), length pack("s!");
printf "format l is %d, l! is %d\n",
length pack("l"), length pack("l!");
"i!" and "I!" are also allowed, but only for completeness'
sake: they are identical to "i" and "I".
The actual sizes (in bytes) of native shorts, ints, longs, and
long longs on the platform where Perl was built are also
available from the command line:
$ perl -V:{short,int,long{,long}}size
shortsize='2';
intsize='4';
longsize='4';
longlongsize='8';
or programmatically via the "Config" module:
use Config;
print $Config{shortsize}, "\n";
print $Config{intsize}, "\n";
print $Config{longsize}, "\n";
print $Config{longlongsize}, "\n";
$Config{longlongsize} is undefined on systems without long
long support.
* The integer formats "s", "S", "i", "I", "l", "L", "j", and "J"
are inherently non-portable between processors and operating
systems because they obey native byteorder and endianness. For
example, a 4-byte integer 0x12345678 (305419896 decimal) would
be ordered natively (arranged in and handled by the CPU
registers) into bytes as
0x12 0x34 0x56 0x78 # big-endian
0x78 0x56 0x34 0x12 # little-endian
Basically, Intel and VAX CPUs are little-endian, while
everybody else, including Motorola m68k/88k, PPC, Sparc, HP
PA, Power, and Cray, are big-endian. Alpha and MIPS can be
either: Digital/Compaq uses (well, used) them in little-endian
mode, but SGI/Cray uses them in big-endian mode.
The names big-endian and little-endian are comic references to
the egg-eating habits of the little-endian Lilliputians and
the big-endian Blefuscudians from the classic Jonathan Swift
satire, Gulliver's Travels. This entered computer lingo via
the paper "On Holy Wars and a Plea for Peace" by Danny Cohen,
USC/ISI IEN 137, April 1, 1980.
Some systems may have even weirder byte orders such as
0x56 0x78 0x12 0x34
0x34 0x12 0x78 0x56
You can determine your system endianness with this
incantation:
printf("%#02x ", $_) for unpack("W*", pack L=>0x12345678);
The byteorder on the platform where Perl was built is also
available via Config:
use Config;
print "$Config{byteorder}\n";
or from the command line:
$ perl -V:byteorder
Byteorders "1234" and "12345678" are little-endian; "4321" and
"87654321" are big-endian.
For portably packed integers, either use the formats "n", "N",
"v", and "V" or else use the ">" and "<" modifiers described
immediately below. See also perlport.
* Starting with Perl 5.10.0, integer and floating-point formats,
along with the "p" and "P" formats and "()" groups, may all be
followed by the ">" or "<" endianness modifiers to
respectively enforce big- or little-endian byte-order. These
modifiers are especially useful given how "n", "N", "v", and
"V" don't cover signed integers, 64-bit integers, or
floating-point values.
Here are some concerns to keep in mind when using an
endianness modifier:
* Exchanging signed integers between different platforms
works only when all platforms store them in the same
format. Most platforms store signed integers in
two's-complement notation, so usually this is not an
issue.
* The ">" or "<" modifiers can only be used on
floating-point formats on big- or little-endian machines.
Otherwise, attempting to use them raises an exception.
* Forcing big- or little-endian byte-order on floating-point
values for data exchange can work only if all platforms
use the same binary representation such as IEEE
floating-point. Even if all platforms are using IEEE,
there may still be subtle differences. Being able to use
">" or "<" on floating-point values can be useful, but
also dangerous if you don't know exactly what you're
doing. It is not a general way to portably store
floating-point values.
* When using ">" or "<" on a "()" group, this affects all
types inside the group that accept byte-order modifiers,
including all subgroups. It is silently ignored for all
other types. You are not allowed to override the
byte-order within a group that already has a byte-order
modifier suffix.
* Real numbers (floats and doubles) are in native machine format
only. Due to the multiplicity of floating-point formats and
the lack of a standard "network" representation for them, no
facility for interchange has been made. This means that packed
floating-point data written on one machine may not be readable
on another, even if both use IEEE floating-point arithmetic
(because the endianness of the memory representation is not
part of the IEEE spec). See also perlport.
If you know exactly what you're doing, you can use the ">" or
"<" modifiers to force big- or little-endian byte-order on
floating-point values.
Because Perl uses doubles (or long doubles, if configured)
internally for all numeric calculation, converting from double
into float and thence to double again loses precision, so
"unpack("f", pack("f", $foo)") will not in general equal $foo.
* Pack and unpack can operate in two modes: character mode ("C0"
mode) where the packed string is processed per character, and
UTF-8 mode ("U0" mode) where the packed string is processed in
its UTF-8-encoded Unicode form on a byte-by-byte basis.
Character mode is the default unless the format string starts
with "U". You can always switch mode mid-format with an
explicit "C0" or "U0" in the format. This mode remains in
effect until the next mode change, or until the end of the
"()" group it (directly) applies to.
Using "C0" to get Unicode characters while using "U0" to get
non-Unicode bytes is not necessarily obvious. Probably only
the first of these is what you want:
$ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
perl -CS -ne 'printf "%v04X\n", $_ for unpack("C0A*", $_)'
03B1.03C9
$ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
perl -CS -ne 'printf "%v02X\n", $_ for unpack("U0A*", $_)'
CE.B1.CF.89
$ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
perl -C0 -ne 'printf "%v02X\n", $_ for unpack("C0A*", $_)'
CE.B1.CF.89
$ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
perl -C0 -ne 'printf "%v02X\n", $_ for unpack("U0A*", $_)'
C3.8E.C2.B1.C3.8F.C2.89
Those examples also illustrate that you should not try to use
"pack"/"unpack" as a substitute for the Encode module.
* You must yourself do any alignment or padding by inserting,
for example, enough "x"es while packing. There is no way for
pack() and unpack() to know where characters are going to or
coming from, so they handle their output and input as flat
sequences of characters.
* A "()" group is a sub-TEMPLATE enclosed in parentheses. A
group may take a repeat count either as postfix, or for
unpack(), also via the "/" template character. Within each
repetition of a group, positioning with "@" starts over at 0.
Therefore, the result of
pack("@1A((@2A)@3A)", qw[X Y Z])
is the string "\0X\0\0YZ".
* "x" and "X" accept the "!" modifier to act as alignment
commands: they jump forward or back to the closest position
aligned at a multiple of "count" characters. For example, to
pack() or unpack() a C structure like
struct {
char c; /* one signed, 8-bit character */
double d;
char cc[2];
}
one may need to use the template "c x![d] d c[2]". This
assumes that doubles must be aligned to the size of double.
For alignment commands, a "count" of 0 is equivalent to a
"count" of 1; both are no-ops.
* "n", "N", "v" and "V" accept the "!" modifier to represent
signed 16-/32-bit integers in big-/little-endian order. This
is portable only when all platforms sharing packed data use
the same binary representation for signed integers; for
example, when all platforms use two's-complement
representation.
* Comments can be embedded in a TEMPLATE using "#" through the
end of line. White space can separate pack codes from each
other, but modifiers and repeat counts must follow
immediately. Breaking complex templates into individual
line-by-line components, suitably annotated, can do as much to
improve legibility and maintainability of pack/unpack formats
as "/x" can for complicated pattern matches.
* If TEMPLATE requires more arguments than pack() is given,
pack() assumes additional "" arguments. If TEMPLATE requires
fewer arguments than given, extra arguments are ignored.
Examples:
$foo = pack("WWWW",65,66,67,68);
# foo eq "ABCD"
$foo = pack("W4",65,66,67,68);
# same thing
$foo = pack("W4",0x24b6,0x24b7,0x24b8,0x24b9);
# same thing with Unicode circled letters.
$foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
# same thing with Unicode circled letters. You don't get the
# UTF-8 bytes because the U at the start of the format caused
# a switch to U0-mode, so the UTF-8 bytes get joined into
# characters
$foo = pack("C0U4",0x24b6,0x24b7,0x24b8,0x24b9);
# foo eq "\xe2\x92\xb6\xe2\x92\xb7\xe2\x92\xb8\xe2\x92\xb9"
# This is the UTF-8 encoding of the string in the
# previous example
$foo = pack("ccxxcc",65,66,67,68);
# foo eq "AB\0\0CD"
# NOTE: The examples above featuring "W" and "c" are true
# only on ASCII and ASCII-derived systems such as ISO Latin 1
# and UTF-8. On EBCDIC systems, the first example would be
# $foo = pack("WWWW",193,194,195,196);
$foo = pack("s2",1,2);
# "\001\000\002\000" on little-endian
# "\000\001\000\002" on big-endian
$foo = pack("a4","abcd","x","y","z");
# "abcd"
$foo = pack("aaaa","abcd","x","y","z");
# "axyz"
$foo = pack("a14","abcdefg");
# "abcdefg\0\0\0\0\0\0\0"
$foo = pack("i9pl", gmtime);
# a real struct tm (on my system anyway)
$utmp_template = "Z8 Z8 Z16 L";
$utmp = pack($utmp_template, @utmp1);
# a struct utmp (BSDish)
@utmp2 = unpack($utmp_template, $utmp);
# "@utmp1" eq "@utmp2"
sub bintodec {
unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
}
$foo = pack('sx2l', 12, 34);
# short 12, two zero bytes padding, long 34
$bar = pack('s@4l', 12, 34);
# short 12, zero fill to position 4, long 34
# $foo eq $bar
$baz = pack('s.l', 12, 4, 34);
# short 12, zero fill to position 4, long 34
$foo = pack('nN', 42, 4711);
# pack big-endian 16- and 32-bit unsigned integers
$foo = pack('S>L>', 42, 4711);
# exactly the same
$foo = pack('s<l<', -42, 4711);
# pack little-endian 16- and 32-bit signed integers
$foo = pack('(sl)<', -42, 4711);
# exactly the same
The same template may generally also be used in unpack().
unpack TEMPLATE,EXPR
unpack TEMPLATE
"unpack" does the reverse of "pack": it takes a string and expands
it out into a list of values. (In scalar context, it returns
merely the first value produced.)
If EXPR is omitted, unpacks the $_ string. See perlpacktut for an
introduction to this function.
The string is broken into chunks described by the TEMPLATE. Each
chunk is converted separately to a value. Typically, either the
string is a result of "pack", or the characters of the string
represent a C structure of some kind.
The TEMPLATE has the same format as in the "pack" function. Here's
a subroutine that does substring:
sub substr {
my($what,$where,$howmuch) = @_;
unpack("x$where a$howmuch", $what);
}
and then there's
sub ordinal { unpack("W",$_[0]); } # same as ord()
In addition to fields allowed in pack(), you may prefix a field
with a %<number> to indicate that you want a <number>-bit checksum
of the items instead of the items themselves. Default is a 16-bit
checksum. Checksum is calculated by summing numeric values of
expanded values (for string fields the sum of "ord($char)" is
taken; for bit fields the sum of zeroes and ones).
For example, the following computes the same number as the System
V sum program:
$checksum = do {
local $/; # slurp!
unpack("%32W*",<>) % 65535;
};
The following efficiently counts the number of set bits in a bit
vector:
$setbits = unpack("%32b*", $selectmask);
The "p" and "P" formats should be used with care. Since Perl has
no way of checking whether the value passed to "unpack()"
corresponds to a valid memory location, passing a pointer value
that's not known to be valid is likely to have disastrous
consequences.
If there are more pack codes or if the repeat count of a field or
a group is larger than what the remainder of the input string
allows, the result is not well defined: the repeat count may be
decreased, or "unpack()" may produce empty strings or zeros, or it
may raise an exception. If the input string is longer than one
described by the TEMPLATE, the remainder of that input string is
ignored.
AUTHOR
Elizabeth Mattijsen liz@raku.rocks
Source can be located at: https://github.com/lizmat/P5pack . Comments and Pull Requests are welcome.
If you like this module, or what I'm doing more generally, committing to a small sponsorship would mean a great deal to me!
COPYRIGHT AND LICENSE
Copyright 2018, 2019, 2020, 2021, 2023, 2024 Elizabeth Mattijsen
Re-imagined from Perl as part of the CPAN Butterfly Plan and an earlier version that only lived in the Raku Ecosystem.
This library is free software; you can redistribute it and/or modify it under the Artistic License 2.0.