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pack TEMPLATE,LIST

Takes an array or list of values and packs it into a binary structure, returning the string containing the structure. 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   An ascii string, will be space padded.
Z   A null terminated (asciz) string, will be null padded.

b   A bit string (ascending bit order, like vec()).
B   A bit string (descending bit order).
h   A hex string (low nybble first).
H   A hex string (high nybble first).

c   A signed char value.
C   An unsigned char value.

s   A signed short value.
S   An unsigned short value.
      (This 'short' is _exactly_ 16 bits, which may differ from
       what a local C compiler calls 'short'.)

i   A signed integer value.
I   An unsigned integer value.
      (This 'integer' is _at least_ 32 bits wide.  Its exact
       size depends on what a local C compiler calls 'int',
       and may even be larger than the 'long' described in
       the next item.)

l   A signed long value.
L   An unsigned long value.
      (This 'long' is _exactly_ 32 bits, which may differ from
       what a local C compiler calls 'long'.)

n   A short in "network" (big-endian) order.
N   A long in "network" (big-endian) order.
v   A short in "VAX" (little-endian) order.
V   A long in "VAX" (little-endian) order.
      (These 'shorts' and 'longs' are _exactly_ 16 bits and
       _exactly_ 32 bits, respectively.)

q   A signed quad (64-bit) value.
Q   An unsigned quad value.
      (Available only if your system supports 64-bit integer values
       _and_ if Perl has been compiled to support those.
       Causes a fatal error otherwise.)

f   A single-precision float in the native format.
d   A double-precision float in the native format.

p   A pointer to a null-terminated string.
P   A pointer to a structure (fixed-length string).

u   A uuencoded string.

w   A BER compressed integer.  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.
X   Back up a byte.
@   Null fill to absolute position.

The following rules apply:

  • Each letter may optionally be followed by a number giving a repeat count. With all types except "a", "A", "Z", "b", "B", "h", "H", and "P" the pack function will gobble up that many values from the LIST. A * for the repeat count means to use however many items are left.

  • 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 necessary. When unpacking, "A" strips trailing spaces and nulls, "Z" strips everything after the first null, and "a" returns data verbatim.

  • Likewise, the "b" and "B" fields pack a string that many bits long.

  • The "h" and "H" fields pack a string that many nybbles long.

  • The "p" type packs a pointer to a null-terminated string. You are responsible for ensuring the string is not a temporary value (which can potentially get deallocated before you get around to using the packed result). The "P" type 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.

  • The integer formats "s", "S", "i", "I", "l", and "L" are inherently non-portable between processors and operating systems because they obey the native byteorder and endianness. For example a 4-byte integer 0x87654321 (2271560481 decimal) be ordered natively (arranged in and handled by the CPU registers) into bytes as

            0x12 0x34 0x56 0x78     # little-endian
            0x78 0x56 0x34 0x12     # big-endian
     

    Basically, the Intel, Alpha, and VAX CPUs and little-endian, while everybody else, for example Motorola m68k/88k, PPC, Sparc, HP PA, Power, and Cray are big-endian. MIPS can be either: Digital used it in little-endian mode, SGI uses it in big-endian mode.

    The names `big-endian' and `little-endian' are joking references to the classic "Gulliver's Travels" (via the paper "On Holy Wars and a Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and the egg-eating habits of the lilliputs.

    Some systems may even have weird byte orders such as

            0x56 0x78 0x12 0x34
            0x34 0x12 0x78 0x56
     

    You can see your system's preference with

    print join(" ", map { sprintf "%#02x", $_ }
                        unpack("C*",pack("L",0x12345678))), "\n";

    The byteorder on the platform where Perl was built is also available via Config:

    use Config;
    print $Config{byteorder}, "\n";

    Byteorders '1234' and '12345678' are little-endian, '4321' and '87654321' are big-endian.

    If you want portable packed integers use the formats "n", "N", "v", and "V", their byte endianness and size is known.

  • Real numbers (floats and doubles) are in the native machine format only; due to the multiplicity of floating formats around, and the lack of a standard "network" representation, 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 (as the endian-ness of the memory representation is not part of the IEEE spec).

    Note that Perl uses doubles internally for all numeric calculation, and converting from double into float and thence back to double again will lose precision (i.e., unpack("f", pack("f", $foo)) will not in general equal $foo).

Examples:

$foo = pack("CCCC",65,66,67,68);
# foo eq "ABCD"
$foo = pack("C4",65,66,67,68);
# same thing

$foo = pack("ccxxcc",65,66,67,68);
# foo eq "AB\0\0CD"

$foo = pack("s2",1,2);
# "\1\0\2\0" on little-endian
# "\0\1\0\2" 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)));
}

The same template may generally also be used in unpack().