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25.2. Endianness and Number Width
Computers store integers and floating-point numbers in different
orders (big-endian or little-endian) and different widths (32-bit and
64-bit being the most common today). Normally, you won''t have to
think about this. But if your program sends binary data across a
network connection, or onto disk to be read by a different computer,
you may need to take precautions.Conflicting orders can make an utter mess out of numbers. If a
little-endian host (such as an Intel CPU) stores 0x12345678 (305,419,896
in decimal), a big-endian host (such as a Motorola CPU) will read it
as 0x78563412 (2,018,915,346 in decimal). To avoid this problem in
network (socket) connections, use the pack and unpack formats
n and N, which write unsigned short and long numbers in
big-endian order (also called "network" order) regardless of the
platform.You can explore the endianness of your platform by unpacking a
data structure packed in native format such as:
print unpack("h*", pack("s2", 1, 2)), "\n";
# ''10002000'' on e.g. Intel x86 or Alpha 21064 in little-endian mode
# ''00100020'' on e.g. Motorola 68040
To determine your endianness, you could use either
of these statements:
$is_big_endian = unpack("h*", pack("s", 1)) =~ /01/;
$is_little_endian = unpack("h*", pack("s", 1)) =~ /^1/;
Even if two systems have the same endianness, there can still be
problems when transferring data between 32-bit and 64-bit platforms.
There is no good solution other than to avoid transferring or storing
raw binary numbers. Either transfer and store numbers as text instead
of binary, or use modules like Data::Dumper or Storable to do
this for you. You really want to be using text-oriented protocols in any event--they''re more robust, more maintainable, and more extensible than binary protocols.Of course, with the advent of XML and Unicode, our definition of text
is getting more flexible. For instance, between two systems running
Perl 5.6.0 (or newer), you can transport a sequence of integers
encoded as characters in utf8 (Perl''s version of
UTF-8). If both ends are running on an architecture with 64-bit
integers, you can exchange 64-bit integers. Otherwise, you''re limited
to 32-bit integers. Use pack with a
U* template to send, and unpack
with a U* template to receive.