Programming Perl, 4th Edition

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:

say unpack("h*", pack("s2", 1, 2));
# '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 ...