ENDIAN(1)ENDIAN(1)NAMEendian - Report endianness of a system.
SYNOPSIS
#!/bin/csh -f
if ( `endian` == 'little' ) then
...
endif
SUMMARY
The endian command reports the endianness of memory on the computer
running it. Note that different endianness may be employed for partic‐
ular I/O operations, such as network packets, without regard for the
endianness used in memory storage.
BACKGROUND
Endianness is defined as the order in which the bytes of an integer
value are stored or transmitted. Binary integer values are regarded as
a simple string of bits by the CPU. For example, the 8-bit binary
integer
10101101
= 1*2^7 + 0*2^6 + 1*2^5 + 0*2^4 + 1*2^3 + 1*2^2 + 0*2^1 + 1*2^0
= 128 + 32 + 8 + 4 + 1
= 173
Most CPUs can process integer values of 8, 16, 32, or 64 bits at once,
but each memory address holds only 8 bits (1 byte). Hence, integers
must span multiple memory addresses, and an order for the bytes must be
chosen.
Most computers are classified as either little-endian or big-endian.
Little-endian machines store the least significant byte (i.e. the 8
bits representing the lowest powers of 2) at the lowest memory address,
i.e. the "little end" first. For example, if we stored the 4 byte
(long) integer value 1 on a little endian machine at address 1000, it
would appear in memory as the following hexadecimal bytes:
1000 | 01 |
1001 | 00 |
1002 | 00 |
1003 | 00 |
Conversely, big-endian machines store the most significant byte (the
"big end") first. Hence the same value above would be stored in memory
as follows:
1000 | 00 |
1001 | 00 |
1002 | 00 |
1003 | 01 |
A few rare CPUs may used other schemes, where the bytes are not stored
in order of significance. For example, the least significant byte may
be stored in position 2 or 3 of a 4 byte integer word. These machines
are referred to as mixed or middle endian. For example:
1000 | 00 |
1001 | 01 |
1002 | 00 |
1003 | 00 |
Much effort has gone into determining what order is generally more
efficient or useful, but given the wide range of ever-changing demands
placed on computers, this is difficult to generalize, and as a result,
both schemes have been widely adopted by computer architects.
Programmers often make erroneous assumptions about endianness based on
the operating system or the processor type. For example, most Linux
systems run on Intel processors, so it may be tempting to assume that
Linux systems are little-endian. However, many operating systems,
especially open-source systems such as the BSDs and Linux, run on a
wide variety of platforms with different endianness, so it is by no
means safe to assume anything based on the OS. As an extreme example,
NetBSD, as of January 2007, supports 17 different CPU types on 58 dif‐
ferent system architectures.
Furthermore, many CPUs, including popular ones such as the DEC Alpha,
PowerPC, DEC MIPS, and some Sun Sparc processors, are capable of oper‐
ating in either big or little-endian mode. Hence, even checking the
processor type will not reveal the correct endianness.
The only sure way to discover the endianness is by storing an specially
crafted integer value (e.g. 0x01020304) at a particular memory address,
and then checking the component bytes individually. In a C program,
this is easily accomplished using a union as follows:
union
{
unsigned long intval;
struct
{
unsigned char byte0;
unsigned char byte1;
unsigned char byte2;
unsigned char byte3;
};
}
or by using a character pointer to examine the bytes at an integer
address:
unsigned long intval;
unsigned char *p;
p = (unsigned char *)&intval;
Ideally, all programs should be written to use portable data formats,
independent of endianness, so that end-users need not be concerned with
the endianness of their hardware. However, many existing programs dump
binary data from memory to files without any formatting, and in cases
where this has been done, it is necessary to know the endianness of the
machine that wrote the data, as well as any other that reads it.
Compiled programs can use techniques like those above. If checking
from the command line or a shell script is necessary, the endian com‐
mand will perform the check and provide the results in a usable format.
DESCRIPTION
Endian reports the endianness to the standard output as "little",
"big", or "mixed". As there is no standard terminology for the various
possible mixed modes, and very few such machines even exist, endian
does not distinguish between various mixed modes.
EXIT VALUES
0 for little-endian
1 for big-endian
2 for mixed-endian
FILES
sys/endian.h
AUTHOR
Jason W. Bacon
HISTORY
The endian command was created in January, 2007, as a FreeBSD port.
ENDIAN(1)
