Network byte order, host byte order, and size issues

Different CPUs have different sort of bytes. These sort of bytes refer to the order in which integers are stored in the memory. This is called the host order.
There are two most common
1. little endian: stores low-order bytes at the starting address
2. Big endian: stores High-Order bytes at the starting address.

Le little-Endian
The byte sequence that best fits people's thinking
Low-level address storage value
High address storage value
This is the byte sequence that best fits people's thinking, because it is from the perspective of human first.
If the low value is small, it should be placed where the memory address is small, that is, the low value of the memory address.
Otherwise, the high value should be placed in the place where the memory address is large, that is, the memory address is high.

Be big-Endian
The most intuitive byte order
High level of the low-level address storage value
The low storage value of the high address
Why is it intuitive? Do not consider mappings.
Write the memory address from left to right in ascending order.
Write the value in the order of high to low.
By contrast, one byte and one byte are filled in.

Example: Memory dual-word 0x01020304 (DWORD) Storage Method

Memory Address
4000 4001 4002 4003
Le 04 03 02 01
Be 01 02 03 04

Example: If we write 0x1234abcd to the memory starting with 0x0000, the result is
Big-Endian little-Endian
0x0000 0x12 0xcd
0x0001 0x23 0xab
0x0002 0xab 0x34
0x0003 0xcd 0x12
X86 series CPUs are in the byte order of little-Endian.

The Network byte sequence is a data representation format specified in TCP/IP. It has nothing to do with the specific CPU type and operating system, this ensures that data can be correctly interpreted during transmission between different hosts. The Network byte sequence adopts the big endian sorting method.

The following four conversion functions are provided for BSD socket conversion:
Htons converts the unsigned short type from host to Network
Htonl converts the unsigned long type from the host sequence to the network Sequence
Ntohs converts the unsigned short type from the network sequence to the host Sequence
Ntohl converts the unsigned long type from the network sequence to the host Sequence

In systems using little endian, these functions convert the byte order.
In systems using the big endian type, these functions are defined as empty macros.

During network program development or cross-platform development, you should also ensure that only one byte sequence is used. Otherwise, different interpretations of the two parties may cause bugs.

Note:
1. Network and host byte Conversion Function: htons ntohs htonl ntohl (s means that short L is long H is host N is network)
2. different operating systems run on different CPUs in different bytes. See the following table.
Processor operating system byte sorting
Alpha all little endian
HP-PA nt little endian
UNIX big endian HP-PA
Intelx86 all little endian <----- x86 systems are small-end bytecode Systems
Motorola680x () All big endian
MIPs nt little endian
MIPs UNIX big endian
PowerPC nt little endian
PowerPC non-nt big endian <----- PPC system is a large-end bytecode System
RS/6000 UNIX big endian
Linux UNIX big endian
IXP1200 ARM core all little endian

 

 

 

 

Talk about byte order
It must involve two CPU factions. That is, Motorola's PowerPC series CPU and Intel's X86 series CPU. PowerPC series
While the x86 series uses little endian to store data. So what is big endian?
What about little endian?

In fact, big endian is used to store the highest valid byte (MSB) at a low address, while little endian is used to store the lowest valid byte (LSB) at a low address ).
Text descriptions may be abstract. The following uses images to describe them. For example, the storage Order of the number 0x12345678 in two different bytes of CPU is as follows:

Big endian

Low address and high address
----------------------------------------->
+-+
| 12 | 34 | 56 | 78 |
+-+

Little endian

Low address and high address
----------------------------------------->
+-+
| 78 | 56 | 34 | 12 |
+-+

From the above two figures, we can see that using the big endian method to store data is in line with our human thinking habits. And little endian ,! @ # $ % ^ & *, Go to hell-_-|

Why should we pay attention to the issue of byte order? You may ask this question. Of course, if the program you write runs only in a standalone environment and does not deal with other programs, you can ignore
Yes. But what if your program needs to interact with other programs? Here I want to talk about two languages. The data storage sequence in a program written in C/C ++ is related to the CPU of the compilation platform,
Java programs use the big endian method to store data. Imagine if you use C/C ++ to communicate with other Java programs written on the x86 Platform
What results will be generated? Take the above 0x12345678 as an example, your program passes a data to someone else and passes the pointer pointing to 0x12345678 to the Java program.
Java uses the big endian method to store data. Naturally, it translates your data into 0x78563412. What? Is it actually another number? Yes.
Result. Therefore, it is necessary to convert the byte order before your C program passes on to the Java program.

Coincidentally, all network protocols use big endian to transmit data. So sometimes we call the big endian method the byte order of the network. When two
For communication between hosts in different bytes, data must be converted to network bytes before transmission. Ansi c provides the following four macros for converting the byte order.

There is another interesting difference between Intel 8080 and Motorola 6800: in both microprocessors, lda commands are loaded from a specific address to the accumulators. For example, in 8080, the following sequence of bytes:

The Bytes stored in the 347bh address are loaded to the accumulators. Now we compare the above commands with the LDA commands of 6800. The latter adopts an extended address mode called 6800:

This byte sequence loads the bytes stored at the address 7b34h To The accumulators.

This difference is very subtle. Of course, you may think they have different operation codes: 3ah for 8080 and b6h for 6800. However, these two microprocessor processes are closely following
The address after the operation code is different. 8080 thinks that the low position is in the front, and the high position is in the back; 6800 thinks that the high position is in the front, and the low position is in the back. This intel and Motorola microprocessor saves multi-node
The fundamental differences have never been solved. Until now, when Intel microprocessor saves the number of multi-word segments, it is still the lowest valid byte (that is, at the lowest storage address), while Motorola microprocessor
When saving the number of multi-word segments, the maximum number of valid bytes is still in front.

These two methods are called Little-Endian (Intel mode) and big-Endian (Motorola mode) respectively ). The advantages and disadvantages of the two methods of debate may be
Interesting, but before that, you need to know that the term big-Endian comes from the gulliver 'stravels of jonathanswift, which refers
Lilliput and blefuscu must touch each other before each meal. Such a debate may have no purpose. Not to mention which method is essentially true, but the difference is true
Additional compatibility issues may occur when information is shared between systems based on little-Endian and big-Endian.