Determination of host byte order

1.1 Basic knowledge of byte order determination and determination

Different types of CPU In the mainframe of , There are two ways to store multi byte integer sequences in memory , It is called host byte order (HBO)：

Small end of the sequence （little-endian） - The order bytes of are stored in the low address

Store the low byte at the starting address , be called “Little-Endian” Byte order ,Intel、AMD And so on in this way ;

big-endian （big-endian）- High order bytes are stored in low address

Store the high byte at the starting address , be called “Big-Endian” Byte order , from ARM、Motorola Etc

1.1.1 Schematic diagram of byte order determination

 1.1.2 Method of byte order determination

#include <stdio.h>

int main(int argc, char const *argv[])
{
    int num=0x12345678;
    char *p=(char *)&num;// This place strengthens the explosion prevention warning 

    if(*p==0x12){
        printf(" This is big endian \n");
    }else if(*p==0x78){
        printf(" This is the small endian byte order \n");
    }
    return 0;
}

union The consortium shares a piece of memory , So you can use this method to test

#include <stdio.h>
typedef union {
    char a;
    int b;
}msg_t;

int main(int argc, char const *argv[])
{
    msg_t num;

    num.b=0x12345678;

    if(num.a==0x78){
        printf(" This is the small endian byte order \n");
    }else if(num.a==0x12){
        printf(" This is big endian \n");
    }

    return 0;
}

2.2 Under what circumstances do we need to consider the problem of byte order conversion

2.2.1 Judge the situation

If the data is 1 Bytes of , There is no need to consider the problem of byte order, which is the same as if the byte order of the hosts on both sides of the communication is clearly known , You can also ignore , If you exceed 1 Bytes of data as a whole , You need to think about it .

2.2.2 Convert the unsigned four byte integer of small endian to large endian , That is, the principle of transformation

#include <stdio.h>

int main(int argc, char const *argv[])
{
    unsigned int num=0x12345678;
    char *p=(char *)&num;
    char *q=p+3;
    unsigned char temp;
    
    temp=*p;
    *p=*q;
    *q=temp;
    
    p++;
    q--;

    temp=*p;
    *p=*q;
    *q=temp;
    
    printf("%#x\n",num);

    return 0;
}

2.3 Byte order conversion function

2.3.1 function

h host host n network The Internet l Long s short

uint32_t htonl(uint32_t hostlong);// Host to network 4 byte

uint16_t htons(uint16_t hostshort);// Host to network 2 byte

uint32_t ntohl(uint32_t netlong);// Network to host 4 byte

uint16_t ntohs(uint16_t netshort);// Network to host 2 byte

2.3.2 Function USES

#include <arpa/inet.h>
#include <stdio.h>

int main(int argc, char const *argv[])
{
    unsigned int num=0x12345678;
    unsigned int m=htonl(num);

    printf("%#x\n",m);
    return 0;
}