Confusion in interpreting 32 bit integer in Big Endian and Little Endian Machines

Question

I am in process of rewriting code from Big Endian Machine to Little Endian machine.

Let's say there is a variable called a , which is a 32 bit integer which holds current timestamp(user request's current timestamp).

In Big Endian machine, right now the code is this way:

uint32 a = current_timestamp_of_user_request;
uint8 arr[3] = {0};
arr[0] = ((a >> (8 * 2)) & 0x000000FF); 
arr[1] = ((a >> (8 * 1)) & 0x000000FF);
arr[2] = ((a >> (8 * 0)) & 0x000000FF);

Now, when I am writing the same logic for little endian machine, can I use the same code(method a), or should I convert the code this way(let's call this method b)?

uint32 a = current_timestamp_of_user_request;
uint32 b = htonl(a);
uint8 arr[3] = {0};
arr[0] = ((b >> (8 * 2)) & 0x000000FF); 
arr[1] = ((b >> (8 * 1)) & 0x000000FF);
arr[2] = ((b >> (8 * 0)) & 0x000000FF);

I wrote this program to verify:

#include<stdio.h>
#include<stdlib.h>


void main() {
    long int a = 3265973637;
    long int b = 0;
    int arr[3] = {0,0,0};

    arr[0] = ((a >> (8 * 2)) & 0x000000FF); 
    arr[1] = ((a >> (8 * 1)) & 0x000000FF);
    arr[2] = ((a >> (8 * 0)) & 0x000000FF);

    printf("arr[0] = %d\t arr[1] = %d\t arr[2] = %d\n", arr[0], arr[1],   arr[2]);

    b = htonl(a);

    arr[0] = ((b >> (8 * 2)) & 0x000000FF); 
    arr[1] = ((b >> (8 * 1)) & 0x000000FF);
    arr[2] = ((b >> (8 * 0)) & 0x000000FF);

    printf("After htonl:\n");
    printf("arr[0] = %d\t arr[1] = %d\t arr[2] = %d\n", arr[0], arr[1],   arr[2]);

}

Results:

Result with little endian machine:

bgl-srtg-lnx11: /scratch/nnandiga/test>./x86
arr[0] = 170     arr[1] = 205    arr[2] = 133
After htonl:
arr[0] = 205     arr[1] = 170    arr[2] = 194

Result with big endian machine:
arr[0] = 170     arr[1] = 205    arr[2] = 133
After htonl:
arr[0] = 170     arr[1] = 205    arr[2] = 133

Looks like without conversion to big endian order, the same logic(without htonl() ) gives exact results in filling the array arr . Now, can you please answer should I use htonl() or not if I want the array to be the same in both little endian and big endian machines(little endian result should be exact as big endian result).

Answer 1

Your code as originally written will do what you want on both big endian and little endian machines.

If for example the value of a is 0x00123456 , then 0x12 goes in arr[0] , 0x34 goes in arr[1] , and 0x56 goes in arr[2] . This occurs regardless of what the endianness of the machine is.

When you use the >> and & operators, they operate on the value of the expression in question, not the representation of that value.

When you call htonl , you change the value to match a particular representation. So on a little endian machine htonl(0x00123456) will result in the value 0x56341200 . Then when you operate on that value you get different results.

Where endianness matters is when the representation of a number using multiple bytes is read or written as bytes, ie to disk, over a network, or to/from a byte buffer.

For example, if you do this:

uint32_t a = 0x12345678;
...
write(fd, &a, sizeof(a));

Then the four bytes that a consists of are written to the file descriptor (be it a file or a socket) one at a time. A big endian machine will write 0x12 , 0x34 , 0x56 , 0x78 in that order while a little endian machine will write 0x78 , 0x56 , 0x34 , 0x12 .

If you want the bytes to be written in a consistent order then you would first call a = htonl(a) before calling write . Then the bytes will always be written as 0x12 , 0x34 , 0x56 , 0x78 .

Because your code operates on the value and not the individual bytes of the value, you don't need to worry about endianness.

Answer 2

You should use htonl() . On a big-endian machine this does nothing, it just returns the original value. On a little-endian machine it swaps the bytes appropriately. So by using this, you don't have to concern yourself with the endian-ness of the machine, you can use the same code after calling it.