c,hex,endianness , Explanation of HEX value representation and Endianess


Explanation of HEX value representation and Endianess

Question:

Tag: c,hex,endianness

I was working on a script to basically output some sample data as a binary blob. I'm a new intern in the software field and vaguely remember the idea of endianness.

I realize that the most significant bits for big-endian starts at the top and works down the memory block.

If I have 0x03000201 and the data is being parsed to output 0 1 2, how does this happen and what is being done to make that work in terms of bits, bytes, etc.

I am wondering, in the example posted below, how the numbers are extracted to form 0 1 2 when printing out the data stored in the variables.

For example: I am creating a couple lines of the binary blob using this file:

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

int main(void)
{
        FILE *file;
        int  buffer = 0x03000201;
        int  buffer2= 0x010203;
        file = fopen("test.bin", "wb");

        if (file != NULL)
        {
                fwrite(&buffer, sizeof(buffer), 1, file);
                fwrite(&buffer2, sizeof(buffer2), 1, file);
                fclose(file);
        }

        return 0;
}

I then created a Python script to parse this data:

Info About Parse

import struct
with open('test.bin','rb') as f:
    while True:
        data = f.read(4)
        if not data: break
        var1, var2, var3 = struct.unpack('=BHB', data)
        print(var1, var2, var3)

Answer:

Big or little endianness defines how to interpret a sequence of bytes longer than one byte and how to store those in memory. Wikipedia will help you with that.

I was really just looking to understand how 0x0300020 when read 2 bytes at a time and reprinted yields 0 1 2.

You don't read 2 bytes at a time, you read 4 bytes: data = f.read(4)

f.read(size) reads some quantity of data and returns it as a string.

You unpack data using =BHB - byte, 2 bytes, byte. Endianness comes into play only when you unpack data, all other IO calls in your code deal with byte sequences.

Experiment with unpack() Byte Order, Size, and Alignment You may also look at file data with a HEX editor of your choice.

And if, after your research, you have a concrete question, ask here.


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