I C (and most other languages) you can enter a value like thirty-two using many different number bases: ‘ ‘ as a char, 32 in decimal, 0x20 is hex, 040 in octal or 0b00010000. All of these mean thirty-two, just written differently using different characters.

Note that each of these representations use different digits and/or letters to specify the same number - the mathematical entity thirty-two. These letters and digits are characters stored as numbers, eg the digit zero (0) is coded in ASCII as 48.

In the end, the C compiler will compile the text ‘32’, ‘0x40’ or ‘0b00010000’ into binary and store it in memory the single byte where the 5th bit set to 1 and the others to 0. This is the way computer memory is used to represent the number, and it’s linked to the way the electric circuits do things like add numbers together.

The compiler is the thing that understands that the characters like ‘3’, ‘+’ or ‘F’ and works out what they represent (a number, a variable name or a sum). When it understands that, it will turn the code into bits that the CPU can work with, for example passing it to the compare instruction CMP used to test whether the input is less than the number thirty-two.

This is very meta, and I hope this clarifies rather than confuses. Compilers are notoriously confusing, such as the thought ‘the compiler compiles the compiler code into an executable that can compile itself’.

You are inspecting a byte, which contains 0x39. That it corresponds to "9" doesn't matter at all to the compiler at all. If you were on a system that didn't match "9" <=> 0x39 it would still replace those byte values.

Maybe this is a bit too much if you are just starting but read the first paragraph at least and then also look at "Relationship to ASCII" and "Relationship to Unicode"

https://en.wikipedia.org/wiki/Code_page

You can actually change this.

The compiler has an 'execution' character set that describes how characters should be interpreted at run time, and a 'source' character set, and they need not be the same.

Check this out

$ cat test_ebdic.c ; gcc -fexec-charset=EBCDIC-US test_ebdic.c ; ./a.out | xxd
#include <stdlib.h>
#include <stdio.h>
int main(void)
{
        char data[] = "Hello";
        printf("%s\n", data);
        return EXIT_SUCCESS;
}
00000000: c885 9393 960a                           ......

The file was typed in UTF-8, and the compiler automatically translated the literal to EBCDIC-US to emit at run time. My terminal can't display this, so I view it with a hex editor.

@akunohana Apologies if I'm missing something, but 0x30, 0x39, and 0x18 are merely hex notation for integers. You could have written 48, 57, and 24 instead for the same effect. Or you (probably?) could have used char literals like '0', '9' (I guess there isn't one for U+0018).

UTF-8 determines how the characters are encoded, i.e. what sequence of numbers (ints) they are represented by. So there's no special understanding of Unicode or hex going on here; you're comparing numbers (as you should).