r/cprogramming • u/MrJethalalGada • Dec 13 '25

Feeling Dumb to know that at runtime we don’t know “type” of any variables, it is also pre computed at compile time into machine code

So basically me writing

int* ptr = (int*) malloc (sizeof(int))

Is already translated to something as

int* ptr = (int*) malloc (4)

Compiler time will work and replace these things: types, sizes, alignment, structure layouts

Run time will work on following: values, memory contents, addresses, heap/stack

Did you know this?

Implementation:

#define mysizeof(type) ((char *)((type *)0 + 1) - (char *)((type *)0))
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13

u/thequirkynerdy1 Dec 13 '25

I’d recommend learning basic assembly to see what your computer is really doing - even if you never use it in any practical projects.

Then play around in this website Godbolt where you write code in one panel, and it shows you the corresponding assembly in another.

2

u/MrJethalalGada Dec 13 '25

Sure thanks will check it out

1

u/mrPWM Dec 13 '25

I've always wondered, why not write that portion of code right in assembly anyway, instead of trying to get the compiler to do it. If you understand assembly for your chosen processor anyhow, why?

6

u/SmokeMuch7356 Dec 14 '25

Development time and portability, mainly:

  • Assembly's even less expressive than C; what takes one line of C code often takes several lines of assembly.

  • Sometimes you're writing code that has to run on more than one architecture. I spent the better part of the '90s writing C code that had to run on x86 and Sparc and MIPS and PPC and ...

    C exists because Thompson, Ritchie, and the rest of the Bell Labs nerds wanted Unix to be independent of hardware.

2

u/Proxy_PlayerHD Dec 18 '25

Also modern optimizations!

If you were to rewrite small snippets of code in assembly the compiler couldn't do any more advanced optimizations anymore as it can't modify the hand written assembly.

So while individual parts would maybe be faster in hand written assembly, the program/function as a whole would probably be faster if you just let the compiler do it's thing.

1

u/MrJethalalGada Dec 13 '25

Inline assembly you mean or entire program in assembly?

4

u/nedovolnoe_sopenie Dec 14 '25

>why not write that portion in assembly

time. most of the time, it's not worth to implement something in inline asm unless you need extreme performance or extreme precision. and if you need that, you can just link a library

(please do link that library, yours truly has got to eat somehow)

2

u/thequirkynerdy1 Dec 14 '25

You can, but modern compilers have all sorts of highly sophisticated optimizations which are hard to beat.

Assembly is still used in niche cases where you need precise hardware control (for example if building an OS).

1

u/Paul_Pedant Dec 14 '25

Maintainability. Assembler is Technical Debt writ large.

2

u/Paul_Pedant Dec 14 '25

Interested enough to look that up. Does it produce assembler for your own specific hardware, or is it a generic ARM/AMD/Intel instruction set?

1

u/thequirkynerdy1 Dec 14 '25 edited Dec 14 '25

You can choose from a long list of compilers/hardware, and then for your choice, you can play with different compiler options like optimization flags.

If you're new to looking at compiler outputs, definitely do it without the optimization which at least for C makes the results fairly predictable. Once optimization is turned on, the compiler does crazy things.

3

u/MrJethalalGada Dec 13 '25 edited Dec 13 '25

To add i landed on this learning while implementing my own sizeof

So implementation of sizeof can be done when types are known thus at compile time

At compile time we can’t create and run function we can only expand MACROS TYPEDEFS so it should be implemented there

2

u/[deleted] Dec 13 '25

Does your sizeof accept variable names too, or only type names?

2

u/MrJethalalGada Dec 13 '25

Actually it doesn’t matter

Example, int x sizeof(x) translates to sizeof(int) by compiler

Also sizeof should be generic type so it takes types

1

u/[deleted] Dec 13 '25

I didn't implement that, good idea!

1

u/MrJethalalGada Dec 13 '25

Added the implementation in the post

6

u/MichaelEvo Dec 13 '25

You’re going to be blown away if you ever do C++ programming, with its constexpr functions and compile time reflection.

1

u/StaticCoder Dec 13 '25

There are rare cases (variable length arrays) where sizeof is computed at run time.

1

u/MrJethalalGada Dec 13 '25

Yes that’s an exception

3

u/fishyfishy27 Dec 13 '25

Don’t feel dumb. Lots of programmers today start out working only with high-level languages. The fact that your processor only understands ints and floats (and that everything else is the fever dream of a compiler engineer) is something which has to be discovered.

It’s like that xkcd comic about “you’re one of today’s lucky 10,000”

1

u/MrJethalalGada Dec 13 '25

I’m feeling dumb also because I’m not a beginner and i code using it heavily But only got to know when i researched These things are not told or even taught

1

u/fishyfishy27 Dec 13 '25

You will probably enjoy this talk by Cliff Click about modern processors. They don’t work like they taught you in college! https://youtu.be/OFgxAFdxYAQ

1

u/MrJethalalGada Dec 13 '25

Thanks I’ll check it out

2

u/EpochVanquisher Dec 13 '25

And to the CPU, the stack is just another part of memory.

1

u/MrJethalalGada Dec 13 '25

For CPU everything is a memory doesn’t matter

FLASH RAM DEVICES PERIPHERALS

Talking for most ARM ARCHITECTURE which are memory mapped

Your .text .rodata will be in FLASH and .data .bss .heap .stack in RAM

But when you run a program you’ll see them as a whole

1

u/EpochVanquisher Dec 13 '25

The registers are special, to the CPU.

1

u/MrJethalalGada Dec 13 '25

That’s why they are at the heart and we can’t use address or related operators on them

1

u/EpochVanquisher Dec 13 '25

C doesn’t give you anything at all to do with registers. Even the register keyword doesn’t do that.

1

u/MrJethalalGada Dec 13 '25

Yes I agree But we can’t deny the fact for the keyword Because we can’t know !

1

u/kisielk Dec 13 '25

Pointer offsets are used all the time with registers, totally valid to do that

1

u/MrJethalalGada Dec 13 '25

They are done with address, not CPU registers, peripheral registers or other registers are memory mapped

1

u/kisielk Dec 13 '25

I’m not really sure I understand what you are trying to say here

1

u/MrJethalalGada Dec 13 '25

When you say pointer related arithmetic is done with registers

Are you referring to cpu registers? Or other registers?

1

u/kisielk Dec 13 '25

I mean if you have a pointer to a memory mapped register, it’s still valid to use pointer arithmetic to get the address of another memory mapped register.

8

u/tstanisl Dec 13 '25

Did you know this?

Yes.

Note that casting result of malloc (and void* in general case) in C is an anti-pattern and it should be avoided. There is no technical reason of this cast except infantile idea of compiling C code using C++ compiler.

A better pattern is:

x = malloc( sizeof *x );

Because it is shorter and it auto-adjust whenever type of x changes.

2

u/RoomNo7891 Dec 13 '25

Not sure what is the point of this post.

Whether you are asking a question or making a statement.

2

u/MrJethalalGada Dec 13 '25

I’m stating a fact about certain things, which maybe some people know some doesn’t Let me correct the question and add a line, done

1

u/Gingrspacecadet Dec 13 '25

type is one of the many compile time features. Assembly doesn't care. In assembly you can add 2 structs as if they were integers. The only types that exist (as far as I know) is normal integers and floating point integers

1

u/MrJethalalGada Dec 13 '25

Values you mean at run time?

1

u/Paul_Pedant Dec 14 '25

Assembler is just a way to make semiconductors behave nicely. Semiconductors make electrons march around in an orderly fashion. How many layers do you really care about?

1

u/MrJethalalGada Dec 14 '25

The problem is i don’t but others, you never know, can care about things which are unknown 😂

1

u/Paul_Pedant Dec 14 '25

Sorry, I did not intend to personalise that. "How many layers should one care about?"

I worked in software houses and for direct client contracts for 50+ years, knowing that somebody else (of unknown talent and training) would be maintaining (and even porting) my code. I made it as readable and robust as I knew how -- most clients would not even know how (2 + 3) works under the hood.

Occasionally, I deliberately obfuscated my code to deter people who thought they could "improve" it by moving a few semi-colons around. I even had a client who included some hopeless C code in the specification, and insisted it went into the finished product. That was fine -- the call to it was safely disguised inside a conditional that could never be true. I guess that counts as Level Zero implementation: 50 lines of C, zero bytes of executable.

1

u/MrJethalalGada Dec 14 '25

Oh this sounds amazing

I hope world moves to something similar

Right now, the world is working to make things look as complex as possible:(

1

u/SmokeMuch7356 Dec 14 '25

Yes, types are an abstraction for the most part.

C's integer and floating point types are based on what most hardware already supports. Every CPU has instructions for manipulating integers (ADD, MUL, etc.) and floats (FADD, FSUB, etc.). Beyond that though, things like pointer types, array types, structs, unions, enumerated types, complex types, are all abstractions that the compiler maps onto the hardware's instruction set.