How constant is const in C?

Let's look at the simple statement

const int a = 2;

If you come from a C# perspective, you expect the keyword const to mean that the value in a will never change. It's a way to help the compiler optimise the code.

In C, however, const is not exactly what you would expect.
For sure if you later wrote

a = 4;

then the compiler will throw an error, something along the lines of:

error: assignment of read-only variable ‘a’

But the thing is, this is C, so if you want to shoot yourself in the foot, you can quite easily do it.

(In C# it is possible to change the value of a const with reflection, or unsafe code. It's a fair amount of code, but it can be done.)

This is all the work it takes:

#include <stdio.h>
int main()
{
  const int a = 2;
  printf("a: %d\n", a);
  int* x = &a;
  *x = 4;
  printf("a: %d\n", a);
  return 0;
}

Just like that. Create a pointer, give it the address of your constant and set the value via the pointer.

Now, depending on the compiler, and your compiler options, you will likely get a warning, something like:

warning: initialization discards ‘const’ qualifier from pointer target type

So, it doesn't go by unnoticed, but it compiles and the code will run just fine.

This is part of the reason I use #define for constants. The other is it's just what I've always done.

I think, for C, it's helpful to think of const as only referring to the variable name, and not the data behind the variable. It's akin to a promise that the variable won't be used to change the value, and not that the value will never change.