Question

So I need to remove constness from some variables in C (I know what I’m doing). So I wrote a little macro (UNCONST) which lets me assign an new value to a const value. This works just fine for normal variables of types like int. But this doesn’t work for pointers. So I can’t let a pointer point to a different position using my macro UNCONST without getting an compiler warning.

Here a little test program unconst.c:

#include <stdio.h>

#define UNCONST(type, var, assign) do { \
  type* ptr = (type*)&(var); \
  *ptr = (assign); \
} while(0)

struct mystruct {
  int value;
  char *buffer;
};

int main() {
  // this works just fine when we have an int
  const struct mystruct structure;
  UNCONST(int, structure.value, 6);
  printf("structure.value = %i\n", structure.value);

  // but it doesn't when we have an char *
  char *string = "string";
  UNCONST(char *, structure.buffer, string);
  printf("structure.buffer = %s\n", structure.buffer);

  // this doesn't work either, because whole struct is const, not the pointer.
  structure.buffer = string;
  printf("structure.buffer = %s\n", structure.buffer);
}

compiling & executing

$ LANG=en gcc -o unconst unconst.c
unconst.c: In function ‘main’:
unconst.c:21:3: warning: assignment discards ‘const’ qualifier from pointer target type [enabled by default]
unconst.c:25:3: error: assignment of member ‘buffer’ in read-only object

Is there a way to optimize my macro so this warning doesn’t show up?

Answer 1

The problem in:

const char *string1 = "string1";
UNCONST(char *, string1, string2);

is that string1 is not really const, it can be assigned to, but points to an array of const characters.

Actually, doing:

string1 = string2;

compiles just fine.

Other thing is if you want to copy the array. Then I’d write your macro as this:

#define UNCONST(type, var) (*(type*)&(var))

And so:

UNCONST(int, i) = 42;

To copy the array of chars you could do:

strcpy(UNCONST(char**,string1), string2);

FOOTNOTE: actually I find this macro rather useless: I find:

*(int*)&i = 42;

as cumbersome as this kind of operation should be.