offsetof

<<split from https://cboard.cprogramming.com/showthread.php?t=40936>>

If I call this offsetof() macro in a function:

Code:

---

int func([type] struct_name, [type] member)
{
  ...
  x = offsetof(struct_name, member);
  ...
}

---

What should be the [type]s in the function prototype? That is, what are the types of the parameters of offsetof()?

> What should be the [type]s in the function prototype?
You can't.

How are we supposed to know?
Post the real code.

offsetof is a macro, not a function, and the first argument is a struct or union, the second a member of said struct/union.

Eg.

Code:

---

struct blah
{
  int x;
  int y;
};

int main()
{
  printf("offsetof(blah, y)= %d\n", offsetof(blah, y));
  return 0;
}

---

Pedantisizm prevention: If we are usign C99, we probably should use %zd as the format specifier, as offsetof forms a size_t value, and that should use the z modifier to tell printf that it is of "size_t" size, rather than regular int.

--
Mats

Just some messing about...

Code:

---

#include <stddef.h>
#include <stdio.h>

struct T
{
  int i;
  double d;
  char c;
  char s[10];
  float f;
};

int Toffset(int member)
{
  switch ( member )
  {
  case 0: return offsetof(struct T, i);
  case 1: return offsetof(struct T, d);
  case 2: return offsetof(struct T, c);
  case 3: return offsetof(struct T, s);
  case 4: return offsetof(struct T, f);
  default: break;
  }
  return 0;
}

int main(void)
{
  int i;
  for ( i = 0; i < 6; ++i )
  {
      printf("Toffset(&#37;d) = %d\n", i, Toffset(i));
  }
  return 0;
}

/* my output
Toffset(0) = 0
Toffset(1) = 8
Toffset(2) = 16
Toffset(3) = 17
Toffset(4) = 28
Toffset(5) = 0
*/

---

Quote:

---

Originally Posted by matsp

Pedantisizm prevention: If we are usign C99, we probably should use %zd as the format specifier, as offsetof forms a size_t value, and that should use the z modifier to tell printf that it is of "size_t" size, rather than regular int.

---

Pendant attack!

d is for signed integrals, u is for unsigned integrals, so the correct C99 format specifier for size_t would be %zu. In strict C89, it would be better to use %lu and cast to unsigned long instead of using %d.

Pedantisizm accepted. However, it is HIGHLY unlikely that the offset within a struct is bigger than 2^n-1 where n is the number of bits in size_t. So signed or unsigned will make no difference.

--
Mats

Agreed, it likely wouldn't make a difference.

Quote:

---

Originally Posted by robwhit

Pendant attack!

---

I hope we're not being attacked by necklaces! You mean pedant not pendant...

QuantumPete

Quote:

---

Originally Posted by QuantumPete

I hope we're not being attacked by necklaces! You mean pedant not pendant...

QuantumPete

---

LOL!!! I didn't even see that 'n' there.

no, I meant necklaces.

they'll choke you, they will.

edit: off to change my user title...