is there a simple way (macro?) to tell structure alignment?

Question

I understand the structure alignment is 'implementation specific', but just wondering if there is any simple way to calculate the structure alignment, for example:

typedef struct
{
    char c;
    int  i;
} test;

if sizeof(test) - (sizeof(char) + sizeof(int)) == 0 means alignment is  1 byte;
if sizeof(test) - (sizeof(char) + sizeof(int)) == 1 means alignment is  2 bytes;
// ...etc...

Is the above assumption reliably true?

Answer 1

If all you want is a good first guess, then transitively find the maximum size of the largest non-aggregate member in the structure and any nested structures

Answer 2

Your method is not reliable in the general case: if you tried it on this struct:

typedef struct
{
    int i1;
    int i2;
} test;

it would give you zero as the result, which is probably not the alignment.

Here is a method that works, though:

#include <stddef.h>

struct dummy
{
    char x;
    test t;
};

size_t test_alignment = offsetof(struct dummy, t);

Answer 3

Hi Caf, thanks for your post, but does the following work?

#include <stddef.h>

struct dummy
{
    char x;
    int  i;
};

size_t test_alignment = offsetof(struct dummy, i);

Answer 4

And here's the macro I have in my toolbox (using caf's technique) to get the alignment of a type:

#define ALIGNMENT_OF(t) offsetof( struct { char x; t dummy; }, dummy)

So now you can use such expressions as:

typedef struct
{
    char c;
    int  i;
} test;

size_t x = ALIGNMENT_OF( test);    // will likely set x = 4

Note that a particular compiler might support a non-standard intrinsic that you might prefer to use (for example, MSVC at some point started supporting an __alignof() intrinsic). My header defines ALIGNMENT_OF() using that intrinsic if it detects a compiler that supports it (though I honestly don't know what advantage the intrinsic provides).

Answer 5

Make alligned structures

#pragma pack(1)
struct tight{           
   short element_1;       
   int *element_2;
};
#pragma pack(pop)