OFFSET of Structure Member

[RAKHE]

struct emp
{
int a;
char b;
int c;
};



I want to find offset of the structure member a, b, c, please anyone
explain about this

 

[Alexander Klauer]

struct emp
{
int a;
char b;
int c;
};

I want to find offset of the structure member a, b, c, please anyone
explain about this

http://c-faq.com/struct/offsetof.html

 

[Mark Bluemel]

struct emp
        {
                int a;
                char b;
                int c;
        };

I want to find offset of the structure member a, b, c, please anyone
explain about this

What do you need explained?

 

[Keith Thompson]

struct emp
{
int a;
char b;
int c;
};



I want to find offset of the structure member a, b, c, please anyone
explain about this

Why? If you have the offset, what do you intend to use it for?

 

[Uno]

/* BEGIN new.c */

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

int main(void)
{
struct emp {
int a;
char b;
int c;
};

printf("offsetof(struct emp, a) is %lu\n",
(long unsigned) offsetof(struct emp, a));

printf("offsetof(struct emp, b) is %lu\n",
(long unsigned) offsetof(struct emp, b));

printf("offsetof(struct emp, c) is %lu\n",
(long unsigned) offsetof(struct emp, c));

return 0;
}

/* END new.c */

$ gcc -Wall -Wextra p2.c -o out
$ ./out
offsetof(struct emp, a) is 0
offsetof(struct emp, b) is 4
offsetof(struct emp, c) is 8
$

In this case, my implementation's macro is essentially the same as
Plauger's.

Linux:
#ifdef __compiler_offsetof
#define offsetof(TYPE,MEMBER) __compiler_offsetof(TYPE,MEMBER)
#else
#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)
#endif

Plauger:
#define offsetof(T, member) ((_Sizet)&((T *)0)->member)

Can you say a few words about how it works?

 

[Peter Nilsson]

Linux:
...
#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)
...
Plauger:
#define offsetof(T, member)     ((_Sizet)&((T *)0)->member)

Can you say a few words about how it works?

<http://c-faq.com/struct/offsetof.html>

The two implementations above assume null pointers map to
'address zero'.

 

[Uno]

[snipping Plauger.]
[Please refer to the above macro herewith.]

<http://c-faq.com/struct/offsetof.html>

The two implementations above assume null pointers map to
'address zero'.

Can you "talk through" the whole thing? It takes me a bit with the logic.

 

[Peter Nilsson]

[snipping Plauger.]
[Please refer to the above macro herewith.]

Can you "talk through" the whole thing?

Suppose you have...

struct {
T1 elm1;
T2 elm2;
T3 elm3;
};

An instance will be layed out as...

[elm1][elm2][elm3]
^ ^ ^
| | |
addr1 addr2 addr3

....where addr2 and addr3 will be a fixed number of bytes
(offset) from addr1.

Now suppose you could lay out the same instance at
address 0...

[elm1][elm2][elm3]
^ ^ ^
| | |
0 addr2 addr3

At a guess, what do you suppose addr2 and addr3 will
represent in the second case?

 

[Uno]

Linux:
...
#define offsetof(TYPE, MEMBER) ((size_t)&((TYPE *)0)->MEMBER)

[snipping Plauger.]
[Please refer to the above macro herewith.]

Can you "talk through" the whole thing?

Suppose you have...

struct {
T1 elm1;
T2 elm2;
T3 elm3;
};

An instance will be layed out as...

[elm1][elm2][elm3]
^ ^ ^
| | |
addr1 addr2 addr3

...where addr2 and addr3 will be a fixed number of bytes
(offset) from addr1.

Now suppose you could lay out the same instance at
address 0...

[elm1][elm2][elm3]
^ ^ ^
| | |
0 addr2 addr3

At a guess, what do you suppose addr2 and addr3 will
represent in the second case?

They are where struct.elm2 and struct.elm3 respectively begin.