How are struct mem in C?

Question

Possible Duplicate:
Why isn't sizeof for a struct equal to the sum of sizeof of each member?

I can't understand struct mem alloc.

typedef struct a{
} a;

sizeof(a) is 0

typedef struct b{
    int bb
} b;

sizeof(b) is 4

typedef struct b2{
    int *bbbb
} b2;

sizeof(b2) is 8

typedef struct d{
    int x;
    int *y;
} d;

sizeof(d) is 16!

Why sizeof is 16?Is it 12?(int+int point=4+8);

I guess sizeof(int) is 4,if int var in struct,mem is 8?

http://stackoverflow.com/questions/119123/why-isnt-sizeof-for-a-struct-equal-to-the-sum-of-sizeof-of-each-member — Prasoon Saurav, Sep 09 '12 at 16:42

score 4 · Accepted Answer · answered Sep 09 '12 at 16:43

4

Why sizeof is 16?Is it 12?(int+int point=4+8);

Since the pointer is 8 bytes wide on your machine, the compiler felt obliged to align it to an 8 byte boundary, for performance reasons. On some architectures it may not even be strictly a performance problem: unaligned access to certain types can be outright illegal.

In other words the compiler is allowed to add padding everywhere, except before the first element of the structure.

answered Sep 09 '12 at 16:43

cnicutar

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I understand!then struct must is 8byte multiple? – SamPeng Sep 09 '12 at 16:45
thank your answer.I understand your mean.this is Machine-dependent. – SamPeng Sep 09 '12 at 16:49
@SamPeng It can also be compiler-dependent. – cnicutar Sep 09 '12 at 16:51
yes,I find this wiki [Data structure alignment](http://en.wikipedia.org/wiki/Data_structure_alignment) through your help. – SamPeng Sep 09 '12 at 16:55

score 1 · Answer 2 · answered Sep 09 '12 at 16:42

1

Depending on what platform you are on, sizeof(int *) could be 4 or 8 (or other). Also, your compiler has the freedom to add unused padding bytes to the struct if it wants to.

answered Sep 09 '12 at 16:42

David Grayson

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score 0 · Answer 3 · answered Sep 09 '12 at 16:43

0

This is due to padding and memory alignment. Refer to this link for further explanations.

answered Sep 09 '12 at 16:43

Deepanjan Mazumdar

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score 0 · Answer 4 · answered Sep 09 '12 at 16:45

0

Memory aligment is at work. Things need to be aligned.

answered Sep 09 '12 at 16:45

Ed Heal

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How are struct mem in C?

4 Answers4