Why is the size of this struct 12 and not 10?

Question

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

I'm trying to parse ID3v2 tags, at least get important data like artist, title, and album, but I'm having trouble setting up the necessary structures.

struct ID3v2_frame{
  char id[4];
  unsigned int size;
  bool flags[2];
};
ID3v2_frame frame;
cout<<sizeof(frame)<<endl;

It's a problem with how I'm setting up a 4 byte int I think. When I output sizeof(frame) it outputs 12, my intended output is 10. I'm running on a 64 bit linux machine.

Yea it is pretty much, didn't come across it in my search, got the answer though, thanks — carboncomputed, Mar 14 '12 at 05:23
@leppie: `bool` was not a standard type in C89, but it was in C99 after you `#include ` (and `_Bool` is a type in C99 at all times). — Jonathan Leffler, Mar 18 '12 at 21:22

score 0 · Answer 1 · answered Mar 14 '12 at 05:19

0

Please check in your main program:

sizeof(bool) //should return 2 my guess
sizeof(char) //should return 1
sizeof(unsigned int) //should return 4

Every compiler can have different sizes for data types.

answered Mar 14 '12 at 05:19

hrishikeshp19

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score 0 · Answer 2 · answered Mar 14 '12 at 05:21

0

Note That bool size on x64 is 2 bytes

answered Mar 14 '12 at 05:21

Muhannad A.Alhariri

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score 0 · Answer 3 · edited May 23 '17 at 11:51

0

You may find your answer from here or here.

Please check What packing? default packing (8)

edited May 23 '17 at 11:51

Community

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answered Mar 14 '12 at 05:26

Divyang Mithaiwala

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score 0 · Answer 4 · edited May 23 '17 at 11:43

It is because of structure packing. Compiler would make the size of structure a multiple of 4 bytes. If you do not want this to happen and see size as 10 bytes in your case, use __attribute__((packed))

struct ID3v2_frame{
char id[4];
unsigned int size;
bool flags[2];
}__attribute__((packed));

Note that this might affect the performance of the program. See this SO question:

Can __attribute__((packed)) affect the performance of a program?

score -1 · Answer 5 · answered Mar 14 '12 at 05:17

-1

My guess would be the terminator character in the flags and id array.

answered Mar 14 '12 at 05:17

joebro

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I seem to remember in my data structures class when you declare in array, the compiler always throws in a /n at the end to let the compiler know the string is done. So id would look like `id = [],[],[],[],/n` but i could be way off base too. – joebro Mar 14 '12 at 14:52

Why is the size of this struct 12 and not 10?

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