why the size of this struct is not correct

Question

I have this sample code which I decorated with pack to make sure that its size if 5 byte (4 for int and 1 for char).

but it print out that the size of struct is 8 byte.

#pragma pack push
#pragma pack 1
struct mystruct
{
    int x;
    char y;
};
#pragma pack pop

//static_assert(sizeof(mystruct) == 5, "Size of mystruct should be 5 byte.");

int _tmain(int argc, _TCHAR* argv[])
{
    int x=sizeof(mystruct);
    printf("size of struct is %d\n",x);
    return 0;
}

Why te pck is not working?

How can make sure that size of struct is always 5.

Please read about [Padding](http://stackoverflow.com/questions/6968468/padding-in-structures-in-c). your question already have more than 100 duplicates on SO only. — Dayal rai, Apr 25 '14 at 10:51
`C` or `C++`? There's a difference. Please, just pick the one you use. — stefan, Apr 25 '14 at 10:51
@mans That's one of my favourite typos ever ;-) (You'd never afford to do it though.) — molbdnilo, Apr 25 '14 at 10:56
And what are you trying to do with this? Why is it a problem if the size is not 5? — James Kanze, Apr 25 '14 at 11:16

score 8 · Accepted Answer · answered Apr 25 '14 at 10:57

8

Your pragma syntax is wrong.

Try this:

#pragma pack(push,1)
...
#pragma pack(pop)

Demo.

answered Apr 25 '14 at 10:57

n. m. could be an AI

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This is not guaranteed to work in all implementations – pmg Apr 25 '14 at 11:00
@pmg: The OP specified which implementation he's using. This is the correct approach. Using `char[N]` is frankly ridiculous. – Lightness Races in Orbit Apr 25 '14 at 11:01
@pmg I already tested and it should work on GNU compiler too. – mans Apr 25 '14 at 11:02
@pmg Pragmas are by definition not guaranteed to work in all implementations. They give implementations implementation-specific directives. – n. m. could be an AI Apr 25 '14 at 11:44

score 0 · Answer 2 · answered Apr 25 '14 at 10:51

0

I believe the compiler is permitted to ignore the pack pragma; perhaps it can be overridden with compiler switches.

answered Apr 25 '14 at 10:51

Codor

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"Permitted" in what sense? The standard doesn't even define that pragma. – Lightness Races in Orbit Apr 25 '14 at 10:52
@Lighness Races in Orbit Well that puts it better. – Codor Apr 25 '14 at 10:56

score 0 · Answer 3 · edited May 23 '17 at 12:04

The compiler can always ignore your attempts to stop packing. This, in some ways, is a good thing, as the packing allows a higher level of efficiency... Think of an array, when you access element 1, it is efficiently traversed to as the distance, in terms of memory, between the elements is uniform. This is also performed with structures in order to ensure efficiency is maintained.

I direct you to this stack overflow Disable structure padding in C without using pragma

score 0 · Answer 4 · answered Apr 25 '14 at 10:59

Strange- I have structs in a 3rd party program that are packed on 1-byte boundaries and they do work as expected (though not liked!).

If the compiler is ignoring your pragmas, try setting the alignment compiler options as well - C/C++ | Code Generation | Struct Member Alignment to 1.

If the compiler is doing an "I know best" optimisation, perhaps putting the char first would stop it.

why the size of this struct is not correct

4 Answers4