Locally disable padding

Question

I write a parser for some data structure, after hours of debugging I found out that the problem is Visual Studio doesn't interpret the structures as I tell it. It seems some "padding" is used

struct foo { 
unsigned char a; //0x00
unsigned char b; //0x01
unsigned int c; //0x02
unsigned int d; //0x06
unsigned int e; //0x0A
unsigned int f; //0x0E
//0x12
};

I expected "sizeof(foo)=4*4+2=18" but I get "sizeof(foo)=20". Is there any possibility to turn padding off just for this special struct? I tried

__declspec(align(1)) struct foo { ...

but it does not work. Thank you for your help.

Dinse

I think this article on MSDN will help. – Mcintire

Are you compiling with /Zp 1? – Ankerite

@MartinBroadhurst That would globally disable padding, The solution of hans works. – Dinse

Answer 1

Use the #pragma pack directive for that:

#pragma pack(push, 1)
struct foo { 
  // etc..
};
#pragma pack(pop)

Cosec

Answer 2

Visual Studio 2010 has #pragma pack to do what you're looking for.

Familist

But wouldn't that affect all structs? I just want it locally for that one struct. – Dinse

You can revert it afterwards - did you even read that documentation? – Familist

You can use #pragma pack push/pop to get around that. Just push before and pop after the struct. – Vibrator