Why g++ isn't performing structure packing here?

Question

Consider following program:

#include <iostream>
struct __attribute__((__packed__)) mystruct_A
{
   char a;
   int b;
   char c;
}x;
int main()
{
    std::cout<<sizeof(x)<<'\n';
}

From this I understood following:

Structure packing suppresses structure padding, padding used when alignment matters most, packing used when space matters most.
Structure Packing, on the other hand prevents compiler from doing padding

I am on 32 bit environment & using Windows 7 OS. The 1st answer of linked question says that above code would produce structure of size 6 on a 32-bit architecture.

But when I compiled it using g++ 4.8.1 it gives me 9 as an output. So, is structure packing not happening completely here? Why extra 3 bytes are there in output? sizeof char is always 1. Sizeof int is 4 on my compiler. So, sizeof above struct should be 1+4+1=6 when structure is packed.

I tried it on here. It gives me expected output 6.

Is there any role of processor or it depends only on Compiler?

@JohnZwinck: please attach the link if you test it on clang. — Destructor
are you on x86 CPU? Also, which compiler? (there are two different ports of g++ to windows 32, mingw and mingw-w64) — M.M
Using mingw-w64-i686-gcc 5.2 with any of the __attribute__((packed)) variants mentioned here, I get size 9. Using mingw-w64-i686-clang 3.7 and the same attributes I get size 6. Using g++ again with #pragma pack(push, 1) / #pragma pack(pop) around the struct, I also get size 6. — melak47

Unknown Unknown · Accepted Answer · 2016-05-13T00:10:50

attribute packed is broken on mingw32 compilers. Another option is to use pragma pack:

#pragma pack(1)
struct mystruct_A {
  char a;
  int b;
  char c;
} x;

Why g++ isn't performing structure packing here?

2 Answers