Why #pragma pack also affects structs' own alignment?

Question

I noticed that when #pragma pack is used around a struct, the alignment inside of it is not the only one that is affected, but also the alignment of the struct itself changes. Consider the following:

#include <stdio.h>
#include <stdint.h>

#pragma pack(1)
typedef struct _TEST
{
    uint32_t a;
} TEST;
#pragma pack()

volatile uint8_t n;
TEST b;

int main()
{

    printf("Address %lX rem %lu\n", (long unsigned int)&b, (long unsigned int)(&b)%(sizeof(int)));  
    return 0;
}

You can try this code is here: https://onlinegdb.com/BkebdxZEU

The program returned Address 601041 rem 1, which means that the pragma also had an effect of aligned(1) on the struct.

Why is that? Is this a defined behavior?

C standard doesn't define pragmas, so pragmas are compiler-specific. In other words - this is not a defined behaviour. — qrdl
@Lundin I think the documentation is quite clear: 1) #pragma pack(n) simply sets the new alignment. (gcc.gnu.org/onlinedocs/gcc-4.4.4/gcc/…) — Ctx
@Ctx Yes it clearly changes the alignment but that doesn't answer why it went off by 1 on that online compiler. There's nothing else allocated save for the struct variable, so why does the compiler pick a misaligned address just for the sake of it? — Lundin
@Lundin The compiler does not deliberately choose a misaligned address, in my case the variable b is simply located adjacent to the transactional memory, which happens to end at 404030 <__TMC_END__>. The next address is then used: 404031 <b> — Ctx
@Ctx That's probably the correct answer then: .bss layout on the specific system. Because when I change it to TEST b={1}; and thereby move the variable to .data, the misaligned address goes away. — Lundin

th33lf th33lf · Accepted Answer · 2020-02-24T08:51:40

The alignment of a structure is affected by the alignment requirements of its members. A structure as a whole is usually aligned to the alignment of its largest member. Because your struct contained a uint32, it would have been aligned to four bytes, had you not invoked the #pragma before.

However, with #pragma pack(1), you force the alignment required for all its members to 1-byte (or no alignment) and hence the structure can now start at any address in memory, not necessarily at a multiple of four bytes.

Why #pragma pack also affects structs' own alignment?

2 Answers