I’m unsure on whether it’s normal or it’s a compiler bug but I have a C struct with lot of members. Among of them, there’s, :
struct list {
...
...
const unsigned char nop=0x90; // 27 bytes since the begining of the structure
const unsigned char jump=0xeb; // 28 bytes since the begining of the structure
const unsigned char hlt=0xf4; // 29 bytes since the begining of the structure
unsigned __int128 i=0xeb90eb90eb90eb90f4f4 // should start at the 30th byte, but get aligned on a 16 byte boundary and starts on the 32th byte instead
const unsigned char data=0x66; // should start at the 46th byte, but start on the 48th instead.
}; // end of struct list.
I had a hard time to find out why my program wasn’t working, but I finally found there’s a 2 bytes gap between hlt
and i
which is set to 0x0. This means that the i
is getting aligned.
This is very clear when I printf that part of the structure, because with :
for(int i=28;i<35;i++)
printf("%02hhX",buf[i]);
I get EBF40000EB90EB90
on the screen.
I tried things like volatile struct list data;
in my program, but it didn’t changed the alignment problem.
So is there a #pragma
or a __attribute__
to tell gcc to not align i
inside struct list
type ?
In GCC you can use __attribute__((packed))
like this:
// sizeof(x) == 8
struct x
{
char x;
int a;
};
// sizeof(y) == 5
struct y
{
char x;
int a;
} __attribute__((packed));
See doc.
Also if you rely on the addresses of struct fields, take a look at the offsetof
macro. Maybe you don't need to pack the structure at all.