GCC implicit alignment problem. (64-bit code)

How can I explicitly disable alignment on defined variable in gcc?

Take this code:

typedef struct{
  unsigned long long offset;
  开发者_开发百科unsigned long long size;
  unsigned long type;
  unsigned long acpi;
}memstruct;

memstruct *memstrx;

This would define an structure having a size of 24 bytes.

I tried doing:

memstrx=(void*)(0x502);

So

&memstrx[0] should have an value of 0x502

&memstrx[1] , 0x51A

&memstrx[2] , 0x532

... and so on and so forth.

But things doesn't seem to be right.

Instead, the

&memstrx[1] , displays an address of 0x522

&memstrx[2] , 0x542

&memstrx[3] , 0x552

... and so on and so forth.

I suspect GCC has implicitly re-sized the structure to 32 bytes (from 24 bytes), forcing a (64-bit alignment of each entry). And I don't really want this behavior only for this structure. How should I tell GCC to not align that structure?

No it can't be done.

The minimal size of the structure you show is 8*4 = 32 bytes.

sizeof(unsigned long) = 8 on 64 bit architecture (Linux)

Edit: if you would use

-unsigned instead of unsigned long

or

• uint32_t and uint64_t instead of unsigned long and unsigned long long

you would get expected alignment.

#pragma pack(x) can change the structure alignment restrictions on both GCC and MSVC.

GCC uses the LP64 model for 64bit builds - which means longs and pointers are 64bits. You need to change to unsigned int for your 32bit fields, OR use and uint32_t and uint64_t for stable field sizes.

#pragma pack(1)

typedef struct{
  unsigned long long offset;
  unsigned long long size;
  unsigned int type;
  unsigned int acpi;
}memstruct;

#pragma pack()

here's one option to control alignment using gcc:

http://gcc.gnu.org/onlinedocs/gcc/Structure_002dPacking-Pragmas.html