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Removing padding from structure in kernel module

Answer 1

+1 A:

If using GCC, you can use the packed attribute on your structure to prevent padding:

struct foo
{
    void * bar;
}
__attribute__( ( packed ) );

Macmade 2010-05-04 07:21:07

Dunno why this didn't work out. Still got size as 36.

dexkid 2010-05-04 08:02:23

Answer 2

+1 A:

#pragma pack might work

drawnonward 2010-05-04 07:21:09

This worked for me. Thanks.

dexkid 2010-05-04 08:03:34

Answer 3

A:

I suspect that GCC is forcing the total structure to be aligned onto a 32 bit boundary, so its size is a multiple of 4.

Imagine the following. struct foo {

void * bar ; some other stuff ..... };

struct foo my_foo_array[10];

Then if the sizeof(struct foo) is not a multiple of 4 then. my_foo_array[0].bar has a different memory alignment to my_foo_array[1].bar. The processor would need to perform 2 32 bit memory accesses in order to access all four bytes of my_foo_array[1].bar. x86 processors will do this reassembly of misaligned 32 bit values but most other processors will throw some form of bus error exception which is not good.

The packed attribute signals how the elements of the structure are packed with respect to each other, but in normal operation the start of structure needs to placed on a 32 bit aligned address.

I hope this explains things a little better.

Andrew Roca 2010-05-14 18:12:20

Hi Andrew,GCC generates a 32 bit aligned address on x86 by default as it is optimum for x86 32 bit processor to fetch data. I only wanted to know how to solve to padding issue with GCC.The pragma pack worked for me.

dexkid 2010-05-15 10:20:43

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