Data Alignment Issue
If a data structure will be used in both a 32bits binary and a 64bits
binary, e.g., an ioctl structure which is passed from a 32bit
application to the 64bit kernel, then we need to consider the code
alignment issue. Because:
- The structure size might be different for a same structure.
- The offset of each structure member might be different for a same
structure.
The alignment can be decided by compiler options. The default compiler behavior will conform to the top rule: all structure members must start at its alignment addresses, and, the whole structure should start at the alignment address of its largest member.
For example, uint16_t will always start at even address, uint32_t will always align at 4 bytes boundary, etc. There are also some points as following:
-
The whole structure's size will be according to the biggest member's
alignment. e.g., if the biggest member is uint32_t, then the whole
sizeof(struct) will be divided exactly by 4bytes. -
If there are "long", "pointer", or "long double" types in the
structure
members, then the alignment in 32bit and 64bit binaries will be
different. Because they are different length. For example, if there is
a pointer in a structure, then, in a 32bits binary, it aligns at 4
bytes
address; in a 64bits binary, it aligns at 8 bytes address. -
The max alignment of 32bits binary is 32bits, The max alignment of
64bits binary is 64bits. So, if there is a "long long" in a 32bits
binary, it aligns at 4 bytes address; however, the same type aligns at
8 bytes address in a 64bits binary. -
"enum" type is considered as uint_t.
Code alignment feature can be disabled by setting specific compiler options, such as, gcc -fpack-struct, which will remove the code alignment feature, that is, there are no gaps between structure members.
And "pack" pragma can also do the same job, details refer to the related section of "C User's Guide". Note that the pragma might cause problems on different hardware platforms.
The following table is comparing the data types between 32bits and 64bits binaries.
C Type ILP32 LP64
char 8 8
short 16 16
int 32 32
long 32 64
long long 64 64
float 32 32
double 64 64
long double 96 128
pointer 32 64
-----------------------------
An data structure Example:
typedef enum list {
a0 = 0,
a1 = 1,
a2 = 2,
} list_t;
struct align {
uint16_t a;
uint8_t b;
list_t list;
uint8_t c;
uint8_t d;
} sa;
offset_a=fffffd7fffdff954, offset_b=fffffd7fffdff956,
offset_list=fffffd7fffdff958, offset_c=fffffd7fffdff95c,
offset_d=fffffd7fffdff95d, sizeof_allign=12
Posted by david mcdaniel on July 18, 2007 at 04:27 AM CST #
Posted by DP on July 20, 2007 at 03:33 AM CST #
Posted by Colin on August 03, 2007 at 05:51 PM CST #
this article is about memory alignment - not *code* alignment, as the title suggests.
Posted by dila on September 18, 2007 at 07:15 AM CST #
i mean it's about data alignment - "memory" is just as ambiguous, heh
Posted by dila on September 18, 2007 at 07:17 AM CST #
Yes, I think "data alignment" is better. I've modified the title of this article. Thanks for your comments. :-)
Posted by Colin on September 18, 2007 at 09:08 PM CST #
Hi
I have a query on following structure
struct sachin
{
char a;
short int b;
int c;
}
What to do with this structure that every field of this structure will aligned to 4 byte ? And sizeof () will come as 12.
Thanks
Sachin
Posted by Sachin Rastogi on June 04, 2008 at 09:42 PM CST #
Sachin,
I tested your structure, the sizeof() is 8. If your test result comes to 12, I guess is because of compiler options.
Posted by Colin Zou on July 13, 2008 at 04:21 PM CST #
Colin,
My query is "How to align every parameter of the above structure to 4 byte so that sizeof(sachin) will come to 12".
Sachin
Posted by Sachin on July 14, 2008 at 01:00 AM CST #
Move int c to the second element as following:
struct sachin
{
char a;
int c;
short int b;
}
Then the size will be 12.
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