On Friday 02 January 2009 18:48:50 Wolfgang Denk wrote:
In message 200901021742.05363.vapier@gentoo.org you wrote:
+__u8 get_vfatname_block[MAX_CLUSTSIZE] __attribute__ ((aligned(sizeof(__u16))));
...
+__u8 get_dentfromdir_block[MAX_CLUSTSIZE] __attribute__ ((aligned(sizeof(__u32))));
...
+__u8 do_fat_read_block[MAX_CLUSTSIZE] __attribute__ ((aligned(sizeof(__u32))));
What makes you sure that a 16 resp. 32 bit alignment is sufficient, and that gcc does not decide to align such structures even stricter?
read the structures in question. the largest member is 16bit/32bit. i really dont know what other alignment gcc could randomly generate.
It is not (only) the size of the "largest" member in a struct which determins global alignment, but also the size of the struct itself.
See for example http://gcc.gnu.org/onlinedocs/gcc-4.3.2/gcc/Type-Attributes.html
Read especially this part:
In the example above, if the size of each short is 2 bytes, then the size of the entire struct S type is 6 bytes. The smallest power of two which is greater than or equal to that is 8, so the compiler sets the alignment for the entire struct S type to 8 bytes.
i was not aware of this, thanks for the highlight
Wouldn't it make more sense to use "__alignof__ ()" here to be on the safe side?
i found no such alignment attribute in the gcc docs which is why i suggested Bryan use aligned(sizeof(...)). http://gcc.gnu.org/onlinedocs/gcc-4.3.2/gcc/Function-Attributes.html http://gcc.gnu.org/onlinedocs/gcc-4.3.2/gcc/Variable-Attributes.html http://gcc.gnu.org/onlinedocs/gcc-4.3.2/gcc/Type-Attributes.html
Maybe you should have searched for the (IMHO pretty obvious) term "Alignment" as well, i.e. look at http://gcc.gnu.org/onlinedocs/gcc-4.3.2/gcc/Alignment.html#Alignment
Quote:
The keyword __alignof__ allows you to inquire about how an object is aligned, or the minimum alignment usually required by a type. Its syntax is just like sizeof.
well i guess we cant all be "Wolfgang Denk"s -mike