🚀 go-pugleaf

Jason Nunn wrote:
> 
> Chia-shyan Chou (cschou@csie.nctu.edu.tw) wrote:
> : I had write a structure as the following under linux:
> 
> : unsigned short No1; // 2 byte
> : unsigned char No2; //1 byte
> : unsigned char No3; //1 byte
> : unsigned char No4; //1 byte
> : unsigned char No5; //1 byte
> : unsigned char No6; //1 byte
> : unsigned short No7; //2 byte
> 
> : The length of this structure should be 9 bytes, however,
> : the sizeof function return 10.
> 
> : How can I avoid this problem ? Does gcc compiler provide the conditional
> : switch to do byte alignment ?
> 
> Your problem lies in how gcc aligns your data. Intel 32 bit machines
> running in Protected Mode, optimal alignment is 4 bytes. Therefore, bytes
> will be stored in a 4 byte space (i can't explain why your struct is 10
> bytes know?).. anyway, you need byte alignment. 

Pure speculation: 1st 3 take one 32-bit word, next 3 take another and
then you need 2 more bytes for ushort#7.  In other words, ushort#7
starts at word boundary and uchars 4,5 & 6 are padded with an extra
byte.

BTW you lose speed when you pack your bytes -- you gain space, of
course.  Also, packing doesn't make your life easier if you intend
to e.g. read it with [f]getc() or recvfrom() -- they return int
which you may have to split into bytes; that's platform dependent, 
too.

Just a thought... 
-- 
Dimitri
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