Re: newby.-interpreting C

[Jan-Benedict Glaw <jbglaw@lug-owl.de>]

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On Mon, 2004-12-20 20:07:54 +0100, soraberri <421246@posta.unizar.es>
wrote in message <cq77vi$1ej$1@sea.gmane.org>:
> Hi all,
> 
> Anyone could give me the meaning of this definition? Is the 
> __attribute__ stuff what scares me. If you feel like I'm too 
> desoriented, please tell me what should I know first.
> 
> typedef struct {
> 	uint8_t b[6];
> } __attribute__((packed)) bdaddr_t;

Compilers do use __attribute__ to allow programmers to modify the
compiler's behavior in certain areas. "packed" tells the compiler to not
start each variable at a natural alignment (ie. 4 bytes on a 32bit
machine, 8 bytes on a 64bit machine). (That is, the address of a given
variable must be a multiple of 4 (or 8) bytes.)

A "packed" structure is usually used for two things:

	- In device drivers to fit a hardware device's memory-mapped
	  register structure
	- In poorly written programs to store variables into on-disk
	  files to be read back later on by other programs.

General rule: if you don't *need* this for a good reason, or if you even
don't know what it does, you don't need it, since it also introduces a
performance penalty: CPUs tend to be fast at naturally aligned memory
accesses, but quite slow on non-aligned accesses.

MfG, JBG

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