AVR-Chat

On Wed, Mar 25, 2009 at 04:06:21PM -0000, Graham Davies wrote:
> --- In AVR-Chat@yahoogroups.com, David VanHorn <microbrix@...> wrote:
> 
> > Casting is something that I'm not up on yet.
> 
> Casting is taking a variable of one type and treating it as if it were
> of a different type.  You do this in assembler all the time.  A
> strength of C is that the compiler is supposed to protect you from
> doing it when you don't mean to, as in the above case.  An "implicit"
> cast is one that you don't write but that happens anyway to match up
> the types.  These can be benign (such as treating an 8-bit integer as
> a 16-bit integer when necessary), wasteful and confusing (such as
> treating an 8-bit integer as a 16-bit integer when required by the
> language definition but not strictly necessary), or dangerous and
> probably wrong, as in your case.
> 
> With decades of high-level language programming experience behind me,
> I have a policy of avoiding implicit casts.  In other words, I write
> my casts out in the code.  Then, I can see what the compiler is going
> to do because I'm telling it exactly.  These are called "explicit"
> casts.  You might want to consider this.

Casts can get ugly pretty quick, especially when using pointers. I
usually end up defining a UNION32_t typedef before a project is finished
so that I can mix types and reach inside values easier.

typedef union {
	struct {
		uint8_t	a,
			b,
			c,
			d;
	} u8;
	struct {
		uint16_t ab,
			 cd;
	} u16;
	uint32_t	u32;
	int32_t		i32;
} UNION32_t;

Then I can declare "UNION32_t tmp;" and use it as tmp.u32 or tmp.i32 or
just use parts from inside with tmp.u8.a, or whatever.

Before copying this into your own code spend some time thinking about
tmp.u16.ab and whether its spelled right and in the right location to
pick up bytes a and b, or b and a, or whether or not the first should be
c and d. Am pretty sure as shown its not right for an AVR.

-- 
David Kelly N4HHE, dkelly@HiWAAY.net
========================================================================
Whom computers would destroy, they must first drive mad.