Function returning Structure: How does it work?

[Paul John Falstad]

In article <1990May28.215331.29333 at agate.berkeley.edu> m100-2ai at WEB.berkeley.edu () writes:
>in ANSI C, I can define a function returning STRUCT, such as the following:
>
>STRUCT function(void)
>{
>	STRUCT structure;
>
>	structure.num = 0;
>	/* Set other fields as well */
>	return structure;	
>}
>
>Now, it seems that the storage space for 'structure' has to be allocated
>on the heap (not on stack).  However, wouldn't this create a lot of

No, that's not how it's done.  I thought it was handled by having the
whole structure pushed on the stack, but it turns out that's not how its
done either.

I tried this program with Amiga Manx 5.0a:

struct foo { int a,b; };
struct foo fooer(void)  { struct foo bar = {1,2}; return bar; }
main()
{
struct foo bar;

  bar = fooer();
}

Manx compiled it the same way as the following:

struct foo fooer(struct foo *ptr)
{
struct foo bar = {1,2};

 *ptr = bar;
 return ptr;
}
main()
{
struct foo bar;

  fooer(&bar);
}

So that's one way to do it.  I tried this gcc on a VAX, and it handled it by
returning the structure in r0 and r1.  I made the structure much bigger
(too big to fit in registers), and gcc compiled it pretty much the same
way as above.

-- 
Paul Falstad  PLINK:Hypnos GEnie:P.FALSTAD net:pfalstad at phoenix.princeton.edu
Disclaimer: My opinions, which belong to me and which I own, are mine.
-Anne Elk (not AN elk!)  The sun never set on the British empire because
the British empire was in the East and the sun sets in the West.