Value, value, who's got the value?

[Geoff Rimmer]

In article <1044 at itivax.iti.org> scs at vax3.iti.org (Steve Simmons) writes:

> int	func1() { int b ; b = 2 ; } 
> int	func2() { int c=3 ; c; }
> 
> main() 
> {
> 	   int a = 1 ;
> 	   printf( "Value of a is %d\n", a ) ;
> 	   a = func1() ;
> 	   printf( "Value of a is %d\n", a ) ;
> 	   a = func2() ;
> 	   printf( "Value of a is %d\n", a ) ;
> }
> 

Get a compiler that loves to complain!  gcc will do just fine.  It'll
print a warning saying that func1 and func2 are *supposed* to return
ints, but that nothing is in fact returned.  The idea of this warning
is to stop you thinking that func1() should always return 2 on every
compiler.  It may happen, it may not.  On our system, the return value
will simply be whatever is left in the d0 register.  For example, the
following program:

int func() { int b; b = 2; }
int main() { int a; a = func(); }

produces the following assembler code for func():

_func:
	link a6,#-4         ; get 4 bytes for b.
	moveq #2,d0         ; set register d0 to 2.
	movel d0,a6@(-4)    ; move 2 into b.
L1:
	unlk a6		    ; cleanup heap.
	rts		    ; return to main()

and this for main()

_main:
	link a6,#-4         ; get 4 bytes for a.
	jbsr _func	    ; jump to func()
	movel d0,a6@(-4)    ; move d0 into a.  So a becomes 2.
L2:
	unlk a6	
	rts

The compiler writers *could* have implemented this in a totally
different way.  They could easily have chosen a different register to
return values from functions, and then, you would get unpredictable
values.

"Glad to be of service!"
Geoff

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