Re: Simple signal/event question

["Peter Garner" <peter garner toward com>]

Hi Havoc, Hi All! :-)

>The return value has to be stored in memory somewhere; if the 
>compiler thinks there is no return value when invoking the 
>function pointer it doesn't have to make that memory.  But if the 
>function being invoked has a return value it will put the return 
>value there anyway.

I was going to take this to private email with Havoc, but I 
decided it MIGHT be of interest to the list.  The reason being is 
that some gtk signal CALLBACK can return values and others may 
not.  In fact one of the MOST powerful features of GTK is the 
almost infinite variety of signatures that one can use with 
callback functions.  Thus we all might run into a situation where
we have a mismatch with the return type.

Merely calling a function with that returns a value when the 
compiler thinks it returns void will NOT cause a problem on ANY 
machine/compiler combination of which I am aware.  C/C++ 
compilers generally just place the return value in a register and 
leave it at that.  On the ix86 series, calling "return 0" merely
loads EAX with 0.  In the case of structures, things are a little 
more complicated, but the same principle applies.

All that matters is what you DO with the return 
value.  If your function returns void, and the caller THINKS a 
value is being returned, and attempts to use the return value, 
it will use whatever random garbage is in the accumulator (EAX) 
register.  If the caller THINKS the function returns void (which 
is what we were originally discussing) I cannot think of ANY
situation where this would cause problems since the caller will 
simply ignore the [valid] data in EAX.  Of course if you are 
returning the address of some malloced structure [in EAX] and 
ignore that, you will get a memory leak, but that is ENTIRELY 
another story! ;-)

In summary, substituting a non void function in place of a 
function that returns void should not cause a problem.  I 
apologise to anyone who thinks this OTO for this list.  However, 
given the almost unlimited flexibility of gtk signals, this may 
become an issue for some of us.  In addition, understanding how 
return values work is essential for calling and 
interfacing assembler routines from C/C++.  In otherwords to 
return a value to a C caller from an assembly routine, simply
load EAX with the value before returning from the function.
(And of course, if you are using C++, do not forget to declare 
the asm routine as extern "C"! ;-)