On Thu, Dec 02, 1999 at 17:49:09 -0500, Robin Kirkman wrote:
>
> Johan wrote:
> > structs are returned in a bizarre way that you don't want to deal with.
> > (I can explain if anyone cares to know...)
>
> Yes, please do explain...
> I am writing a tios function call wrapper for regular gcc, and i haven't
> got it to return values yet, just do the calls.
> --robin
Ok, you've been warned ;)
struct S {
...;
};
struct S rets_struct(...) {
...;
}
void gets_struct(...) {
struct S foo;
...;
foo=rets_struct(...);
...;
}
A struct is returned by letting the callee write it to the top of the
caller's local variable area.
When rets_struct() is called, a6 points *immediately after* where
rets_struct() should write the returned struct.
Note that this will clobber local variables if there's no memory reserved in
the stack frame for the struct-being-returned.
When rets_struct() returns and gets_struct() gets in charge again, it should
copy the returned struct to its final position (to 'foo' in my example above).
Stack when rets_struct() is running, assuming both functions use real stack
frames:
- - - - - - - - - - - - - - -
gets_struct():
| arg n |
| : |
| arg 1 |
+--> | old a6 |
| | STRUCT | <- returned struct will be written here by rets_struct()
| | foo |
+----| ... |---+
| ... | |
- - - - - - - - -|- -|- - - -
rets_struct(): |
| arg m | |
| : | |
| arg 1 | |
a6 -> | old a6 | >-+
| local |
| : |
| local |
| ... |
sp -> | ... |
- - - - - - - - - - - - - - -
(Does my explaination make any sense?)
//Johan