Re: [cc65] returning structs

Groepaz wrote:
> On Montag 10 August 2009, Oliver Schmidt wrote:
>> Hi Groepaz,
>>
>>>> I know that you cant return a struct by value.  Is there any way around
>>>> this?  If I return a pointer to the struct, the values are lost upon
>>>> leaving the function.  Can anyone provide an example workaround?
>>> make it return a pointer, and then copy it to a local struct:
>>>
>>> memcpy(&localstruct,functhatreturnsstruct(),sizeof(localstruct));
>>>
>>> (thats pretty much almost exactly what a compiler would do too)
>> Maybe it's woth mentioning that your proposal presumes that memcpy()
>> a) is inlined / is a specially designed intrinsic / <you name it>
>> b) doesn't use the stack itself
>> because otherwise calling it could overwrite the data you want to
>> copy. From that perspective I'd rather classify this approach as hack
>> than as workaround.
> 
> hu? how would calling memcpy destroy the data that was passed to it?

It does not destroy the data passed to it, but may destroy the data the
pointer returned by `functhatreturnsstruct()` points to.  Example:

typedef struct {
    char x, y;
} bar;

*bar foo() {
    bar result;
    result.x = 42;
    result.y = 23;
    return &result;
}

int main(void) {
    bar local;
    memcpy(&local, foo(), sizeof(local));
    return 0;
}

`foo()` is called and returns a pointer to `result` on the stack and
before `memcpy()` is called this area is destroyed by overwriting it
with `memcpy()`\s arguments.

Ciao,
	Marc 'BlackJack' Rintsch


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