Purebasic interface to Box2D / Chipmunk2D [solved. again]

[Lunasole]

And again there are problems making me stuck. Don't want to make another dll-import-problem thread so leaving it here.

Why does this code leads to data corruption (or stack corruption, not sure) and how it is possible to handle returning structure (not pointer) correctly in PB?

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; Create an empty space.
Define *space.cpSpace = cpSpaceNew()	; cpSpaceNew returns a pointer to structure stored inside external lib

; it is all OK, the *space data is valid
Define *testG.cpVect = cpSpaceGetGravity (*space) ; cpSpaceGetGravity returns a whole cpVect structure, not just a pointer
; now the first fields of *space data structure are corrupted [seems one first field which size is 4 bytes, as pointer size]

The called C function:

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typedef double cpFloat;
typedef struct cpVect{cpFloat x,y;} cpVect; 

cpVect
cpSpaceGetGravity(const cpSpace *space)
{
	return space->gravity; 
}

That's how it is declared in PB:

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Macro cpFloat			: d : EndMacro
Structure cpVect ; Align not needed, cause lib temporary re-compiled with 1 byte align
	X.cpFloat
	Y.cpFloat
EndStructure

cpSpaceGetGravity.i   (*space.cpSpace) As "_cpSpaceGetGravity";  = cpVect

[infratec]

Hi,

I'm not really the specialist for such cases, but in my opinion:

It returns a pointer, but you have to copy the structure immediately to a 'real' structure.
Maybe it is later no longer available.

Bernd

[Lunasole]

Hi,

I'm not really the specialist for such cases, but in my opinion:

It returns a pointer, but you have to copy the structure immediately to a 'real' structure.
Maybe it is later no longer available.

Bernd

Just tried something like this, all the same:

[Lunasole]

It seems to be another annoying PB limitation. And I am not familiar with ASM to fix it.
If I haven't access to a lib sources it would be total damn disaster. But fortunately I have, so modified called function this way and it works fine:

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void
cpSpaceGetGravity(const cpSpace *space, cpVect *lp_out)
{
	*lp_out = space->gravity;
}

Now calling this way

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Define Gout.cpVect
cpSpaceGetGravity (*space, Gout)

[DontTalkToMe]

It returns a pointer, but you have to copy the structure immediately to a 'real' structure.

Unfortunately (for us using PB) in C you can return structures by value and I think (not sure) the way to do so it's not specified in the language.

EDIT
http://stackoverflow.com/questions/2155 ... structures
http://stackoverflow.com/questions/9653 ... ction-in-c

[Lunasole]

It returns a pointer, but you have to copy the structure immediately to a 'real' structure.

Unfortunately (for us using PB) in C you can return structures by value and I think (not sure) the way to do so it's not specified in the language.

EDIT
http://stackoverflow.com/questions/2155 ... structures
http://stackoverflow.com/questions/9653 ... ction-in-c

> The address of the caller's Data return value is actually passed as a hidden argument to the function, and the createData function simply writes into the caller's stack frame.

Sounds interesting. So technically there is no difference between

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Define ReturnStruct
someFunct (@ReturnStruct)

and returning structure byval in C/C++ compilers style. And no arguments against using such return.

I'd like to experiment trying passing that "hidden pointer" (it might be possible without using ASM) but it is too late, I've already changed all 41 problematic functions to void and using *lp_out as return