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DLL function: returning a string -confusion-

Hi.
I've written a dll, I want that dll to return a string.
example:

extern "C" __declspec(dllexport) char* getString()
{
//do stuff

return "a string";
}

My question is about memory. I _think_ in this code I am returning a pointer to the string but if the memory is allocated in the dll, when is it deleted?

If the pointer to the string and the memory for the string is allocated within the dll, why is it that my application (that uses the dll/function) has access to this locally allocated value?

As you can see, I'm confused. How SHOULD I return a string? If I allocate it on the heap, can I delete it in the calling process?

Brief explaination would be appreciated.

Regards,
David.

ASKER CERTIFIED SOLUTION

AlexFM

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dave_p_r_b

ASKER

thank you.

How is it possible to delete a value that is not on the local heap? surely this would cause an exception? No?

Regards,
David.

dave_p_r_b

ASKER

(I was refering to your second example).

stsanz

The EXE and the DLL access the same memory space : the process memory space.

A DLL can allocate space that will be deleted by the EXE, and the opposite is also true.

dave_p_r_b

ASKER

//DLL function :

extern "C" __declspec(dllexport) char* getString()
{
char* s = new char[10];
strcpy(s, "a string");
return s;
}

//exe function :

int _tmain(int argc, _TCHAR* argv[])
{
//stuff

s = getString(); // s receives the same value as it is given in the dll

delete s; //ASSERTs here
}

//-----------------

in dbgheap.c
/*
* If this ASSERT fails, a bad pointer has been passed in. It may be
* totally bogus, or it may have been allocated from another heap.
* The pointer MUST come from the 'local' heap.
*/
_ASSERTE(_CrtIsValidHeapPointer(pUserData));

This assert fires. I assume as the addresses of s are the same, it is not a bad pointer. Is that a fair assumption?

Thanks for your help
Regards,
David.

chofreither

You should do "delete [] s;" since s is an array.

GloomyFriar

>A DLL can allocate space that will be deleted by the EXE, and the opposite is also true.
NO!
If you allocate in DLL you have to free in the same DLL
If you allocate in EXE you have to free in the same EXE

SOLUTION

macrom

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dave_p_r_b

ASKER

Thanks to all.

Brilliant answer macrom.... thank you.