New memory in DLL that using it in another projects

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Hi,

In a Dll project (VC++) , I defined a function that get wchar_t* and return wchar_t* in a parameter.
My Question is this: Is it correct to create the pointer(new wchar_t*) with specified size then return it as parameter refernce ?
like "sret" in my code:

extern "C"
{

	__declspec(dllexport) int  Function(wchar_t*  s, wchar_t*  sret,  int len)
	{
		int sLen = wcslen(s);
		sret = new wchar_t(sLen+4);
		wcscpy_s(sret, sLen, s);
		wcscat_s(sret, 4, L"_456");
		len = wcslen(sret);
		return 1;
	}
}

I want to use this dll in other windows projects . They's memory doesn't occur with problems?

What I have tried:

new the wchar_t* into a dll function then return it

Posted 20-Jan-18 23:29pm

Zon-cpp

Updated 21-Jan-18 4:13am

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Richard MacCutchan · Answer 1 · 2018-01-21T02:35:00

This will not work because sret is a local pointer, so the calling application will still have it pointing somewhere else. You need the address of the pointer, something like:

C++

__declspec(dllexport) int  Function(wchar_t*  s, wchar_t**  sret,  int len)
{
    int sLen = wcslen(s);
    *sret = new wchar_t(sLen+4); // should be sLen + 5
    wcscpy_s(*sret, sLen, s);
    wcscat_s(*sret, 4, L"_456");
    len = wcslen(*sret); // what is this line for?

    return 1; // what is this line for?
}

However, a much better way would be to just return the pointer, something like:

C++

__declspec(dllexport) wchar_t* Function(wchar_t*  s)
{
    int sLen = wcslen(s);
    wchar_t* sret = new wchar_t[sLen + 5];
    wcscpy_s(sret, sLen, s);
    wcscat_s(sret, 4, L"_456");

    return sret;
}

Is there a specific reason that you think this needs to be in a DLL?

CPallini · Answer 2 · 2018-01-21T04:13:00

Solution 2

In addition to Richard solution, have a look at c++ - Is it bad practice to allocate memory in a DLL and give a pointer to it to a client app? - Stack Overflow[^].
Unfortunately mismatches between allocator and deallocator do happen and are not pleasant.

Posted 21-Jan-18 4:13am

CPallini

Comments

Richard MacCutchan 21-Jan-18 12:07pm

Ha, ha. That told me :(