How to release memory allocate in c++ dll in delphi application

Question

I have a problem with mem allocate in c/c++ dll and call it with delphi, sth like this:

create a dll with c/c++

#include "MemTestDll.h"

extern "C" EXPORTAPI char* __cdecl Test()
{
    char* str=new char[1024*1024*2];
    return str;
}

then in delphi:

function Test():PAnsiChar;  cdecl; external 'MemTestDll.dll';

procedure TForm3.btn3Click(Sender: TObject);
var
  ptr:PAnsiChar;
begin
   ptr:=Test();
  //FreeMem(ptr); // crash
  //SysFreeMem(ptr) //crash too
end;

I see the taskmanager,each click will leak 8 KB memory.

How can i release ptr? FreeMem this pointer will crash the application
I allocate 1024*1024*2 bytes in C/C++ dll,why it shows leak 8KB?

paddy · Accepted Answer · 2013-08-15 02:20:30Z

7

The rule for using dynamic memory across DLL boundaries is that whoever allocated the memory must also free it. You cannot allocate memory in the DLL and then free it outside the DLL. So you should provide another function in your DLL that will free a pointer.

answered Aug 15, 2013 at 2:20

paddy

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4 Comments

Haozes Over a year ago

Is the rule of windows dll or delphi rule? When i use c++ call a c/c++ dll, delete that pointer in caller,that's ok. I don't know the size i should allocate in caller,so i must free it in caller

paddy Over a year ago

This is a fundamental rule of using DLLs anywhere

Móż Over a year ago

Haozes: you can get away with that if you have a shared memory manager. But it's a bad idea, since someone else might import your dll and not use the memory manager. This is why, for example, a lot of windows API calls take a buffer and the buffer size as parameters. You're saying "tell me this, put the answer into this chunk of memory I allocated for you". Most of those, if you set the size to zero, will return the size they need.

dummzeuch Over a year ago

In theory it should be possible for the dll to use an OS API call for allocating the memory which could then be released by the application using the corresponding OS call to free the memory. Usually this isn't what happens, though, because all programming languages come with their built in memory managers which add another layer on top of the OS API calls and the dlls use them.

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