从delphi调用c ++函数

Question

I wrote a DLL in C++ whose functions will be called from a Delphi application.

One of the functions in the DLL takes a Pointer to a buffer where an XML string should be written. But, when I write a string into the buffer, after returning from the function the application crashes with an "Access violation at address 0048B... in module ....exe. Write of address 3030D..." error.

The calling convention of the function declarations are the same, both in the DLL and the application.

I've made a simple application in Delphi to simulate the behavior of the application and it works fine. The biggest problem is that I don't have any information about the application internals: no sources, no documents, not even logs. Just function declarations and parameter descriptions.

Function declaration in delphi:

function functionName(var Buffer: Pointer; var BuffLen: Integer): Integer; stdcall;

Function Declaration in the DLL:

extern "C" int WINAPI functionName(char*, int*);

Does someone know how to solve this?

From my tests, I have a feeling that the problem is in the application, not in the DLL. However, I'm not completely sure about this. Are there any possible tests I can do at the DLL site to either solve the problem or locate the issue?

I'd really appreciate any help in this matter.

As a side note, the DLL is compiled with Visual Studio. Can this cause the problem?

Answer 1

The DLL function you showed is declared wrong in your Delphi code. var Buffer: Pointer is equivilent to void** in C, or void*& in C++, but certainly not to char* like the DLL function is expecting. Using a void** / void*& parameter would be useful if the DLL were allocating memory to return to the application, but from your description that is not the case.

Use this Delphi declaration instead:

function functionName(Buffer: PAnsiChar; var BuffLen: Integer): Integer; stdcall;

PAnsiChar in Delphi is equivalent to char* in C/C++.

You should read the following blog article about the gotchas to watch out for when converting C/C++ declarations to Delphi:

Rudy's Delphi Corner: Pitfalls of converting