Today I had another encounter with Win32::API. I have some C code I want to access from Perl. So, I compiled it in Visual C++ into a DLL, but Win32::API kept segfaulting, although loading the same DLL from another C++ program worked fine. Another round of investigation began…

To cut a long story short, here is the correct way to compile C code into a DLL and access it from Perl.

The C code

I’m going to write a simple C function that demonstrates some interesting concepts like passing data in and out with pointers. Here is the .h file:

int __stdcall test1(char* buf,
int num, char* outbuf);

This is a (almost) normal declaration of a function named test1 that takes two pointers to a char and one integer as arguments, and returns an integer.

__stdcall is a Visual C++ compiler keyword that specifies the stdcall calling convention. The stdcall convention is used by Windows API functions.

There’s another common calling convention - __cdecl which is usually used for “normal” (not Windows API) code. The Win32::API Perl module supports only __stdcall, so while we could use __cdecl for binding this DLL to another piece of C / C++ code, it doesn’t work with Win32::API.

The .c file provides the implementation:

#include "dll_test.h"

int __stdcall test1(char* buf,
int num, char* outbuf)
{
int i = 0;

for (i = 0; i < num; ++i)
{
outbuf[i] = buf[i] * 3;
}

return num;
}

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