This is a debugging story and the lessons learned.
I was writing an application using a
MailSwitchToAccount API new in WIndows Mobile 5.0. According to docs it’s defined in cemapi.h and an app needs to link with
cemapi.lib.
That’s how I called it:
hr = MailSwitchToAccount(_T("SMS"));
Visual Studio 2005 refused to link the app, claiming that:
TestDevice.obj : error LNK2019: unresolved external symbol
"long __cdecl MailSwitchToAccount(wchar_t const *,unsigned long)"
(?MailSwitchToAccount@@YAJPB_WK@Z) referenced in function "void
__cdecl OnMailSwitchToAccount(void)" (?OnMailSwitchToAccount@@YAXXZ)
My first thought was that this symbol is simply missing from cemapi.lib. So I used dumpbin.exe (part of Visual Studio tools) to dump all symbols exported from cemapi.lib:
dumpbin -exports cemapi.lib
I found that it does have MailSwitchToAccount:
?MailSwitchToAccount@@YAJPBGK@Z (long __cdecl
MailSwitchToAccount(unsigned short const *,unsigned long))
Howerver, if you look closely, you’ll see that signatures don’t match: expected type for the first parameter is unsigned short * while I’m calling it as wchar_t const *. Now, unsigned short is the WCHAR Windows UNICODE type. C++ also defines wchar_t which is also UNICODE char.
I vaguely remembered that some C++ compilers have an option to treat wchar_t as a native language type (as opposed to just a typedef for existing type, unsigned short in Windows’ case).
And indeed, there it was, in project properties, C/C++/Language there’s an option “Treat wchar_t as Built-in Type”, set by default to Yes.
You can set it to “No”, which corresponds to passing /Zc:wchar_t- to cl.exe.
It seems wrong that you have to do that. It seems like cemapi.dll/cemapi.lib were compiled with /Zc:wchar_t- which forces everyone who links to them also be compiled like that.
Lessons learned:
- C++ is evil
- dumpbin.exe is your friend
- Visual Studio’s code browsing is your friend (it’s easy to find out what a given typedef or #define really is)
- be aware of “wchar_t as built-in type or not” issue
Update: turns out it’s a known problem and has been blogged about on
official Visual Studio blog. Good news is that it will probably be fixed in future versions of Visual Studio.