Home / Debugging adventure

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@@[email protected]) 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@@[email protected] (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). At 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.