在Visual Studio 2017中的errno上获取链接错误lnk2001

Question

I have some C code that's compiled in every version of Visual Studio until I upgraded to 2017. For some reason, the upgrade process broke my linker references which resulted in over 400 link errors for things like HWND, SendMessage etc.

I slogged through the linker properties input list and added user32.lib, gdi32.lib and a few others to resolve most of the missing external symbol errors but can't get rid of the last 2 errors. Both of them are looking for _errno which old C folks will recognize as a file system error result variable. I explicitly added libcmtd.lib to my linker input list and still no joy.

How do I get around this last link error in VS2017?

Answer 1

The answer turned out to be that VS2017 altered the errno.h file to also declare errno. In past Visual Studio implementations, errno.h was just a list of defines for the various return values of errno. In VS2017, errno.h conditionally declares errno.

My file that made an extern reference to errno only cared if errno was zero or not so I didn't bother including errno.h in the file.

All that was required was to include errno.h in the file that was making an extern reference to errno.

The change in VS2017 makes sense so it's a bit odd it wasn't done much earlier.