Runtime Library mis-matches and VC++ - Oh, the misery!

It seems that all my adult life I've been tormented by the VC++ linker complaining or balking because various libraries do not agree on which version of the Runtime library to use. I'm never in the mood to master that dismal subject. So I just try to mess with it until it works. The error messages are never useful. Neither is the Microsoft documentation on the subject - not to me at least.

Sometimes it does not find functions - because the name-mangling is not what was expected? Sometimes it refuses to mix-and-match. Other times it just says, "LINK : warning LNK4098: defaultlib 'LIBCMTD' conflicts with use of other libs; use /NODEFAULTLIB:library" Using /NODEFAULTLIB does not work, but the warning seems to be benign. What the heck is "DEFAULTLIB" anyway? How does the linker decide? I've never seen a way to specify to the linker which runtime library to use, only how to tell the compiler which library to create function calls for.

There are "dependency walker" programs that can inspect object files to see what DLL's they depend on. I just ran one on a project I'm trying to build, and it's a real mess. There are system .libs and .dll's that want conflicting runtime versions. For example, COMCTL32.DLL wants MSVCRT.DLL, but I am linking with MSVCRTD.DLL. I am searching to see if there's a COMCTL32D.DLL, even as I type.

So I guess what I'm asking for is a tutorial on how to sort those things out. What do you do, and how do you do it?

Here's what I think I know. Please correct me if any of this is wrong.

  1. The parameters are Debug/Release, Multi-threaded/Single-threaded, and static/DLL. Only six of the eight possible combinations are covered. There is no single-threaded DLL, either Debug or Release.

  2. The settings only affect which runtime library gets linked in (and the calling convention to link with it). You do not, for example, have to use a DLL-based runtime if you are building a DLL, nor do you have to use a Debug version of runtime when building the Debug version of a program, although it seems to help when single-stepping past system calls.

Bonus question: How could anyone or any company create such a mess?


Your points (1) and (2) look correct to me. Another thing to note with (2) is that linking in the debug CRT also gives you access to things like enhanced heap checking, checked iterators, and other assorted sanity checks. You cannot redistribute the debug CRT with your application, however -- you must ship using the release build only. Not only is it required by the VC license, but you probably don't want to be shipping debug binaries anyway.

There is no such thing as COMCTL32D.DLL. DLLs that are part of Windows must load the CRT that they were linked against when Windows was built -- this is included with the OS as MSVCRT.DLL. This Windows CRT is completely independent from the Visual C++ CRT that is loaded by the modules that comprise your program (MSVCRT.DLL is the one that ships with Windows. The VC CRT will include a version number, for example MSVCRT80.DLL). Only the EXE and DLL files that make up your program are affected by the debug/release multithreaded/single-threaded settings.

The best practice here IMO is to pick a setting for your CRT and standardize upon it for every binary that you ship. I'd personally use the multithreaded DLL runtime. This is because Microsoft can (and does) issue security updates and bug fixes to the CRT that can be pushed out via Windows Update.

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