What's the difference between linking against libeay32MD.lib and libeay32MT.lib?

Question

I have third-party application with source, which is currently set to be built against libeay32MD.lib. But this is application, not library. Shouldn't it be built against libeay32MT.lib then? What's the difference between two?

There are the following variants of libraries:

• libeay32MD.lib
• libeay32MDd.lib
• libeay32MT.lib
• libeay32MTd.lib

and "static" ones with the same name. Can you explain the difference between all of them?

Answer 1

These variations determine which C++ library is used, and what type of code is generated, although only 'M' is available meaning multi-threaded, there are no single-threaded options any more.

• MTd = Multi threaded debug code, and linking to C++ static debug library
• MDd = Multi threaded debug code, and linking to C++ dynamic (DLL) debug library
• MT = Multi threaded, linking to C++ static library
• MD = Multi threaded, linking to C++ dynamic (DLL) library

Just edited - sorry, codes were in wrong order.

Edit 2: More info...

These flags are C++ options and nothing to do with requiring ssleay32.dll and libeay32.dll. There are 8 version of libeay32 - 4 for a static build (no dependency on ssl/libeay32.dll) and 4 for dynamic build (requires ssl/libeay32.dll). Each of the 4 are divided into the type of C++ library required...

C++ is available to link statically or dynamically to your application, and for each of these types you can use debug libraries or release libraries.

/MT and /MTd (static) do not require the C++ redistributable code because all the C/C++ calls are contained inside your compiled program. If every module (not only ssleay & co.) you link to uses these options, your app will be fully stand-alone in terms of C++ dependencies.

/MD and /MDd (dynamic) need the C++ redistributable DLL's installed on the target computer. For /MD the releases are easily downloaded from MSFT, but you also need to note which version of Visual C++ you used - e.g. VC++ 2008, VC++ 2010 etc. there are many versions of the redistributable that you may need. For /MDd, the libraries will be on your development computer but there is no general release from MSFT for this - but you can build your own installer if necessary using Visual Studio; usually /MDd is only used by the developer for testing.

The versions of SSLEAY etc in the original question do not indicate what version 2005/2008/2010 etc of Visual C++ was used to compile the MD versions, but once compiled, it can be noted from the built target using a dependency viewer (e.g depends.exe). E.g. if your app depends on MSVCR90.DLL, then that means VC++ 9 (confusingly, that's the 2008 redistributable).

All developers need the option to choose static or DLL library linkage, here are some notes on each:

Static linkage:

• self contained, easy to install, larger code footprint, duplication of code when building multiple dll's and exe's.

Dynamic linkage:

• smaller footprint, bug fixes from MSFT updates, shared code, a bit harder to install

When you have code in a library that returns a C/C++ object (e.g. allocated memory, std::string, etc) it is mandatory to link your code with the same flags used to compile the library, with no exceptions.

Answer 2

• MD for dynamic-release
• MDd for dynamic-debug
• MT for static-release
• MTd for static-debug

Source article via archive.org[^1].

[^1]: Original Link