Why is runtime library a compiler option rather than a linker option?

Question

I'm trying to build a C/C++ static library using visual studio 2005. Since the selection of the runtime library is a compile option, I am forced to build four variations of my library, one for each variation of the runtime library:

• /MT - static runtime library
• /MD - DLL runtime library
• /MTd - debug static runtime library
• /MDd - debug DLL runtime library

These are compiler options, not linker options. Coming from a Linux background, this seems strange. Do the different runtime libraries have different calling conventions or something? Why can't the different runtime libraries be resolved at link time, i.e. when I link the application which uses my static library?

Answer 1

These options may add defines (__DLL and __DEBUG for example) that are used in the runtime library header files. One common thing to do is to add __declspec(dllimport) to function declarations when linked dynamically.

The compiler also seems to use these to assist the linker in linking to the correct libraries. This is explained in the MSDN.