Should I compile with /MD or /MT?

Question

In Visual Studio, there's the compile flags /MD and /MT which let you choose which kind of C runtime library you want.

I understand the difference in implementation, but I'm still not sure which one to use. What are the pros/cons?

One advantage to /MD that I've heard, is that this allows someone to update the runtime, (like maybe patch a security problem) and my app will benefit from this update. Although to me, this almost seems like a non-feature: I don't want people changing my runtime without allowing me to test against the new version!

Some things I am curious about:

• How would this affect build times? (presumably /MT is a little slower?)
• What are the other implications?
• Which one do most people use?

Answer 1

If you are building executable that uses other dlls or libs than /MD option is preferred because that way all the components will be sharing same library. Of course this option should match for all the modules involved i.e dll/lib/exe.

If your executable doesn't uses any lib or dll than its anyone's call. The difference is not too much now because the sharing aspect is not into play.

So maybe you can start the application with /MT since there is no compelling reason otherwise but when its time to add a lib or dll, you can change it to /MD with that of the lib/dll which is easy.

Answer 2

from http://msdn.microsoft.com/en-us/library/2kzt1wy3(VS.71).aspx:

/MT Defines _MT so that multithread-specific versions of the run-time routines are selected from the standard header (.h) files. This option also causes the compiler to place the library name LIBCMT.lib into the .obj file so that the linker will use LIBCMT.lib to resolve external symbols. Either /MT or /MD (or their debug equivalents /MTd or /MDd) is required to create multithreaded programs.

/MD Defines _MT and _DLL so that both multithread- and DLL-specific versions of the run-time routines are selected from the standard .h files. This option also causes the compiler to place the library name MSVCRT.lib into the .obj file.

Applications compiled with this option are statically linked to MSVCRT.lib. This library provides a layer of code that allows the linker to resolve external references. The actual working code is contained in MSVCR71.DLL, which must be available at run time to applications linked with MSVCRT.lib.

When /MD is used with _STATIC_CPPLIB defined (/D_STATIC_CPPLIB) it will cause the application to link with the static multithread Standard C++ Library (libcpmt.lib) instead of the dynamic version (msvcprt.lib) while still dynamically linking to the main CRT via msvcrt.lib.

So if I am interpreting it correctly then /MT links statically and /MD links dynamically.

Answer 3

I believe the default for projects built through Visual Studio is /MD.

If you use /MT, your executable won't depend on a DLL being present on the target system. If you're wrapping this in an installer, it probably won't be an issue and you can go either way.

I use /MT myself, so that I can ignore the whole DLL mess.

P.S. As Mr. Fooz points out, it's vital to be consistent. If you're linking with other libraries, you need to use the same option they do. If you're using a third party DLL, it's almost certain that you'll need to use the DLL version of the runtime library.

Answer 4

If you are using DLLs then you should go for the dynamically linked CRT (/MD).

If you use the dynamic CRT for your .exe and all .dlls then they will all share a single implementation of the CRT - which means they will all share a single CRT heap and memory allocated in one .exe/.dll can be freed in another.

If you use the static CRT for your .exe and all .dlls then they'll all get a seperate copy of the CRT - which means they'll all use their own CRT heap so memory must be freed in the same module in which it was allocated. You'll also suffer from code bloat (multiple copies of the CRT) and excess runtime overhead (each heap allocates memory from the OS to keep track of its state, and the overhead can be noticeable).

Answer 5

By dynamically linking with /MD,

• you are exposed to system updates (for good or ill),
• your executable can be smaller (since it doesn't have the library embedded in it), and
• I believe that at very least the code segment of a DLL is shared amongst all processes that are actively using it (reducing the total amount of RAM consumed).

I've also found that in practice, when working with statically-linked 3rd-party binary-only libraries that have been built with different runtime options, /MT in the main application tends to cause conflicts much more often than /MD (because you'll run into trouble if the C runtime is statically-linked multiple times, especially if they are different versions).

Answer 6

The problem you will run into with /MD is that the target version of the CRT may not be on your users machine (especially if you're using the latest version of Visual Studio and the user has an older operating system).

In that case you have to figure out how to get the right version onto their machine.