What is the C runtime library?

Question

OK, I know this is maybe the most stupid question ever asked here, but what actually is C runtime library and what is used for? I was searching, googling like devil, but I could find better than Microsofts: "The Microsoft run-time library provides routines for programming for the Microsoft Windows operating system. These routines automate many common programming tasks that are not provided by the C and C++ languages."

OK, I get that, but for example, what is in libcmt.lib ? What functions? I thought that C standart library is part of C compiler, so is libcmt.lib windows implementation of C standart library functions to work under win32?

Answer 1

i think Microsoft's definition really mean:

The Microsoft implementation of standard C run-time library provides...

Answer 2

Hi!

There are three forms of the C Run-time library provided with the Win32 SDK:

* LIBC.LIB is a statically linked library for single-threaded programs.
* LIBCMT.LIB is a statically linked library that supports multithreaded programs.
* CRTDLL.LIB is an import library for CRTDLL.DLL that also supports multithreaded programs. CRTDLL.DLL itself is part of Windows NT. 

Microsoft Visual C++ 32-bit edition contains these three forms as well, however, the CRT in a DLL is named MSVCRT.LIB. The DLL is redistributable. Its name depends on the version of VC++ (ie MSVCRT10.DLL or MSVCRT20.DLL). Note however, that MSVCRT10.DLL is not supported on Win32s, while CRTDLL.LIB is supported on Win32s. MSVCRT20.DLL comes in two versions: one for Windows NT and the other for Win32s.

see: http://support.microsoft.com/?scid=kb%3Ben-us%3B94248&x=12&y=9

Answer 3

C is a language and in its definition, there do not need to be any functions available to you. No IO, no math routines and so on. By convention, there are a set of routines available to you that you can link into your executable, but you don't need to use them. This is, however, such a common thing to do that most linkers don't ask you to link to the C runtime libraries anymore.

There are times when you don't want them - for example, in working with embedded systems, it might be impractical to have malloc, for example. I used to work on embedding PostScript into printers and we had our own set of runtime libraries that were much happier on embedded systems, so we didn't bother with the "standard".

Answer 4

If you use a tool like Dependency Walker on an executable compiled from C or C++ , you will see that one of the the DLLs it is dependent on is MSVCRT.DLL. This is the Microsoft C Runtime Library. If you further examine MSVCRT.DLL with DW, you will see that this is where all the functions like printf(), puts(0, gets(), atoi() etc. live.

Answer 5

Yes, libcmt is (one of several) implementations of the C standard library provided with Microsoft's compiler. They provide both "debug" and "release" versions of three basic types of libraries: single-threaded (always statically linked), multi-threaded statically linked, and multi-threaded dynamically linked (though, depending on the compiler version you're using, some of those may not be present).

So, in the name "libcmt", "libc" is the (more or less) traditional name for the C library. The "mt" means "multi-threaded". A "debug" version would have a "d" added to the end, giving "libcmtd".

As far as what functions it includes, the C standard (part 7, if you happen to care) defines a set of functions a conforming (hosted) implementation must supply. Most vendors (including Microsoft) add various other functions themselves (for compatibility, to provide capabilities the standard functions don't address, etc.) In most cases, it will also contain quite a few "internal" functions that are used by the compiler but not normally by the end user.

If you want to get a complete list of the functions in "libcmt" (to use your example) you could open one of the Visual Studio command prompts (under "Visual Studio Tools", normally), switch to the directory where your libraries were installed, and type something like: lib -list libcmt.lib and it'll generate a (long) list of the names of all the object files in that library. Those don't always correspond directly to the names of the functions, but will generally give an idea. If you want to look at a particular object file, you can use lib -extract to extract one of those object files, then use `dumpbin /symbols " to find what function(s) is/are in that particular object file.