C [http://cm.bell-labs.com/cm/cs/cbook/index.html] is a programming language 
found first
[http://cm.bell-labs.com/cm/cs/who/dmr/chist.html]
on Unix systems but now found on many platforms.
The [FSF]'s [gcc] compiler is a quite popular C compiler found on many
platforms and operating systems.

Also, most Unix systems have vendor supported compilers.  They are often unbundled products these days. Sun is one of the vendors who have this type of product.

On [Microsoft Windows] there are many options,
including
   * Visual C++ from [Microsoft] (commercial),
   * [C++Builder] from [Borland] (commercial; an older command-line version is still available as a free download),
   * the [Cygwin] C compiler (free; a version of [gcc]; for some strange reason it cannot be used to compile the [Tcl] source code out-of-the-box - but Cygwin comes with its own version of Tcl at least),
   * the Mingw C compiler [http://www.mingw.org/] (free; another version of [gcc] for Windows with slightly different goals),
   * CodeWarrior [http://www.metrowerks.com/MW/Develop/Desktop/Windows/default.htm] (commercial),
   * Digital Mars C [http://www.digitalmars.com/] (free), which was once Zortech C and later became Symantec C,
   * LCC-Win32 [http://www.cs.virginia.edu/%7Elcc-win32/] (free) [[Can that compile Tcl itself?]],

etc.

The number of C compilers available on [MacOS] is more limited.  There is

   * CodeWarrior by Metrowerks [http://www.metrowerks.com] (commercial) for [MacOS Classic] and [MacOS X],
   * Apple's MPW for [MacOS Classic] (free download),
   * [gcc] for [MacOS X] (free; download from Apple); either used stand-alone or with Apple's ProjectBuilder IDE (included in the download).

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