Version 92 of C

Updated 2014-05-13 10:44:30 by RLE

C is a programming language, most notable as the implementation language of Tcl ;)


1% the code


C99 Draft (N869)
C99 draft specification
big HTML file
The C Programming Language, by The C Programming Language, Second Edition , by Brian W. Kernighan and Dennis Ritchie
The Development of the C Language* , Dennis M. Ritchie
Programming in C
a list of various C papers and resources


The ANSI committee as well as the ISO committee have approved a standard reference for C.

Support for C is traditionally provided by a compiler. Some efforst at a C interpreter also exist.

Most Unix systems provide vendor-supported compilers. They are often unbundled products these days.


gcc, by the FSF
a quite popular C compiler found on many platforms and operating systems.
C language family frontend for LLVM [L1 ]
the "Tiny C Compiler"
The SubC Compiler , by Nils M Holm
a fast and simple public domain compiler for a clean subset of the C programming language. It can compile itself and passes gcc -Wall -pedantic. Companion to Practical Compiler Construction: A Non-nonsense Tour Through a C Compiler .

[In the following list, be certain to indicate whether or not you have built Tcl and Tk with the compiler in question.] On Microsoft Windows there are many options, including

Visual C++
from Microsoft (commercial) (but, in summer 2003, tclguy says VC7 is "free from ms - it just has the opts turned off "), PT NB: as of Apr 2004 the command line version with optimizations is now free (see below).
from Borland (commercial; an older command-line version is still available as a free download),
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),
C compiler [L2 ] (free; another version of gcc for Windows with goals slightly different from Cygwin),
Digital Mars C
(free), which was once Zortech C and later became Symantec C
(free) [Can that compile Tcl itself?]
Intel C++ compiler
OpenWatcom C++ compiler
(free?) [No idea if it can compile Tcl.], etc.

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

by Metrowerks (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 XCode IDE (included in the download)
IBM's XL C/C++
(commercial), intended to be optimized for recent machines.


RFox: a (free) interpreter, originally from Masaharu Goto, and apparently currently maintained by the CERN Root team which uses CInt as its scripting language. I do not know if Tcl can be interpreted under this interpreter
a C/C++ interpreter that runs cross-platform on a large number of operating systems and hardware. It provides not only standard language features, but also support for 2 and 3 dimensional plotting, shell programming, and numerical programming. It supports 1990 ISO C standard as well as C99 features. In Jan, 2003, the product page for Ch Standard says that the product is free for use on Unix, Linux, and Mac OS X.
another C interpreter.

Linkable Compilers

Support for C can also take the form of a run-time compiler.

tcc provides a libtcc library, which allows a running program to compile and link C code without touching disk. In milliseconds for a small chunk of code. (x86 only as of Jun 2003)


C Annoyances

AMG: One annoying thing about C is that, like Fortran, it uses = as its assignment operator. This is very easy to confuse with the comparison operator ==, resulting in either of the following two coding errors:

x == 5;           /* Nothing happens, except maybe a warning if -Wall is on. */
if (x = 6) {...}  /* Unintended assignment and incorrect logic. */

Algol uses := as its assignment operator, and C is ancestrally derived from Algol, so I was always mystified about why C reverted to Fortran's assignment syntax. But I just now figured it out, I think. A := assignment operator looks almost identical to C's != inequality operator, so using := instead of = for assignment simply trades one potential coding error for another. To fix that problem, != would have to be replaced with, say, /= (borrowed from Ada), but that (1) collides with the compound division and assignment operator /= and (2) breaks symmetry with the logical negation operator !. Or instead use \=, but that will behave unexpectedly when placed inside double quotes. Or use <> and be like BASIC and SQL. Shrug!

(PL: the change from := to = was made during the transition from BCPL to B (and simply kept when going from B to C), and IIRC the reason was that Ken Thompson felt that an operator that was going to be used as often as assignment was should be as short as possible. The reasons you mention may of course also have affected his decision, but there was never any secondary ambiguity with "divide-assign", which was written as =/ in B.)

bch: One thing that I do in C (indeed, Perl too) is use typical rvalues on the left side when doing comparisons, which will fail compilation if the assignment operator is mistakenly used instead of the comparison operator. Eg: use if(9==a){...} instead of if(a==9){...}; in the case of mistakenly using assignment operator, this will fail if(9=a){...}, which is good; the programmer will be notified that his intent was mis-typed. If the rvalue is indeed on the right, though, one would have typed if(a=9){...}, which will compile, but is unlikely to represent the programmer's intent, and lead to hard-to-find bugs. Reading expressions written this way (w/ rvalues on left) may seem "backward" to some people, and the practice is affectionately dubbed Yoda Conditions , [L3 ]

AMG: Cute. I've seen that style too. Unfortunately, it doesn't help when both sides of the comparison (or unintended assignment) are valid lvalues. Thankfully, gcc -Wall provides useful warnings:

x == y; warning: statement with no effect
if (x = y) {...} warning: suggest parentheses around assignment used as truth value

To silence the latter warning on a case-by-case basis, type: if ((x = y)) {...}. In my opinion, the extra parentheses highlight the fact that this expression isn't Business As Usual, so the reader will look at it more carefully and is more likely to notice that it uses assignment instead of comparison. Of course, only silence this warning when the code really is doing what you intend, and please put a comment nearby to testify to this fact and explain why. Also, watch your warnings like a hawk, and don't release code that emits any warnings in your chosen build environment(s). (You can't control if some alien compiler puts out weird warnings because it used MS-Word to grammar-check your comments.)

Sometimes I really do want my conditional expression to have the side effect of assigning into a variable. I need this when I want to save some intermediate value of a complex conditional for later processing. A simple example is calling a syscall (read()) that either returns a useful result (number of bytes read) or an error flag (-1, check errno for details).

if ((ret = read(fd, buf, max)) == -1) {
} else {
    printf("read %d bytes\n", ret);

This style is also available in Tcl:

proc getline {} {...}
while {[set line [getline]] ne ""} {
    puts "got header line: $line"
puts "got blank line, the header is now done"

As you can see, Tcl makes it impossible to mix up assignment and comparison. ;^)

Questions about C vs Tcl

How does performance in Tcl compare to comperable programs in C?

For instance, certainly there are types of programs which can likely be developed more quickly in Tcl than in C.

Are there types of things that run faster in Tcl than C? If not, how much slower than C is Tcl? 10 times slower? 50 times slower? 100 times slower?

Miscellaneous Thoughts

AMG: I just thought of a cute alternate name for C: "lightspeed". You know, 'cuz physicists use the constant "c" to represent the speed of light. :^) I leave it up to you to figure out if this name has any deeper meaning. At least it would give C an excuse to have a snazzy, energetic theme song .

See Also

Cameron Laird's personal notes on how to use C with Tcl
What 'embedding' means
How to embed Tcl in C applications
Adding Tcl/Tk to a C application
Extending Tcl
Writing Tcl-Based Applications in C
Building a custom tclsh
Init Scripts in the DLL
Tcl interpreter in C# Application
Adding Tk to an Existing Xt Program
Mixing Tcl/Tk and Xt Event Loops
Using Xlib With Tcl/Tk
3D Text for Tk in Unix
C compiled image processing on an interactive Bwise canvas
automatically generating socket based Tcl / C connection code, 1
automatically generating socket based Tcl / C connection code, 2, using bwise
Connecting Tcl/Tk with GNUstep Objective-C programs
Building Tcl with the free VC++ toolkit
How to invoke a C function from Tcl
Tcl and other languages
C-header Parser
C code generators
C Language