Updated 2018-09-28 12:54:35 by PeterLewerin

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

Background Information  edit

History and Spirit of C and C++ (pdf), by Olve Maudal

Critique  edit

1% the code

Documentation  edit

C99 Draft (N869)
at busybox.net
C99 draft specification
big HTML file
Draft ANSI C: Standard (ANSI X3J11/88-090), C Rationale (ANSI X3J11/88-151)
Draft C99 (N1256)
Draft C1X (N1570)
The C Programming Language [1], 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

Description  edit

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.

Dialects  edit

a safe dialect of C

Free Compilers  edit

gcc, by the FSF
a quite popular C compiler found on many platforms and operating systems. Apple XCode provides a gcc distribution for Mac OS X.
C language family frontend for LLVM [2]
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 [3] (free; another version of gcc for Windows with goals slightly different from Cygwin),
OpenWatcom C/C++ compiler
Free. Microsoft Windows, OS/2, DOS, and Linux.
the "Tiny C Compiler". Free.
The SubC Compiler, by Nils M Holm
Free. 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.

Proprietary Compilers  edit

Commercial. An older command-line version is still available as a free download
freescale CodeWarrior
Commercial. Previously by Metrowerks. Versions for Windows and MacOS Classic and MacOS X previously existed.
Digital Mars C and C++
Commercial. No charge. Was once Zortech C and later became Symantec C
IBM XL C and C++
Commercial. For AIX, IBM i, Blue Gene, Linux, z/VM, Z/OS
Intel C++ compiler
Commercial. No charge for non-commercial use.
Microsoft Visual C++
Commercial. Now only avaialable as a component of Visual Studio.
Microsoft Visual Studio
Commercial. Visual Studio Express at no charge. For Microsoft Windows.

Historical  edit

Macintosh Programmer's Workshop: for MacOS Classic

Interpreters  edit

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  edit

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  edit

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!

[name redacted]: 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.

Larry Smith Frankly, in this day and age, assignment should be "←". We need to start using unicode for programs and frankly, it's past time to worry about the kind nonsense we have had to put with in ascii's limitations. Ascii is so 20th century. Swap should be ↔,less than, <, greater, >, equal, =, multiply and divide, × and ÷ and lessthan or equal and greater or equal should be ≤ and ≥. APL, essentially the only programming languages to get this, was right. J is, in fact, something of a step backward at the very moment in time when APL's disadvantages are now fewer in number and less in nature. In fact, we could do a lot of this for "nearly" free by just bolting in an APL interpreter in place of the expression evaluator (or, more likely, as an alternative for some time). We could use http://www.nars2000.org/ to handle the grunt work. Likely all it would need would be a way to meld their separate notions of variables (perhaps simply by adding the new types into the Tcl type, since NARS2000 can handle some rather unique types) but, as with Tcl, everything is essentially a string because it must, conceptually, pass through the interpreter's line-oriented interface, as unicode.

EMJ unicode for programs? One of these decades, it's too much of a revolutionary change to be viable now. APL is a different country, too many people will not get it.

Larry Smith I try to look beyond my own limitations. I am far from an APL expert, in fact, my APL has largely bit-rotted in my brain - but it remains one of the most popular programming languages for extremely heavy-duty mathematics, and many, many people do "get" that. Rather than sidling up to it unintentionally, it would be good for Tcl, as well as APL, to join the two. Each is especially good at one particular domain that can be hard for the other. Having it available as a back end for heavy math while tcl handles GUI and control functions make for a very powerful combination, especially since previous attempts at adding GUI concepts to APL have not, in my opinion, anyway, been very successful. Since it marries two essentially disjoint programming communities to their mutual advantage, it would change the market dynamics of both significantly.

As for unicode, Tcl is uniquely good with unicode, and should be pushing this boundary as a natural advantage over other programming systems. For all that they are supposedly "based" on unicode, neither java nor javascript are graceful about how they handle unicode, and Tcl's basic rules would adapt to unicode input without so much as a satisfied burp. Actually, as I pointed out in http://wiki.tcl.tk/48736 Tcl is very nearly there already. It cheerfully accepts unicode characters in-line.

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, [4]

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. ;^)

PYK 2015-07-30: Undefined behaviour, in combination with optimizing compilers, leads to unpleasant surprises. Here is one from the Linux kernel, as described by A Guide to Undefined Behaviour in C and C++, Part1, John Regehr, 2015-07-21:
static void __devexit agnx_pci_remove (struct pci_dev *pdev)
  struct ieee80211_hw *dev = pci_get_drvdata(pdev);
  struct agnx_priv *priv = dev->priv; 

  if (!dev) return;
  ... do stuff using dev ...

The error is that since the behaviour of dev->priv is undefined if dev is NULL, a compiler might decide to omit the following check for NULL, leading the program to do stuff using dev when dev is NULL, and even though there is an explicit check in the code for the case that dev is NULL. Apparently this is exactly what some versions of gcc do.

Questions about C vs Tcl  edit

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  edit

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.

Tools  edit

Makeheaders, DRH
a simple utility that will automatically generate all the .h files based on the .c or .cpp source files for a large programming project. Hypnotoad also maintains a fork.
Parsing C
a project to create a C parser in Tcl
Parsing C Types
an offshot of Parsing C, finds and describes global variables and types

See Also  edit

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
the C interface to Tcl
the C interface to Tk
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
a simple parser for C header files
C code generators
C Language