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This is an introductory essay on C programming. It assumes that you know varying amounts about computers and programming in general. First, I recommend that you purchase The C Programming Language, Second Edition by Brian W. Kernighan and Dennis M. Ritchie (referred to by everyone as K&R 2), and also Expert C Programming: Deep C Secrets by Peter van der Linden, and keep both at your side while you program. They are very useful and very handy books.
The C language has changed some since the publication of these books, but overall its flavor is still much the same as it has always been. This essay will focus entirely on "modern C", that is to say, the ANSI Standard C language dating from 1989. A revised standard was created in 1999, incorporating numerous and sometimes significant changes to the language; however, I will not refer to it (much) here since its features are not so much of interest to beginning programmers, and (as I write this, 2001) "C 99 " compilers are not in wide, or any use at all. Beware: Peter van der Linden and K&R 2 occasionally refer to anachronisms which you do not need to know about, and in fact probably should not know about, as they are confusing, useless, and dated. First, C is a compiled language, like C++ or Fortran (speaking of dated, useless anachronisms) and unlike interpreted languages such as BASIC and Dr Scheme. Additionally, C has steps that Java (which occupies a middle ground between compilation and interpretation) lacks.
Conceptually, a programmer creates a series of text files, named in the manner . c and . h, which make up the source code to a program, and then uses a C compiler on them. The compiler shall behave as if first it "p reprocesses" the text files, and then "compiles" them. You do not need to know the magic that actually goes on behind the scenes; in fact, you should stay away from knowing too much about your compiler. Compilers vary on different systems.
If you program in a Windows environment, I suggest DJGPP, which is a port of the freedom software compiler gcc for the Linux platform. Preprocessing only acts on the text files themselves. The preprocessor, and the commands used to instruct it, form a sort of primitive proto-language on top of C. In fact, this is how it historically evolved, if you are curious. The C preprocessor is highly useful, and isn't too "dangerous" if you know how to command it properly.
Then compilation occurs, which transforms the preprocessed source code into a working application destined to be run on a system. Never mind how that works. C programs are built out of "functions." Functions take data passed to them ("arguments" - it is a nearly standard abbreviation to say "args"), and perform operations based on them. Functions can do other things while they " re at it - in fact, most C functions operate in this matter. Functions can also return a value to the original function that executed, or "called" them, but they need not. ("Functional programming" takes this to an extreme and declares that doing anything but returning a value is a "side effect" and thus to be avoided. C is a "procedural" or "imperative" language, if you like.
Do this, do that - it's all a C program really is. ) No functions are inherently "special" - you can call your functions, and basically everything you use whatever you like, as long as you remember that your C compiler wants to use some (many) names for itself. There is one special function, though, and that is the "main" function. The main function is called by whatever magic code gets your program actually up and running, and when the main function returns a value the program is finished. If you are interested, it is perfectly legitimate for a function to call itself. Such "recursive" functions are not commonly seen in C, but are used occasionally. This applies to all functions: even the main function can be recursive.
This is a difference from other languages such as C++. Never mind this too much; our early examples will not use recursion at all. Functions in C use parentheses around a list of the args passed to them. For example: foo (bar, baz) is a function foo that takes the args bar and baz. If we have a function que that takes no arguments at all, we "declare" it que (void) to show that it takes nothing at all, but if we want to actually call the function, we just type the statement:
que ();(statements in C are terminated with semicolons). This is indeed a minor inconsistency.
We cannot declare a function that takes no arguments simply as que () because of an unusual and useless tidbit of history. Additionally, if we have a function properly declared as que (void) and in a program type the statement:
que;Nothing at all will happen, rather than the desired effect of executing whatever statements are in the que function. More precisely, this line "que; " evaluates the "address" of the function que and then discards it, without ever actually calling the function. Addresses will be covered later. One more thing: text inside the delimiters / and / enclose comments in C programs. Comments are replaced by "whitespace", essentially a single space, during a particular phase of compilation.
C comments also do not nest, if you " re curious. I think it's time for our first program. I will include some unnecessary, redundant code in this program, with the intention that it will help you learn how to use other functions than main quickly (which I had trouble figuring out as a beginning C programmer), and will later explain why this quick snippet is unnecessary. Try taking the code below and putting it into a text file called "hello. c" and executing the command
gcc -Wall -o hello. exe hello.or the equivalent on your system:
c
/ Here is a comment - the compiler ignores these / / Begin Hello, world! program / #includeIf you successfully compile and run this program (which K&R 2 correctly notes is the hardest part of learning a language! ) it should print:int main (void); int main (void) { printf ("Hello, world! n"); return 0; } / End Hello, world! program! /
Hello, world!and then return to your operating system. Doubtless you are wondering exactly what you typed.
The line
#includeis a preprocessor directive as men...
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