| Issue 008 |
June 2000
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00436824 loc_436824: ; CODE XREF: start+4Dj 00436824 8B C3 mov eax, ebx 00436826 83 C0 07 add eax, 7 ; switch 7 cases 00436829 83 F8 06 cmp eax, 6 0043682C 0F 87 B2 00 00 00 ja loc_4368E4 ; default 00436832 FF 24 85 39 68 43 00 jmp ds:off_436839[eax*4] ; switch jumpThey should have made the CRC be necessary to decrypt parts of the file.
In the previous installment X-Calibre introduced the basic data types usable in C, now I'm going to start to show you how to create your own programs. We will look at the function printf, useful for all of our early programs to demonstrate what we are doing. Then we will start to look at the data types that are available to us and gradually introduce more control structures as the series continues. I should probably mention that I am not assuming that you are working on any particular platform. All the examples I will use will work identically no matter what platform you use - the only thing that you need to do is use a modern compiler. First we will create the first rather obvious program, depending on how your compiler is set up it might spit out some warning about main not being an int or there being no return value, ignore these if you encounter them.
#include <stdio.h>
void main()
{
printf("If I could program something really neat would happen now...\n");
}
This small piece of code can be saved as a .c file and compiled, and
guess what, it works! It also demonstrates a few useful things to
know about C. The first line 'includes' a set of functions. This is
done to allow us to reuse commonly used functions in our code, C
generally comes with a lot of ready built functions for us to include
in our code. We have included stdio.h which contains a
lot of functions to do with standard input and output, hence it's
name. I will not go into detail about the #include works
at this point, it's not worth looking at until you know how to at
least write a simple program. The void main() followed
by the { and } braces is the function that
gets called as soon as the program is started, we will be putting all
of our code between these two braces until I introduce functions. The
next noteworthy feature is the printf line. This is a
function call - this one displays output to the console. Printf is a
very useful function for displaying just about anything, it's first
parameter is a format string, this specifies what printf will display
and what other parameters will be being passed to printf. To display
simply a string you can use a call like the one above, but this is
certainly not the limit of the function. If you want to display an
int variable then you can use the call printf("%d",
variable). The %d is a format code, there are lots like it for
printf to determine what it will display. Some examples are,
%x - display integer as hex %s - display a string %f - display a floating point number %c - display a character
You will be able to see more of the printf calls in action as I continue to create more examples.
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Escape characters These are used throughout computing. The syntax of most languages will not permit certain characters in their constructs, for example you can't put a " in the middle of a string in C. So for example "and he said "forget that" before turning away in a huff"
would not be valid. This is rather limiting for programs, they want
to be able to display messages as they please and certainly the
machine is capable of it so the languages had to come up with a
work-around for the problem. Escape codes are this work around,
special characters are 'escaped' to prevent them from being
mis-interpreted.
Examples are,
\" - " \\ - \ \n - LF \r - CR \t - TAB
So to make the previous string legal you escape the quotes and put
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Now that we have created our first program, and I assume you managed
to compile it okay, we will create another. As you will remember from
the previous installment we saw the data types available to us, so
lets start to use them. In the previous installment we said that we
could define various pieces of data, in general though we rarely use
such terms, in fact we usually call the data that we are manipulating
variables, they do after all often vary. Of course we also like to
know which variable we are toying with at any time so we give them
names, a useful concept that we all know and love. With the previous
examples of data definition in mind we can see how this works. With
the definition below the name of the variable is myChar.
When we want to use it again in the program we simply use it's name.
The variable name is just a way of referencing a piece of data, when
we use the name we are actually talking about the piece of data that
it represents rather than something of that name. It's just a lot
easier to talk about a name than some meaningless address, especially
since the address may well not correspond to the data's real address
in memory
char myChar = 'a';
Now that the variable is declared we can use it later in the program, we could for example display what the variable contains. Time to use that fabulous function printf again!
printf("myChar is - %c\n", myChar);
Now of course we want to be able to do more than just initialise a variable and displaying it, we want do be able to do, well stuff to them. This is where operators come in, an int variable for example can be added to, set, subtracted etc. An example of this is,
#include <stdio.h>
void main()
{
int a, b, c, d;
a = 0;
b = 2;
c = a + b - 1;
d = c * 3;
printf("D is %d\n", d);
}
As you can see we have used several operators, =,
+, - and *. Operators allow
you to perform basic operations on variables. These are some of the
key building blocks of our C programs and are the most rudimentary
functions we can apply to the variables. I won't explain these simple
operators in any real detail since the way they work is obvious, the
designers of the language where after all human!
Well that's it for this installment, ttfn :)
Credits and thanks for this issue go to: AcidFusion, amante4, Fatboy Joe, Muad'Dib, SantMat,
My personal greetings fly out to: ACiD_BuRN, alpine, amante4, Carpathia, Corn, Fatboy Joe, kaai, Latigo, LaZaRuS, Lord Soth, Lucifer48, Muad'Dib, Neural, _pain, +Sandman, SantMat, S^witz, Tornado, Yoshi, all the regulars in #immortaldescendants, and everyone I forgot (probably MANY)