Programming in C

By Ken Howe <khowe90@entergy.com>

About three months ago I finally gained access to the internet and started browsing around Aminet. I usually look for utilities and source code to help me in my programming efforts on the Amiga. I also read Amiga Report and lo and behold I spotted a copy of the first Issue of Amiga Report Tech Journal. After reading the first and now second edition I thought the concept had great potential.

I know how difficult it is learning to program the Amiga especially as most Amiga magazines only touch the surface of programming, running the same old simple programming tutorials. I used to buy those magazines month in month out hoping to find something useful but no. So I soldiered on as most people have and learned what I know very slowly. I am by no means a great Amiga programmer and alot of people may already know most of the concepts that I will be talking about in the coming issues so if you think you can do better put together an Article and maybe you can teach me something!!

Ive been doing small development on the Amiga on and off for about 3 years mainly just to teach myself C and soon C++. I program Client Server Windows Application for a living, can Program over 10 different programming languages and have been programming for over eight years. I may not be an Amiga expert but I hope I know a bit about programming.

Well thats the background, Ive only had a week to put this article together so Im just going to do a few programming tips, mainly a few tricks I have learn't about ANSI C. I hope to do more article for future issues with a bit more meat in them!

1. Stop yourself from making the if( a = b ) mistake by declaring a define statement as follows
   #define EQU ==

   Then anywhere you would normally use if( a == b ) use if( a EQU b ). Your C compiler may already have this defined. I always include a header file of mine called Kens.h with this define and a few other things just to make sure.

2. Initialised auto variables for strings, arrays or any composite object is very inefficient. If you do not recognise the name it simply put means a variable that is automatically initialised for you. Example:
   int i = 0;

   Each object defined in this way will occupy exactly twice the storage space. Also the compiler will reset the variable every time the function is called. This involves copying the variable contents from a safe location in memory into your working variable.

   This not only wastes CPU time, it also makes for larger memory overheads and larger stack usage. In most cases the variable can be changed to be static if this is not the case then try to use a global buffer shared between all routines, which you can initialise yourself. Example:
   Replace: void func( void ) { char ls_Msg = "This is time consuming";...}
   With: void func( void ) { static char ls_Msg = "Much Better";...}

3. Pre increment/decrement operators are more efficient than post increment/decrement operators. This is valid mostly when they are combined in expressions using compare. Example
   ( ++li_A < li_B )

   There is no saving when they are being used in the simple for of li_A++ or li_B++. The reason for the benefit of ( ++li_A < li_B ) over ( li_A++ <= li_B ) is that the compiler must generate an extra jump instruction to prevent the increment from happening.

4. You can reduce the size of simple if/else blocks, where the instructions are single lines by always performing the else instruction then having a plain if. You remove a JMP instruction from your code. Example:
   Replace: if( li_A < li_B ) li_Z = 10; else li_Z = 0;
   With: li_Z = 0; if( li_A < li_B ) li_Z = 10;
5. You can use register variables to dramatically increase the speed of loops within your program but! you must be careful when declaring more than one register in a single declare statement.

   When you declare the register variables, declare them singularly with the inner most variable declared first. Thus making sure the most used variable is the register variable whenever a spare register is available. Example:
   Replace: register int li_I, li_J;
   With: register int li_J; &47;* J in inner most variable *&47;
register int li_I;

6. In SAS/C you can change the size of the default console window by including the following command just after your last include:
   char __stdiowin[]="CON:0/0/700/550/Window Title";

   This command works the same as the DOS shell program so you can adjust the size and coordinates of the window.

7. Care should be taken when defining a macro that makes up more than one statement, for example:
   #define ABORT (void)printf("Aborting\n");exit(1);

   It is easy then to use ABORT in other parts of the program but it can cause problems when not used in a straight forward manner such as in an if statement. In order to avoid problems like this you should always enclose multi statement macros in a do {} while(0) construct, for example:
   #define ABORT do {(void)printf("Aborting\n");exit(1);} while(0);

8. Parameterized macros should always have parentheses () around the parameters in a parameterized macro. The helps to overcome the following.
   #define SQR( number ) ( number * number )
li_Size = SQR( 5 )
   equates to
   li_Size = ( 5 * 5 )
   but consider
   li_Size = SQR( 5 + 6 )br> equates to
   li_Size = ( 5 + 6 * 5 + 6 )
   

   Therefore if we use the rule of always putting parentheses around parameters we overcome the problem, for example:
   #define SQR( number ) ( ( number ) * ( number ) )
li_Size = SQR( 5 + 6 )
   gives
   li_Size = ( ( 5 + 6 ) * ( 5 + 6 ) )
   

   (NOTE) never use ++ and -- with macros, consider:
   li_Size = SQR( li_Val++ )
   equates to
   li_Size = ( ( li_Val++ ) * ( li_Val++ ) )
   Yuk!

9. The last tip is not much of a tip really if you are already programming in C but! if you are thinking about starting C you will probably want a book to learn from. And being new to C you will probably ask other people for advice and they will say... buy the K&R book The C programming language.

   Well I have to disagree, working in computing I am fortunate to have access to lots of books on programming and if there is a book I want to read I can order it free of charge. Most people recommend the K&R book as they have not tried many other books.

   I found the K&R book very tuff going and Iam not new to programming. Therefore I would like to take this opportunity to recommend the best two book out of the 12 I have at my disposal:

   As a quick reference: C Quick Reference, by QUE, ISBN 0-88022-372-3 ( about $8 ) Is a compact (154 pages) reference to the C language and is great for looking up commands and C syntax.

   As a tutorial book: Using C, by QUE, ISBN 0-88022-571-8 ( about $30 ) Is almost 1000 pages. It is split into 3 books, 1) the tutorial is brilliant with loads of meaning full examples it start easy and will take you through most of the advanced stuff. 2) A complete C library reference. 3) An introduction to C++.

Well thats all for this issue I hope to have a full and hopefully useful routine to pick over next issue.

Ken Howe can be reached at khowe90@entergy.com