Chapter 6: C Functions and Program Structures

One possible source of problems occurs when you reuse a token. You might write:

#define Up 0

And come back a month later, when your header file has 500 lines and forget that you have already defined Up and add:

#define Up 1

The preprocessor uses the last #define and you won’t get any warnings, other than this one. But you probably will get some mysterious bugs that you’ll blame on the hardware until much later you finally see what you’ve done and apply a welldeserved boink to your own head.

Macro Substitution

We can use #define to make a simple token that replaces a complex, or frequently used expression. For example we may want to determine the larger of two variables:

#define larger( x, y) ( (x)>(y) ? (x) : (y) )

Which we would use as:

int a= 9; int b = 7; int c = 0;

c = larger( a, b);

The preprocessor replaces the last statement with:

c = ( (a)>(b) ? (a) : (b) );

Which is what the compiler sees.

The expression larger( a, b) looks like a function but isn’t. A macro is substituted in the code anywhere that it is used, while a function is located in only one place and is called each time it is used. The big difference from a microcontroller perspective is that nothing is pushed on the stack when a macro is used, unlike

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Chapter 6: C Functions and Program Structures

functions, which use extra RAM. Also macros create in-line code that can be faster than function calls (no processor overhead). And finally, macros don’t require formally declared data types:

double da = 12; double db = 14; double dc = 7; double dd = 0;

dd = larger( (db-da), dc);

If larger() was a function, the parameters require a data type such as int or double, but couldn’t use both (okay, there is casting, but that’s another topic).

Conditional Inclusion

Often microcontrollers come in families that differ only in a few features, pinouts, memory size, and register locations. You can write C code for the entire family if you substitute alias for the things that differ. Let’s say that SuprMic16 uses pins 12 and 13 for USART transmit and receive, while SuprMic8 uses pins 6 and 14, and SuprMic4 uses pins 1 and 2. We put the following in our SuprMic.h file:

#if SuprMicX == 16

#define TXD 12

#define RXD 13 #elif SuprMicX == 8

#define TXD 6

#define RXD 14 #elif SuprMicX = 4

#define TXD 1

#define RXD 2

#else

#error “No definition for SuprMicX TXD and RXD pins." #endif

If we are using the SuprMic8 in our Killer Cylon Robot project we should put the following in our CylonKillerRobot.h file:

#ifndef SuprMicX

#define SuprMicX = 8

#include <SuprMic.h>

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#endif

The ifndef means ‘if not defined’ so that the preprocessor will use the #define SuprMicX = 8 and #include <SuprMic.h> lines only the first time it sees the #ifndef SuprMicX line. This prevents the preprocessor from attaching the contents of SuprMic.h in each file that uses CylonKillerRobot.h

As a matter of standard practice, always begin a header file with an #ifndef statement and a #define so that the preprocessor will only use that header’s data once in a project. If you put the header data in more than once you may get a lot of compiler errors about multiple declarations.

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