Having Compiling trouble with definition function in another header

Question

So I was wondering why I am getting the following error:

Error 1 undefined reference to `MIN_COUNTER' C:\Users\Wyatt Crosby\Dropbox\Atmel Studio\ReflowController\ReflowController\ReflowController\Debug/.././ReflowController.c 146 1 ReflowController

When in PID.h I have:

#ifndef PID_H
#define PID_H

#define LCD_SCREEN_SIZE     16
#define MAX_COUNTS          180
#define OVEN_MAX_TEMP       260
#define OVEN_MIN_TEMP       0
#define MIN_COUNTER(x,a)    tempLookupArray[x] < a ? x : MIN_COUNTER(x+1,a)         
#define FIND_COUNTER(a)     a <= OVEN_MIN_TEMP ? MAX_COUNTS : MIN_COUNTER(0,a)
const float tempLookupArray[MAX_COUNTS];
...

Where tempLookupArray is define further in PID.c:

const float tempLookupArray[MAX_COUNTS] = {
    260.00,
    260.00,
    260.00,
    259.99,
    259.98,
    259.96,
          ...

And in ReflowController.c I include PID.h and write:

TriggerCounter = FIND_COUNTER(PIDgain);

Where PIDgain is local and of type 'float', and TriggerCounter is global and of type 'volatile int'

It seems to be the fact that I am trying to call MIN_COUNTER from inside MIN_COUNTER and doesn't have any sort of prototype for it yet (if it were a function) . . .

Any thoughts from you smart guys out there?

Thanks!

score 2 · Accepted Answer · answered Jun 30 '13 at 22:59

After the preprocessor has gone through your code,

TriggerCounter = FIND_COUNTER(PIDgain);

becomes

TriggerCounter = PIDgain <= 0 ? 180 : tempLookupArray[0] < PIDgain ? 0 : MIN_COUNTER(0+1,PIDgain);

At that point, the preprocessor hands the code to the compiler and linker who look for a method called MIN_COUNTER but can't find one. The preprocessor doesn't work recursively in a case like that. Imagine if it did: The code you wrote would send the compiler in an infinite loop; it would need to expand MIN_COUNTER and to do that, it needs to expand another MIN_COUNTER and so on...

Try using actual functions instead of macros

int min_counter(int x, float a) {
    return tempLookupArray[x] < a ? x : min_counter(x+1,a);
}

int find_counter(float a) {
    return a <= OVEN_MIN_TEMP ? MAX_COUNTS : min_counter(0,a);
}

In addition to recursing properly, it will also avoid some of the caveats of macros (For example, FIND_COUNTER(i++) would increment i multiple times, whereas find_counter(i++) wouldn't)

Having Compiling trouble with definition function in another header

1 Answers1