Question

I have this preprocessor directive:

#define INDEXES_PER_SECTOR BYTES_PER_SECTOR / 4

where BYTES_PER_SECTOR is declared in another header file as:

#define BYTES_PER_SECTOR    64

I have this simple math equation that I wrote where after executing I get an assertion error as the value assigned to iTotalSingleIndexes is incorrect.

int iTotalSingleIndexes = (iDataBlocks - 29) / INDEXES_PER_SECTOR;

Now I believe this to be because of the preprocessor directive INDEXES_PER_SECTOR. Upon executing my equation iDataBlocks is 285 which is correct. I have confirmed this with gdb. The problem is that the value that gets assigned to iTotalSingleIndexes is 1 when it ought to be 16. I really have no idea why this is happening.

When I do something like:

int iIndexesInASector = INDEXES_PER_SECTOR;
int iTotalSingleIndexes = (iDataBlocks - 29) / iIndexesInASector;

the correct value gets assigned to iTotalSingleIndexes.

On other notes I use preprocessor directives in other equations and they work just fine so I am even more puzzled.

Any help would be much appreciated.

Answer 1

The preprocessor simply performs token replacement – it doesn’t evaluate expressions. So your line:

int iTotalSingleIndexes = (iDataBlocks - 29) / INDEXES_PER_SECTOR;

expands to this sequence of tokens:

int iTotalSingleIndexes = ( iDataBlocks - 29 ) / 64 / 4 ;

…which, due to the associativity of the / operator, is then parsed by the compiler as:

int iTotalSingleIndexes = ((iDataBlocks - 29) / 64) / 4;

…which results in the value of 1. As leppie says, you want:

#define INDEXES_PER_SECTOR (BYTES_PER_SECTOR / 4)

This makes INDEXES_PER_SECTOR expand to a complete subexpression.