if ifdef ifndef else endif

These are used for conditional compilation, to control which sections of code are compiled.

Example

#if constant_expression

/* If constant_expression is non-zero, compile this */

#else

/* otherwise compile this */

#endif  /* Need this to mark the end of the conditional section */

You can also use #if...#endif without the #else.

The #ifdef is slightly different: you test with

#ifdef identifier

and if the identifier is currently #defined, then the if-block is compiled. #ifndef has the same effect if the identifier is not defined. Both can have #else and both must end with #endif. There is an important debugging use of this facility. Consider:

#ifdef DEBUG
      printf("Have reached position X in the program\n");
#endif

Normally this print statement will not be compiled. We could force it to compile with a #define but this is such a valuable use that the compiler has a switch (-D) to define any identifier directly:

cc -DDEBUG my_prog.c

causes DEBUG to be defined, and therefore forces the compilation of the ``print'' statement.

The variant:

cc -DDEBUG=10 my_prog.c

is the same as having

#define DEBUG 10

in your program. The first version is equivalent to:

#define DEBUG 1

You can narrow down the range of debugging by appropriate use of #undef.