• All preprocessor lines always begin with #. This listing is from Weiss pg. 104. The unconditional directives are as follows:
  • #define - Define a preprocessor macro
  • #include - Insert a particular header from another file
  • #undef - Undefine a preprocessor macro
  The conditional directives are as follows:
  • #ifndef - If this macro is not defined
  • #ifdef - If this macro is defined
  • #if - Test if a compile time condition is true
  • #elif - #else an #if in one statement
  • #else - The alternative for #if
  • #endif - End preprocessor conditional
  Other directives include:
  • ## - Token merge, creates a single token from two adjacent ones
  • # - Stringization, replaces a macro parameter with a string constant

The above code will tell the CPP to replace instances of MAX_ARRAY_LENGTH with 20. To increase readability,use #define for constants.

The above code tells the CPP to get stdio.h from System Libraries and add the text to this file. The next line tells CPP to get mystring.h from the local directory and then add the text to the file.

The above code tells the CPP to undefine MEANING_OF_LIFE and define it for 42.

The above code tells the CPP to define IROCK only if IROCK isn't defined already.

Thed above code tells the CPP to do the following statements if DEBUG is defined.If you pass the -DDEBUG flag to gcc,this is useful .