Header Files

In a large program, it is possible that a header file might get included twice

Use the C preprocessor to avoid double inclusion

      #ifndef COULOMBSLAW_H
      #define COULOMBSLAW_H
      extern double CoulombsLaw (double q1, double q2, double r);
      #endif /* COULOMBSLAW_H */

• #ifndef is preprocessor directive saying "if not defined"

• COULOMBSLAW_H is preprocessor macro variable and is upper case by convention

• #define defines a macro variable, but in this case doesn't give it a value

• #endif ends the #ifndef

• this structure seen in all system header files