C language constants defined

• Use const keyword

  • Format: const constant name Data type = a constant value;
  • About naming constants name: beginning with 'k', mixed case. E.g:const int kHoursInDay = 24
  • Note: In c, const modified constant is unsafe, you can be modified by the pointer.

• Use macro definition

  • Format: #define constant name value

  • Naming convention: all caps, use an underscore between words _separated. E.g:#define HOURS_IN_DAY 24

  • Step-compiled, pre-defined macros will be expanded.

    

    Using gcc -E a.c -o a.ithe source code preprocessing
    
    can see has no macro definition, and 24 is assigned a variable.