Macro

Cited from BTP300 Chris seneca cs Web

• Continuation

• A macro definition may extend over several lines. The backslash character - \ - immediately followed by the end-of-line character identifies a continuation onto the next line. For example,
  

  #define PI 3.141\
  592654
  is the same as
  #define PI 3.141592654

• Efficiency and Flexibility

  Function-like macros provide efficient and flexible solutions. They avoid the overhead of function calls and do not impose type constraints on the parameters in the macro definition.

• #define SQUARE(x)  ((x) * (x)) /* NOTE THE PARENTHESES *// Fardad noted in class

• Side-Effects

  Macro definitions can also generate side effects. Because the pre-processor substitutes textual patterns rather than values, it can generate repeated evaluations of expressions that were intended to be a single evaluation. Such as area(r++).
  

•  #define PI 3.14
  #undef  PI
  #define PI 3.14159    /* OK */
  #define AREA(r) PI * r * r

• Predeined macros

•      printf("The name of the source file is %s\n", __FILE__);
  printf("The date of its translation is %s\n", __DATE__);
  printf("The time of its translation is %s\n", __TIME__);
  if (__STDC__ != 0)
      printf("Compiled under Standard C\n");
  else
      printf("Not compiled under Standard C\n");
  

reference macro