text: The.
  ANSI C defines a number of macros. Although each one is available for use in programming, the predefined macros should not be directly modified.
  Let's try the following example −
  #include <stdio.h>
  The C preprocessor offers the following operators to help create macros −
  A macro is normally confined to a single line. The macro continuation operator (\) is used to continue a macro that is too long for a single line. For example −
  #define message_for(a, b) \ printf(#a " and " #b ": We love you!\n") ("token34 = %d", token34);
  This example shows the concatenation of token##n into token34 and here we have used both stringize and token-pasting.
  The preprocessor defined operator is used in constant expressions to determine if an identifier is defined using #define. If the specified identifier is defined, the value is true (non-zero). If the symbol is not defined, the value is false (zero). The defined operator is specified as follows −
  #include <stdio.h> #if !defined (MESSAGE) #define MESSAGE "You wish!" #endif int main(void) { printf("Here is the message: %s\n", MESSAGE); return 0; }
  When the above code is compiled and executed, it produces the following result −
  Here is the message: You wish!