---

Macros

---

• Macros
  • Macro:

    Macro = a rule or pattern that specifies how a certain input sequence (= a sequence of characters) should be mapped to a replacement sequence

    ---

  • Syntax to define a macro:

    #define <identifier> <replacement token list> #define <identifier>(<parameter list>) <replacement token list>

    Meaning:

    Every occurence of the <identifier> string in the source code will be replaced by the <replacement token list>.

    ---

  • Use of macros in C:

    Defining constant Defining simple functions

  ---

  ---

  ---

  ---

• Defining symbolic constants in C
  • C uses the C pre-processor's #define macro construct to define symbolic constants

    ---

    Example:

    #define PI 3.141592653589793238462643383279502884197 #define MAX 99999 int main( int argc, char* argv[] ) { int x; double y; x = MAX; y = PI; for ( i = 0; i < MAX; i++ ) z = k * sin( i * PI ); }

    After processing with gcc -E constant.c:

    int main( int argc, char* argv[] ) { int x; double y; x = 99999; y = 3.141592653589793238462643383279502884197; for ( i = 0; i < 99999; i++ ) z = k * sin( i * 3.141592653589793238462643383279502884197 ); }

    ---

  • Example Program: (Demo above code)                                                
    • Prog file: /home/cs255001/demo/c/Cprep/define1.c

    How to run the program:

    To compile:   gcc -E define1.c Look at the output on the terminal....

  ---

  ---

  ---

  ---

• Defining simple functions in C
  • You can use the #define macro construct to define simple (one line) functions

    ---

    Example:

    #define square(x) (x*x) // Be careful, this definition had a problem !! int main( int argc, char* argv[] ) { double a, b; b = square(a); }

    After processing with gcc -E define2.c:

    int main( int argc, char* argv[] ) { double a, b; b = (a*a); ( square(a) ---> (a*a) ) }

    ---

  • Example Program: (Demo above code)                                                
    • Prog file: /home/cs255001/demo/c/Cprep/define2.c

    How to run the program:

    To compile:   gcc -E define2.c Look at the output on the terminal....

    ---

    ---

  • Caveat:

    Make very sure that you use brackets around the parameter x to prevent arithmetic errors caused by operator precedence

    Example:

    #define square(x) (x*x) // Becareful !!! int main( int argc, char* argv[] ) { double a, b, z; z = square( a+b ); // a+b*a+b = a + (a*b) + b != (a+b)^2 !! }

    After processing with gcc -E define2.c:

    int main( int argc, char* argv[] ) { double a, b, z; z = a+b*a+b; // a+b*a+b = a + (a*b) + b != (a+b)^2 !! }

    Solution:

    #define square(x) ((x)*(x))

    ---

  • Example Program: (Demo above code)                                                
    • Prog file: /home/cs255001/demo/c/Cprep/define2.c

    How to run the program:

    To compile:   gcc -E define2.c Look at the output on the terminal....

  ---

  ---

  ---

  ---

• Undefining a macro
  • Undefine a macro:

    #undef macro-name

    Example:

    #undef PI #undef MAX #undef square