Overview

• Preliminaries.
  • Admin.
  • Upcoming Work.
  • Extra Credit.
  • Questions.
• Interesting macro examples.
• Requirements for a debugging/logging system.
• Implementation details.

Review

• Macros in C: Look like functions or values, but implemented as textual substitution during preprocessing stage.

• Examples (defining a single value):

  #define ARRAY_SIZE 1000
  int values[ARRAY_SIZE];
  

  • Let's us give names to "magic numbers"
  • Makes it easier for us to change those magic numbers

• Examples (defining something function-like)

  #define SQUARE(X) X*X
  
  
  printf("%d squared is %d\n", i, SQUARE(i));
  

• Why use #define rather than constant?

  • Constant didn't exist in early C.
  • You can change #defines when you compile. -DNAME=VAL
• Why use #define rather than a function call?
  • The #define is more general: It will work with doubles as well as integers.
  • Eliminate the overhead of a function call. In Java, we saw a factor of eight difference for including a function call to swap.

Macros are dangerous

    printf("%d\n", SQUARE(2+3));
    int x = 3;
    printf("%d\n", SQUARE(x++);
    printf("x= %d\n", x);

We should have defined it as

    #define SQUARE(X) ((X)*(X))

Interesting macro examples

• printf-style debugging
• Generic code in C

Problems with Printf-Style Debugging

• Add them, then comment them out later. Lots of effort.

Solving with Macros

• Strategy 1

  #define LOG(message) printf("%s\n", message);
  #define LOG(message)
  

• A cool thing you can do with modern macros (e.g., in GNU CC)

  #PARAM gives you a parameter as a string
  

• Strategy 2

  #ifdef DEBUG
  #define LOG(message) fprintf(stderr, "*** %s ***\n", message);
  #define LOGI(i) fprintf(stderr, " *** %s = %d ***\n", #i, i);
  #else
  #define LOG(message)
  #define LOGI(i)
  #endif
  

Using Macros for Generics

• Another cool thing with modern macros PARAM ## text substitutes and then concatenates.
• In C, when we want linked lists for different types, we generally copy-paste-change. Let's think about generalizing that code.