The Preprocessor

Last time: Miscellaneous C Topics

Prof. Brian D. Davison

Computer Science & Engineering, Lehigh University

Additional Credits

The Preprocessor

• When dealing with multiple files, we need one or more header files to define function signatures as well as structs, global variables, typedefs, etc.
• The header file is loaded using a preprocessor directive #include which loads the specified file
  • Local header files have quotes around the name, and are relative to the current directory
  • System header files have <> around the file name, and are found in a system-defined location (e.g., /usr/include/)
• The C preprocessor supports many other directives.

Defining constants and macros

• #define lets you create constants and macros

  #define PI 3.141592654
  #define TIMES *
  
  double area(double r) {
    return PI TIMES r TIMES r;
  }
  

Complex macros

• Macros can be more complex with arguments

  #define area(x) (PI * x * x)
  

  but this form won't work for area(v + 1) as the compiler will see it as (PI * v + 1 * v + 1). A better macro would be

  #define area(x) (PI * (x) * (x))
  

  so that expected evaluation order is preserved.
• Macros are faster than functions (no stack manipulation) but may use more space (since each function call is expanded in place).

Conditional Compilation

• The C preprocessor can also be used to "turn on" or "turn off" sections of code depending on whether something has been #define'd

#ifdef IS_WINDOWS
/* do something backwards */
....
#    else
/* do regular code */
....
#endif

Conditional Compilation

• #ifndef allows for the reverse test. Commonly used to prevent header files from being loaded more than once.

/* local.h  -- local function definitions, variables, types, etc. */

#ifndef LOCAL_LOADED
#define LOCAL_LOADED
#include <stdio.h>

#define PI the_whole_thing
int eat(int food);
#endif

Common use of conditionals

• Conditional compilation is often used in debugging, and for platform specific code

...
#ifdef DEBUGGING
         printf("Code reached point 14; x=%d, y=%f\n", x, y);
#endif

• Symbols can also be defined at compile time, not just in #define statements

  gcc -D DEBUGGING mycode.c
  

Other conditions

• The preprocessor can also check the value of a constant expression

#if    DEBUG_LEVEL == 1 /* show minimal debugging */
 ...
#elif  DEBUG_LEVEL == 2 /* show more */
 ...
#elif  DEBUG_LEVEL == 9 /* show max */
 ...
#else  /* unsupported debugging level */
 ...
#endif

Defensive Programming

• The larger your project, the more difficult it will be to find subtle errors
• The best approach is to code defensively
• Check every function for failure codes (especially system calls)
• Initialize all variables, especially pointers
• Write testing harnesses for all functions
• Document carefully -- e.g., describe all assumptions for values passed as parameters to your functions
• Verify that parameters are valid before using them
• Use assert(3c) liberally to enforce those assumptions

Using assert()

• We want coding errors to cause the app to fail quickly so that they can be easily detected during debugging.
• C provides a standard assert(3c) macro, which evalutes the expression passed in, and if false, outputs an error message and calls abort(3c)

#include <assert.h>
...
assert(status == 0);

• Failures would look something like:

  % ./program
  source.c:114: failed assertion `status == 0'
  Abort
  %
  

When to use/not use assert()

• Use assert() anywhere that a coding error might cause an invalid condition
  • e.g., when your code is called by someone else!
  • or when one module calls another
• Don't use assert() to validate user input
• since assert causes the program to fail, rather than politely generating an appropriate error message and permitting the user to re-enter valid input
• Don't use assert() when you can recover gracefully
  • Such as when trying to open a file without proper permissions

Looking Ahead

• Friday will be our first exam. Remember, one sheet of notes! (Feel free to use mpage to make them small!)