The C Preprocessor

• Line splicing
  • Lines that end with \ are joined with the next line.

• File inclusion - #include
  • #include <file>
  • "" searches directory of source first, then standard places.
  • <> search standard places only.

• Macro substitution - #define
  • #define <identifier> <text>
  • The macro identifiers are substituted by the following text from the point of definition to the end of the source file.
  • Standardly written in CAPITAL_LETTERS.
  • No ; at the end of the text.
  • FahrCelciusDefine.c - Program to convert fahrenheit to celcius using #define.
  • The expansion is checked for further expansions.
  • Macros may be defined with arguments if the identifier is followed immediately by a (.
    • The formal arguments in the definition are replaced by actual arguments which must match in number.
    • Example

      #define MAX(A,B) ((A) > (B) ? (A) : (B))
                     

      Then

      x=MAX(p+q,r+s); 
                     

      expands to

      x=((p+q) > (r+s) ? (p+q) : (r+s))
                     

    • This is useful as a max for any data type. However beware problems.

      #define SQUARE(X) X*X
      SQUARE(z+1)
                     

      expands to

      z+1*z+1
                     

  • HashDefine.c - Program that uses #define to keep the world clean. Notice the ()s used to avoid precedence errors when the macro is used in an unexpected context.
  • #undef <identifier> causes the expansion to be forgotten.
  • Predefined identifiers
    • __LINE__ = current source line number
    • __FILE__ = current file name
    • __DATE__ = date of compilation
    • __TIME__ = time of compilation
    • __STDC__ = 1 (only in standard conforming implementations)
    • SysDefs.c - Program that prints out system symbol values

• Conditional compilation.
  • The preprocessor allows code to be compiled or ignored, depending on existing defined macro identifiers. The general form is :

    #if??? ?????????
        <code to be compiled if #if is non-0>
    #endif
              

  • #if <constant expression> checks if the <constant expression> is non-0.
  • #ifdef <identifier> checks if the identifier is defined, by a #define.
  • #ifndef <identifier> checks if it is not defined.
  • In between #if??? and #endif there may be #elif <constant expression> or #else with the intuitive meaning.
  • The constant expression may contain expressions of the form defined(<identifier>), which are replaced by 1L if the identifier is defined, 0L if not.
  • IfDef.c - Program that uses #define to keep the world clean
  • MyHeader.c - Avoiding multiple inclusions
  • Use for debugging. You can set a symbol without a value on the command line, for example:

    prompt> gcc -DDEBUG prog.c -o prog 

    Here the value is assumed to be 1. The setting of such flags is useful, especially for debugging. You can put commands like:

    x = y *3;
    
    #ifdef DEBUG
         print("Debugging: Variables (x,y) = \",x,y);
    #endif 

• Line control
  • #line <constant>
  • #line <constant> "<filename>"
  • Causes the compiler to believe that the current line number is <constant>, and it is compiling the file <filename>

• Error generation
  • #error <text>
  • Causes the preprocessor to output an error message that includes the text.

• Pragmas
  • #pragma <text>
  • Causes the preprocessor to perform implementation dependant action, which is ignored if unrecognised.

• The -E compiler option will force the compiler to stop after the preprocessing stage and output the current state of your program. Apart from being debugging aid for preprocessor commands and also as a useful initial learning tool (try this option out with some of the examples above) it is not that commonly used.

Exercises

• Write code that defines a macro CHOOSE. If the symbol LARGE is defined CHOOSE should return the larger of its two arguments, otherwise it should return the smaller.
  Answer: ChooseMacro.c
• Define a preprocessor macro SWAP(t,x,y) that will swap two arguments x and y of a given type t.
• Define preprocessor macros to select:
  • the least significant bit from an unsigned char
  • the nth (assuming least significant is 0) bit from an unsigned char.

Exam Style Questions

• Any of the short exercises.
• Explain the difference between #include <blue.h> and #include "blue.h"?
• Write a program that prints out the date and time is was compiled.
• What does the following macro do?

  #define cc(A,B,C) (A)?(B):(C) 

• Write a conditional compilation statement that, depending on whether or not the symbol LOCAL is defined, includes the file string.h from the current directory (LOCAL is defined) or from the standard splace (LOCAL not defined).