C: Lesson 18

C Preprocessor

• Includes other files being compiled

• Defines symbolic constants and macros

• Processes conditional compilation and conditional execution of preprocessor directives.

• Preprocessor directives begin with # and only white-space characters and comments may appear before a preprocessor directive on a line.

#include Preprocessor Directive

• The #include directive causes a copy of a specified file to be included in place of the directive.

• The two forms of the #include directive are

  • #include <filename"> Finds standard library implementation

  • #include “filename“ Finds user defined implementation - starts searches in the same directory and may search in other locations too

#define Preprocessor Directive for Constants & Macros

• The #define directive creates symbolic constants.

  • format: #define identifier replacement-text

• Constants are represented as symbols

  • eg: #define PI 3.14159

• Macros are operations defined as symbols.

  • eg #define CIRCLE_AREA(x) ( (PI) * (x) * (x) )

• The replacement text for a macro or symbolic constant is normally any text on the line after the identifier in the #define directive.

• We can use backlash (\) at the end of the line if the macro needs to continue on the next line

• Replacements would look something like the following

  • Before processed: area = CIRCLE_AREA(4)

  • After processed: area = ((3.14159) * (4) * (4))

Conditional Compilation

• Controls the execution of preprocessor directives.

• Each of the conditional preprocessor directives evaluates a constant integer expression

• Cast expressions, sizeof expressions and enumeration constants cannot be evaluated in preprocessor directives.

• Common use cases:

  • To comment out portions of the code we want to have in development purposes only

  • To make use of debuggers

# Operator

• The # operator causes a replacement text token to be converted to a string surrounded by quotes.

• The # operator must be used in a macro with arguments because the

  operand of # refers to an argument of the macro.

  • eg: #define HELLO(name) printf("hello " #name "\n")

## Operator

• The ## operator concatenates two tokens.

  • eg. #define TOKEN_CONCAT(A, B) A ## b

Line Numbers

• line_number: Sets the current line number to this value.

• "filename" (optional): Sets the current file name that the compiler uses in messages (e.g., for warnings and errors).

Predefined Symbolic Constants

Assertions

• The assert macro is defined in the <assert.h> header

• Tests the value of an expression.