Preprocessor

• All preprocessor directives or commands begin with a #.– E.g. #include <stdio.h>

C program → Modified C program → Object Code

• Can appear anywhere in the program• No “;” in the end

preprocessor compiler

Preprocessor Directives

• Macro definition– #define, #undef

• File inclusion– #include

• Conditional Compilation– #if, #ifdef, #ifndef, #elseif, #else

• Others

Advantages

• Easy to– Develop program – Read programs – Modify programs

• C code more transportable between different machine architectures

#define

• To define constants or any macro substitution.

    #define <macro> <replacement name>  

E.g.

  #define FALSE 0

#define TRUE !FALSE

• To undefined a macro.

E.g. #undef FALSE

– A macro must be undefined before being redefined to a different value.

Define Functions

• E.g. To get the maximum of two variables:  

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

• If in the C code:    x = max(q+r,s+t);

• After preprocessing:

   x = ( (q+r) > (s+t) ? (q+r) : (s+t));

File inclusion

• #include directive– Include the contents of another file at the point where the

directive appears.

• Why need file inclusion– Use built-in functions

• E.g., printf(), rand();

– Reuse code

defTime.h

struct date{int day;char month[10];int year;

};

struct date{int day;char month[10];int year;

};

struct date today;

#include “defTime.h”→

struct date today;

Your code

File inclusion formats

• #include <file> – Used for system header files– File contain function prototypes for library

functions•<stdlib.h> , <math.h> , etc

• #include "file" – Used for header files of your own program

• looks for a file in the current directory first• then system directories if not found

Standard C libraries

• Build-in with your compiler

• Detailed information– http://www.utas.edu.au/infosys/info/document

ation/C/CStdLib.html

• stdio.h– core input and output capabilities of C

• printf function• scanf function

Math Library Functions

• Math library functions – perform common mathematical calculations

• E.g. exp(), pow(), sqrt(), fabs(), sin(), cos().– #include <math.h>

• Format for calling functions– FunctionName( argument, …, argument );

• All math functions return data type double– E.g.: printf( "%.2f", sqrt( 900.0 ) ); – Arguments may be constants, variables, or

expressions• Compile

– gcc yourfilename.c –lm –o yourfilename.exe

stdlib.h

• A variety of utility functions– rand function

• A function to generate a pseudo-random integer number

• Return value is in the range 0 to RAND_MAX • Example:

#include <stdlib.h> int i = rand();

– memory allocation– process control

string.h

• All the string handling functions – strcpy – strcat – strcmp

Custom header files

• Steps for creating custom header files– Create a file with function prototypes – Save as a .h file. E.g.: filename.h– Load in other files with

•#include "filename.h"

• Advantage– Reuse functions and data structure

declaration

Conditional compilation

• Useful when– machine-dependencies– Debugging– setting certain options at compile-time.

• Expressions– #if expression– #ifdef expression (same as #if defined )– #ifudef– #elif and #else– #endif

Examples

• Example: write programs that are portable to several machines or operating systems– #if defined(WINDOWS)– #elif defined(LINUX)– #elif defined(SOLARIS)– #endif

• Example: Providing a default definition for a macro– #ifndef BUFFER_SIZE– #define BUFFER_SIZE 256– #endif