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The Preprocessor. #include <stdio.h> #define N 10. C program. Preprocessor. Modified C program. Preprocessor. Object codes. Caution. The preprocessor has only a limited knowledge of C. As a result, it’s quite capable of creating illegal programs as it executes directives.

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The Preprocessor

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The preprocessor

The Preprocessor

#include <stdio.h>

#define N 10

C program


Modified C program


Object codes



The preprocessor has only a limited knowledge of C. As a result, it’s quite capable of creating illegal programs as it executes directives.

Preprocessing directives

Preprocessing Directives

  • Macro definition: #define to defines a macro; the #undefine to removes a macro definition.

  • File inclusion: #include causes the content of a specified file to be included in a program.

  • Conditional compilatoin: #if, #ifdef, #ifndef, #elif, #else and #endif allow blocks of text to be either included in or excluded from a program, depending on conditions that can be tested by the preprocessor.

Macro definitions

Macro Definitions

Simple Macros:

#define identifierreplacement-list

#define N = 100


int a[N];

#define N 100;


int a[N];

Simple macro

Simple Macro

Simple macros are primarily used for defining “manifest constants”: giving names to numeric, character, and string values:

#define STR_LEN 80

#define TRUE 1

#define FALSE 0

#define PI 3.14159

#define CR ‘\r’

#define EOS ‘\0’

#define MEM_ERR “Error: not enough memory”

Simple macro advantages

Simple Macro---advantages

  • It makes programs easier to read

  • It makes programs easier to modify

  • It helps avoid inconsistencies and typographical errors

  • Making minor changes to the syntax of C

  • Renaming types: #define BOOL int

  • Controlling conditional compilation

Parameterized macros

Parameterized Macros

#define identifier(x1, x2, … , xn ) replacement-list

#define MAX(x, y) ((x) > (y) ? (x) : (y))

#define IS_EVEN(n) ((n)%2 == 0)

#define TOUPPER(c) (‘a’<=(c)&&(c)<=‘z’?(c) –’a’+’A’:(c))

Parameterized macros advantages

Parameterized Macros---advantages

  • The program may be slightly faster

  • Macros are “generic”

Parameterized macros disadvantages

Parameterized Macros---disadvantages

The compiled code will often be larger

n = MAX(I, MAX(j, k));

n = ((i)>(((j)>(k)?(j):(k)))?(i):(((j)>(k)?(j):(k))));

Arguments aren’t type-checked

It’s not possible to have a pointer to a macro

A macro may evaluate its arguments more than once

n = MAX(i++, j);

n = ((i++) > (j) ? (i++) : (j))

Conditional compilation

Conditional Compilation

The #if and #endif Directives

#define DEBUG 1


printf(“Value of i: %d\n”, i);

printf(“Value of i: %d\n”, i);


Writing large programs

Writing Large Programs

Source file **.c

Header file **.h

#include <filename>: search the directory in which system header files reside(on UNIX, system header files are kept in the directory /usr/include

#include “filename”: search the current directory

#include “c:\cprogs\utils.h” /* windows path */

#include “/cprogs/utils.h” /* UNIX path */

Pointer as arguments

Pointer as Arguments


#define BOOL int

#define TRUE 1

#define FALSE 0


#include “boolean.h”


#include “boolean.h”

Sharing function prototypes

Sharing Function Prototypes


void make_empty(void);

int is_empty(void);

int is_full(void);

void push(int i);

int pop(void);


int contents[100];

Int top = 0;

void make_empty(void)




int is_empty(void);




int is_full(void);

void push(int i);

int pop(void);


#include “stack.h”

int main(void)





Sharing variable declaration

Sharing Variable Declaration

External variables can be shared among files in much the same way functions are.

We can put its definition in one source file, then put declarations in other files that need to use the variable.

int i; /* declares i and defines it as well */

extern int i; /* declares i without defining it */

extern works with variables of all types.

Be careful

Be careful

When declarations of the same variable appear in different files, the compiler can’t check that the declaration match the variable’s definition.

int i;

extern long i;

Protecting header files

Protecting Header Files


#include “Header1.h”

#include “Header1.h”



#include “Header2.h”

#include “Header3.h”

Protecting header files1

Protecting Header Files


#ifndef STACK_H

#define STACK_H

void make_empty(void);

int is_empty(void);

int is_full(void);

void push(int i);

int pop(void);


Building a multiple file program

Building a Multiple-File Program

Compiling: each source file in the program must be compiled separately.

the compiler generates a file containing object code:

.o in UNIX or .obj in Windows

Linking: the linker conbines the object files to produce an executable file.

Most compiler supports a simpler way:

gcc –o justify justify.c line.c word.c



justify: justify.o word.o line.o

gcc –o justify justify.o word.o line.o

justify.o: justify.c word.h line.h

gcc –c justify.c

word.o: word.c word.h

gcc –c word.c

line.o: line.c line.h

gcc –c line.c



  • Each command in a makefile must be preceded by a tab character, not a space.

  • A makefile is normally stored in a file called Makefile (or makefile)

  • To invoke make, use the command

  • make target

  • If no target is specified when make is invoked, it will build the target of the first rule.

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