Macros 97

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Preprocessor

Vijaya Saradhi

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Introduction

� Header files

� Macros

� Conditionals

� Examples

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Header files

� A file containing declarations and macros

� Usage of header files: #include

� # stands for preprocessing directive� Two types:

± System header files stdio, stdlib, stdargs,

± Our own hears containing declarations, macros� System headers resides at designated places

± /usr/include; /usr/local/include; etc

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Headers

� Inclusion:

± #include<stdio.h>

± stdio.h is a file located at: /usr/include directory ± #include min.h

± min.h is a file located in the current directory

� Can specify path explicitly

± #include /home/guest/min.h

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Example

File Name: min.h:

Located at: /home/guest

int min(int, ...);int max(int, ...);

void myprint(char *format, ...);

File Name: min.c:

#include<stdio.h>

#include ³/home/guest/min.h´

int main(int argc, char *argv[])

{

int k = 10, j = 20;

k = min(4, 5, 4, 3, 2);

k = min(6, 7, 6, 5, 4, 3, 2);

k = max(4, 5, 4, 3, 2);k = max(6, 7, 6, 5, 4, 3, 2);

myprint(³%d%d\n´, k, j);

}

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Search Path

� Header files can be located at any place

� They can be included in any C program file

� How the compiler knows where to pick thesefiles?

� standard library files and other systems files

are searched in a specified locations� We can request compiler to search at

specified locations as well

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Example

� You have two directories:

± assignment1

� src

� include

± At the time of compiling:

� gcc min.c I/home/guest/assignment1/include

± Our header files shall be searched in the given path

± Causes to place the contents in the required files

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Macros

� Identifier following #define

� A fragment of code which has been given aname

� Two kinds: ± Object like macro resembles data objects

± Function like macro resembles functions

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Object like macro

� A simple identifier

� This will be replaced by the code fragment

� Creating object like macro: ± #define MAX_ARRAY_SIZE 100

± #define TABLE_SIZE 200

± #define max_characters 1024

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Example

#include<stdio.h>

#define MAX_ARRAY_SIZE 100


{int a[MAX_ARRAY_SIZE];

int a[100];

float f[MAX_ARRAY_SIZE];

float f[100];

..

..

}

Pre-processor recognizes and expands the macro

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Close look

� Macro ends at the #define line

� For a macro running multiple lines:#define NUMBERS 10,\

11,\

12,\

13,\

14

int x[] = {NUMBERS};

Causes: int x[] = {10, 11, 12, 13, 14};

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Function like macro

� Macros whose use looks like a macro

� Creating function like macro:

� Macros with arguments:

� #define identifier( id1, id2, , idk) string

#include<stdio.h>

#define print() printf(³Hello CS101\n´)


{print();

printf(³Hello CS101\n´);print();

printf(³Hello CS101\n´);

return 0;

}

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Example

#include<stdio.h>

#define sq(x) x * x


{int k = 5;

printf(³%3d\n´, sq(k));

printf(³%3d\n´, k * k);

return 0;

}

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Multi-line macro

#include<stdio.h>

#define FILL(array, size) \

for( i = 0; i < size; i++ ) \

array[i] = rand()

#define print(array, size, fmt) \for( i = 0; i < size; i++ ) \

printf(fmt, array[i])


{

int i = 0, a[10];

FILL(a, 10);

for( i = 0; i < 10; i++ )

a[i] = rand();

print(a, 10, ³%3d´);

for( i = 0; i < 10; i++ )

printf(fmt, a[i]);

return 0;}

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Variadic Macros

� A macro can be declared to accept variable

number of arguments#include<stdio.h>

#define eprintf(...) fprintf(stdout, __VA_ARGS__ )


{

int k = 10;

double d = 70.7;eprintf(³k: %-4d d: %4.2\n´, k, d);

fprintf(stdout, ³k: %-4d d: %4.2\n´, k, d);

return 0;

}

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Pitfalls

� Operator precedence:#include<stdio.h>

#define sq(x) x * x

#define sq(x) ((x) * (x))


{

int k = sq( 4 + 5 );

int k = 4 + 5 * 4 + 5;

int k = ((4 + 5) * (4 + 5));

printf(³k: %d\n´, k);

return 0;

}

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Pitfalls

#include<stdio.h>

#define FILL(array, size) \

for( i = 0; i < size; i++ ) \

array[i] = rand();

#define print(array, size, fmt) \

for( i = 0; i < size; i++ ) \

printf(fmt, array[i]);


{int i = 0, a[10], not_filled=1;

if( not_filled == 1 )

FILL(a, 10);

else

printf(³filled\n´);

if( not_filled = 1 )

for( i = 0; i < 10; i++ )

a[i] = rand();;

else

printf(³filled\n´);

Introduction of empty statement

� Swallowing of semicolon

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Pitfalls

� Duplication#include<stdio.h>

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

int main(int argc, char*argv[])

{

int x = 5, y = 8, z = 0;

z = MAX(x++, y++);

z = ( (x++) > (y++ ) ? (x++) : (y++) );

printf(³x: %d, y: %d, z: %d\n´, x, y, z);

return 0;

}

output: x: 6, y: 10, z: 9

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Pitfalls

� Self referential macros: A macro whose name

appears in its definition#include<stdio.h>


{

int x = 120;

#define x (10 + x)

printf(³%d\n´, x);printf(³%d\n´, (10 + x));

return 0;

}

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Self-referral macros

� Indirect self-referral: Does not go into infinite

loop of expanding macros x, y#define x (4 + y)

#define y (2 + x)

x expands to: (4 + y) -> (4 + (2 + x))

y expands to: (2 + x) -> (2 + (4 + y))

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Pitfalls

� Mis-nesting:#include<stdio.h>

#define once(x) x(1)

#define twice(x) (2 * (x))


{

printf(³%d\n´, once(twice));

printf(³%d\n´, twice(1));

printf(³%d\n´, (2 * (1)));

return 0;

}

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At compilation

� One can define object like macro at

compilation time#include<stdio.h>

int main(int argc, char *argv[]){

int a[ MAX_ARRAY_SIZE];

float f[ MAX_ARRAY_SIZE];

for( k = 0; k < MAX_ARRAY_SIZE; k++ )

{a[i] = 10; f[i] = 70.7;

}

return 0;

}

gcc ±D MAX_ARRAY_SIZE=10 macro.c

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Recollect

� Macros

± getchar() : #define getchar() getc(stdin)

± putchar(c) : #define putchar(c) putc( (c) , stdout)

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Conditional Compilation

� Preprocessor has directives for conditionalcompilation

� Use of constructs: ± #ifdef, #ifndef

± #if, #else, #elif

± #defined()#include<stdio.h>


{

#ifdef DEBUGprintf(³Debug: Hello CS101 student\n´);

#endif

printf(³Hello CS101\n´);

return 0;

}

Compile:

gcc macro2.c ±DDEBUG=1Output:

Debug: Hello CS101 studentHello CS101

Compile:gcc macro2.cOutput:

Hello CS101

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Conditional Compilation Contd.

� If a particular macro is *NOT* defined then

take some actions

#include<stdio.h>int main(int argc, char *argv[])

{

#ifndef DEBUGprintf(³No Debug: Hello CS101\n´);

#endifprintf(³Hello CS101 student\n´);

return 0;

}

Compile:

gcc macro2.c ±DDEBUG=1Output:

Hello CS101 student

Compile:gcc macro2.cOutput: No Debug: Hello CS101

Hello CS101 student

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#If

� #ifdef/#if ndef checked for existence of macro

� #if allows evaluate arithmetic expression#include<stdio.h>


{

#if (10 < 20)

printf(³True\n´);

#else

printf(³False\n´);

#endif

return 0;

}

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Arithmetic expression

� Involves

± Integer constants

± Character constants

± Arithmetic operators: +, -, /, *

± Logical: ||, &&

± Bitwise: &, |, !

± Shit operators: <<, >>

± Macros

± defined operator

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defined

� Special operator

± Checks whether a name is a defined macro or not

± Returns 1 if true

± 0 otherwise#include<stdio.h>


{

#if defined( LI NUX ) && !defined(W I NDOWS)

printf(³I am in Linux\n´);#else

printf(³I am in Windows\n´);

#endif

return 0;

}

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Undefine a macro

� We can un-define a (object and/or function

like) macro

� When you include files having macros;

� First un-define them to overwrite the new

definitions#include<stdio.h>

#define MAX_ARRAY_SIZE 100


{

int a[MAX_ARRAY_SIZE];

#undef MAX_ARRAY_SIZE

..

return 0;

}

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#error

� During compilation, #error directive helps

throw errors.

#if 10 < 20

#error ³Incompatible sizes´

#endif

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Pragma

� Causes behavior dependent on particularcompiler

� Want an identifier to not recognize

± Example: printf, scanf

± Poison them using poison pragma#include<stdio.h>

# pragma GCC poison printf scanf


{

int k = 10; double d = 70.7;

printf(³k: %d, d: %f\n´, k, d);

return 0;

gcc macro6.c ±g

macro6.c:9:3: error: attempt to

use poisoned printf

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Line numbers

� Use of #line causes renumbering of lines of

the source fileFile Name: macro7.c

1: #include<stdio.h>2:

3: int main(int argc, char *argv[])

4: {

5: #line 100 ³cs101.c´

100: printf(³Hello CS101 %s::%d\n´, __ FILE__, __ LI NE__ );101: return 0;

102:}

Output: Hello CS101: cs101.c::100

Macros 97

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