C Programming Tutorial
Preprocessor directives performs macro definition,source code file inclusion,and conditional compilation. and line control
it is a seperate program invoked by the C Compiler. C has a Built-in Preprocessor that isolate any system dependent code.
#define LIMIT 100 //Here LIMIT is called as a symbolic constant
Lines that start with # are called Preprocessing Directives
scope of the preprocessing directive the line it starts in a file to end of the file.
Preprocessor doesnot know 'C', it is a independent program invoked by compiler,these independent program replaces all symbolic constants with the macors,after that actual compilartion starts.
Job of Preprocessing
File name inclusion can be done in following ways
searches for file in system standard directories and then other directories
#include <filename>
searches for file first in current directory and then other directories
#include "filename"
Symbolic Constants Given a Name to a value.
#define PI 3.14159
here PI is a symbolic constant
Preprocessing directive program, whereever symbolic constant PI occurs in any expressions in source code ,it replaces with value 3.14454, this process happens before compilation of the C source code
Passing arguments
User can pass arguments with macros
#define MAX(a,b) ((a)>(b))?(a):(b)
Passing arguments to macro,MAX macro takes 2 arguments irrelevant of the type
Calling macros in source code
MAX(10,15)
MAX(10.5f,9.9f)
MAX('Z','z')
MAX(sam,peter)
Calling macros within macros or nested macros
define a simple macro called
#define SQR(X) (x) * (x)
printf("%d",SQR(5)) will print square of 5 is 25.
again calling SQR within SQR , means SQR(SQR(5)) gives 625.
nested macro expanded first, 5*5 25 replaces SQR(5),now SQR(25) expanded, 25 * 25 gives 625
This type of macros has side effects
for ex: int a=10;
printf("%d",SQL(a++)) ,SQR will be expanded as (a++) * (a++) , a has value 10, (11)*(10) gives 110.
now print "a" value, gives 12, because "a" is incremented twice. programmer must avoid such expressions in macros.
comparing 2 strings using macros
define simple macro called COMPARE which accepts 2 arguments string1 and string2, if both strings are equal displays message "strings are equal " otherwise not equal.
#define COMPARE(s1,s2) if(strcmp(s1,s2)==0) printf("strings are equal\n"); \
else printf("strings are not equal\n");
#multi-line macro , end of first line should be backslash \ to indicate continous
COMPARE("sam","sam"); COMPARE("RAM","sam"); prints "strings are equal" and "strings are not equal",respectively.
Swapping 2 numbers using macros
exchange 2 values using third variable. copy x value to temp, now temp hold value of x, so x value can be erasable, means copy y value to x,because temp already has x value, finally copy temp value i.e x value to y
calling SWAP macro
#define SWAP(x,z,temp) {temp=x;x=y;y=temp;}
double x=10.12,y=20.25,temp;
printf("x=%f,y=%f\n",x,y);
SWAP(x,y,temp);
printf("x=%f,y=%f\n",x,y);
#if, #ifdef, #ifndef, #elif, #else, #endif
The unary operator # is used to "stringization" of macro parameter.It is also called as creation of strings
#define TOK(a) printf(#a)
TOK(10);TOK(Hello);
TOK(Hello World);
Preprocessor directive replaces formal parameter of Macro with printf statement,argument sorrounded by double quotes
Whenever TOK(10);is called it is replaced with printf("10"); TOK(Hello) with printf("hello"); TOK(Hello World) with printf("Hello World");10 Hello Hello World
Enum constant values can be converted to String using unary Preprocessor # operator
#define TOK(a) printd(#a)
enum COLOR{RED,BLUE,GREEN}
TOK(RED);
TOK(BLUE);
ouputs RED as a string, BLUE as a String. etc.,
The binary operator ## is used to merge tokens.
Assert macros are defined in "assert.h". assert macros are very usefull in validating data. otherwise control statements should be used to test a data.in that case code becomes cumbersome and error-prone, in order to avoid that assert macro helps us to write clean code for validating input data.
if assertion fails displays error message and exits the application
int a=3,b=4; assert(a+b != 7); Assertion `(a+b)!=7' failed. Aborted (core dumped)
if you call like this assert(a+b == 7); validation successful, it continues with next statementADS