C Programming Day 3

2 Copyright © 2005, Infosys Technologies Ltd ER/CORP/CRS/LA07/003 Version No. 1.0 Storage Class Specifiers Every variable in a C program has a storage class specifier –specifies how the variable is stored in the memory –governs the scope & lifetime of a variable Scope of a variable is the area within a program in which it is accessible Lifetime is the time span for which the variable is alive when the program is in execution Four storage class specifiers available in C –automatic –static –register –extern

3 Copyright © 2005, Infosys Technologies Ltd ER/CORP/CRS/LA07/003 Version No. 1.0 Storage Class Specifiers… automatic –automatic is the default storage class specifier. –Scope & Lifetime is within the functions in which they are declared. –auto int iCount; declares iCount as an automatic integer variable auto is a keyword and its optional

4 Copyright © 2005, Infosys Technologies Ltd ER/CORP/CRS/LA07/003 Version No. 1.0 Storage Class Specifiers… static –Scope of a static variable is within the function in which it is declares –Lifetime of a static variable is throughout the program i.e. static variables retain their values throughout the program –static int siCount; declares siCount as a static integer variable static is a keyword –static variables are automatically initialized to zero

5 Copyright © 2005, Infosys Technologies Ltd ER/CORP/CRS/LA07/003 Version No. 1.0 Storage Class Specifiers… register –Variables are stored in registers if They are frequently referenced they need to be accessed at a faster speed –Use of register variables The loop counter variables are the best candidates –Scope & Lifetime of register variables within the functions in which they are declared

6 Copyright © 2005, Infosys Technologies Ltd ER/CORP/CRS/LA07/003 Version No. 1.0 Storage Class Specifiers… extern –The scope of global variables is within the file in which it is declared –extern is used if a global variable defined in file1.c needs to be made available in file2.c

7 Copyright © 2005, Infosys Technologies Ltd ER/CORP/CRS/LA07/003 Version No. 1.0 Storage Class Specifiers… Storage AreaScopeLife TimeInitial value automaticStack Within the function Garbage externData section Across translation units Through out the program zero registerCPU register Within the function Garbage staticData section Within the function Through out the program zero Storage class

8 Copyright © 2005, Infosys Technologies Ltd ER/CORP/CRS/LA07/003 Version No. 1.0 Enumerated Data Types Enumeration is a list of named integer constants These constants can be used instead of the associated integer Enumerations provide self-documenting code and help in clarifying the structure of the program Example typedef enum _Days { Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, Saturday }eDays;

9 Copyright © 2005, Infosys Technologies Ltd ER/CORP/CRS/LA07/003 Version No. 1.0 Enumerated Data Types… Declaring a enumerated data type –eToggleSwitch is an enumeration i.e. it’s a list of 2 integer constants OFF (0) and ON(1) –TS1 is a variable of type eToggleSwitch and can have either of the values OFF (0) or ON(1) –TS1=ON will toggle the switch ON hence printf (“%d”, TS1) will print 1 –TS1=OFF will toggle the switch OFF hence printf (“%d”, TS1) will print 0 enum eToggleSwitch { OFF, ON } TS1;

Preprocessor Directives

11 Copyright © 2005, Infosys Technologies Ltd ER/CORP/CRS/LA07/003 Version No. 1.0 Program Life Cycle

12 Copyright © 2005, Infosys Technologies Ltd ER/CORP/CRS/LA07/003 Version No. 1.0 #include int main(void ) { int iResult; scanf(“%d%d”, &a,&b); iResult = a + b; printf(“%d”, iResult); return 1; } Life Cycle Edit test.c Compile test.c Gives error, as a and b are not declared EXE not CREATED Example File: test.c

13 Copyright © 2005, Infosys Technologies Ltd ER/CORP/CRS/LA07/003 Version No. 1.0 #include int main(void) { int iResult,iN1,iN2; scanf(“%d%d”, &iN1,&iN2); iResult = diff (iN1,iN2); printf(“%d”, iResult); return 1; } Life Cycle Edittest.c Compile test.c Gives error as function diff() is not declared EXE NOT CREATED Example File: test.c

14 Copyright © 2005, Infosys Technologies Ltd ER/CORP/CRS/LA07/003 Version No. 1.0 #include int sum(int a, int b); int main(void) { int iResult,iN1,iN2; scanf(“%d%d”, &iN1,&iN2); iResult = sum (iN1,iN2); printf(“%d”, iResult); } Life Cycle Edit test.c Compile test.c –create object file test.o Link test.o Gives Link Error as function definition for sum() is missing EXE NOT CREATED Example File: test.c

15 Copyright © 2005, Infosys Technologies Ltd ER/CORP/CRS/LA07/003 Version No. 1.0 The C Preprocessor The C Preprocessor is a program that processes the source code before it is passed to the compiler Each of these preprocessor directives begin with a # symbol The directives provided by the C Preprocessor are for –macro expansion –conditional compilation –file inclusion

16 Copyright © 2005, Infosys Technologies Ltd ER/CORP/CRS/LA07/003 Version No. 1.0 Macro Expansions /* file1.c */ # define PI int main (int argc, char *argv) { float fRadius, fArea; printf("\nEnter the radius of the circle:- "); scanf("%f",& fRadius); fArea =PI * fRadius * fRadius; printf("\n Area of the circle = %f", fArea); retrun 0; } /* file1.i */ # define PI int main (int argc, char *argv) { float fRadius, fArea; printf("\nEnter the radius of the circle:- "); scanf("%f",& fRadius); fArea = * fRadius * fRadius; printf("\n Area of the circle = %f", fArea); return 0; } Preprocessor

17 Copyright © 2005, Infosys Technologies Ltd ER/CORP/CRS/LA07/003 Version No. 1.0 Macro Expansions… Why Macros if the same can be achieved through variables? –Treating a constant as a variable is principally not correct –A variable may get altered somewhere in the program hence it no longer remains constant Advantages of Macro Expansion –The program become more readable –If the macro expansion has to be changed change only the macro definition preprocessor will replace all occurrences with the new macro expansion

18 Copyright © 2005, Infosys Technologies Ltd ER/CORP/CRS/LA07/003 Version No. 1.0 Macro Expansions… Macros with Arguments. # define AREA(x) (3.14 * x * x) int main (int argc, char *argv) { float fRadius, fArea; printf("\nEnter the radius of the circle:- "); scanf("%f",&fRadius); fArea =AREA(fRadius); printf("\n Area of the circle = %f", fArea); return 0; } # define AREA(x) (3.14 * x * x) int main (int argc, char *argv) { float fRadius, fArea; printf("\nEnter the radius of the circle:- "); scanf("%f",& fRadius ); fArea = 3.14 * fRadius * fRadius; printf("\n Area of the circle = %f", fArea); return 0; } Preprocessor

19 Copyright © 2005, Infosys Technologies Ltd ER/CORP/CRS/LA07/003 Version No. 1.0 Macro Expansions… Precautions while writing macros with arguments –Never leave a blank space between the macro template and its argument Wrong: #define ABS (iIntVarx) (3.14*iIntVarx *iIntVarx) Correct: #define ABS(iIntVarx) (3.14*iIntVarx *iIntVarx) –Parenthesize macro parameters Wrong: #define ABS(iIntVarx) iIntVarx>=0? iIntVarx :-iIntVarx Correct: #define ABS(iIntVarx) (iIntVarx)>=0? (iIntVarx) :-(iIntVarx) –Example Usage: ABS( iIntVar a-1) Translated to: iIntVar a-1>=0? iIntVar a-1: - iIntVar a-1 Required: iIntVar a-1>=0? ( iIntVar a-1): -( iIntVar a-1)

20 Copyright © 2005, Infosys Technologies Ltd ER/CORP/CRS/LA07/003 Version No. 1.0 Macro Expansions… Precautions while writing macros with arguments –Avoid using expressions with side effects as arguments Careful: #define ABS(i) ((i) >=0 ? (i) : (-(i)) –Example Usage: ABS(i++) Translates to: ((i++) >=0 ? (i++) : (-(i++)) –Avoid circular definitions in which the symbol being #defined appears in its own definition Wrong: #define INFINITY (INFINITY +1)

21 Copyright © 2005, Infosys Technologies Ltd ER/CORP/CRS/LA07/003 Version No. 1.0 Popular Usage –#define AND && –#define ARANGE (a>25 && a<50) –#define FOUND printf(“lost property”); Macro Expansions…

22 Copyright © 2005, Infosys Technologies Ltd ER/CORP/CRS/LA07/003 Version No. 1.0 Difference between Functions and Macros –A macro is be more generic than a function since it will accept different types of data as arguments Macros execute faster than their equivalent functions Macro Expansions…

23 Copyright © 2005, Infosys Technologies Ltd ER/CORP/CRS/LA07/003 Version No. 1.0 File Inclusion Some functions are frequently required in a program Example: –printf() –scanf() These function’s prototypes can be stored in a file and it can be included in every program Such files are called as header files –“.h” extension for the files The prototypes of all related library functions are stored in a header file

24 Copyright © 2005, Infosys Technologies Ltd ER/CORP/CRS/LA07/003 Version No. 1.0 File Inclusion… Preprocessor directive to include a header file –#include 2 ways to use #include 1.#include “abc.h” Preprocessor would look for the file abc.h in - the current directory - list of directories specified in the system path 2.#include Preprocessor would look for the file abc.h in - list of directories specified in the system path

25 Copyright © 2005, Infosys Technologies Ltd ER/CORP/CRS/LA07/003 Version No. 1.0 Difference between header files and library files A header file is a text file A typical header file contains –prototypes, –definitions –preprocessor directives –by convention it contains no program statements Header files are used by the preprocessor A library file is an object file A library file contains a –collection of compiled modules for frequently used functions Library files are used by the linker

26 Copyright © 2005, Infosys Technologies Ltd ER/CORP/CRS/LA07/003 Version No. 1.0 Conditional Compilation Certain circumstances might require the compiler to skip over a part of source code This can be done by the #ifdef and #endif preprocessor directives Syntax: #ifdef and #endif preprocessor directives is as follows: #ifdef macroname statement1; statement2; #endif Note: If macroname has been #defined, then the block of code between #ifdef and #endif will get compiled

27 Copyright © 2005, Infosys Technologies Ltd ER/CORP/CRS/LA07/003 Version No. 1.0 Example: Conditional Compilation… #define iMAX 10 int main (int argc, char *argv) { #ifdef iMAX printf(“iMAX defined "); #else printf(“iMAX not defined "); #endif return 0; }

28 Copyright © 2005, Infosys Technologies Ltd ER/CORP/CRS/LA07/003 Version No. 1.0 Conditional Compilation… Situations where we go for conditional compilation –To “comment out” lines of code –To make programs portable We might need to write a program which should run on different platforms. In that case certain piece of code will be platform dependent and it should be placed between a #ifdef and #endif directives

29 Copyright © 2005, Infosys Technologies Ltd ER/CORP/CRS/LA07/003 Version No. 1.0 #if, #elif, #else, #endif Conditions are based on system variables, apart from the usual ones #ifdef, #ifndef Conditions test whether a symbol is defined or not #undef Undefine a symbol that was already defined #error Conditional Compilation…

30 Copyright © 2005, Infosys Technologies Ltd ER/CORP/CRS/LA07/003 Version No. 1.0 Example: Conditional Compilation… #define iMAX 10 int main (int argc, char *argv) { #if iMAX==10 printf(“iiMAX defined as 10"); #else printf(“iMAX not defined as 10"); #endif return 0; }

31 Copyright © 2005, Infosys Technologies Ltd ER/CORP/CRS/LA07/003 Version No. 1.0 Makefiles Why do we need them? –Tedious to compile each C file and then link them, especially if you have many source files What is a makefile? –Makefiles are special files used with the make utility –Helps to automatically build and manage projects with many source and header files

32 Copyright © 2005, Infosys Technologies Ltd ER/CORP/CRS/LA07/003 Version No. 1.0 Makefiles… A Makefile contains Dependency lines Executable command lines Macro definitions # A makefile that compiles three C files OBJECTS = try.o add.o sub.o final: $(OBJECTS) gcc $(OBJECTS) -o final add.o: add.c common.h sub.o: sub.c common.h Dependency Line Command Line Macro Definition

33 Copyright © 2005, Infosys Technologies Ltd ER/CORP/CRS/LA07/003 Version No. 1.0 Makefiles… /* mainfile.c */ #include main() { int iSum,iDiff; iSum=add(1,2); printf("Sum is %d\n",iSum); iDiff=sub(10,5); printf("Difference is %d\n",iDiff); } /* add.c */ #include"common.h" int add(int a, int b) { return(a+b); } /* sub.c */ #include"common.h" int sub(int a, int b) { return(a-b); } /* Common.h */ #include int add(int,int); int sub(int,int);

34 Copyright © 2005, Infosys Technologies Ltd ER/CORP/CRS/LA07/003 Version No. 1.0 Makefiles… # makefile to compile and execute three files # mainfile.c add.c and sub.c final: mainfile.c add.c sub.c gcc mainfile.c add.c sub.c –o final # makefile to compile and execute three files # mainfile.c add.c and sub.c OBJECTS=mainfile.o add.o sub.o final: $(OBJECTS) gcc $(OBJECTS) -o final TAB space is mandatory

35 Copyright © 2005, Infosys Technologies Ltd ER/CORP/CRS/LA07/003 Version No. 1.0 Makefiles… # makefile to compile and execute three files # mainfile.c add.c and sub.c OBJECTS=mainfile.o add.o sub.o final: $(OBJECTS) gcc $(OBJECTS) -o final add.o: add.c common.h sub.o: sub.c common.h

36 Copyright © 2005, Infosys Technologies Ltd ER/CORP/CRS/LA07/003 Version No. 1.0 Makefiles… Libraries –Compiled objects modules can be stored in a common library –Using “ar” command object modules can be grouped under a library with a common name Example: ar -r mather.lib add.o sub.o –These libraries can be used by other programs. One Library can be used by different programs

37 Copyright © 2005, Infosys Technologies Ltd ER/CORP/CRS/LA07/003 Version No. 1.0 Makefiles… # makefile to compile and execute three files # mainfile.c add.c and sub.c OBJECTS=mainfile.o final: $(OBJECTS) mather.lib gcc $(OBJECTS) mather.lib -o final mather.lib:add.o sub.o ar -r mather.lib add.o sub.o add.o: add.c common.h sub.o: sub.c common.h

38 Copyright © 2005, Infosys Technologies Ltd ER/CORP/CRS/LA07/003 Version No. 1.0 Thank You!