CSCI 171 Presentation 8 Builtin Functions Preprocessor Directives

CSCI 171 Presentation 8 Built-in Functions, Preprocessor Directives, and Macros

Built - In Functions • C provides many built in functions • stdio. h printf scanf • math. h pow cos sin • For complete list, consult ANSI guide

System functions • All within stdlib. h file (must be included) • exit( ) – terminates execution • atexit( ) – performs functions at program termination • system( ) – executes operating system commands

exit ( ) function • Terminates program execution #include <stdio. h> #include <stdlib. h> void main ( ) { char i; exit(0); printf("Enter a character"); scanf("%c", &i); } //These statements will //not be executed

exit ( ) function continued • If 0 is passed into function it means program executed normally • If a nonzero value is passed into function it means program abnormally terminated (used as error code) • stdlib. h has two symbolic constanst: – #define EXIT_SUCCESS 0 – #define EXIT_FAILURE 1 • can call exit(EXIT_SUCCESS) • can call exit(EXIT_FAILURE)

exit ( ) function continued • exit( ) should only be used for abnormal termination • Normal termination should occur after the last line of main is executed

Structured use of exit() #include <stdio. h> #include <stdlib. h> int main(void) { float * x = NULL; x = (float *)malloc(16 * sizeof(float)); if (x == NULL) { printf("nn***** Memory allocation error *****"); exit(1); } //Additional program code would be put here return 0; }

Unstructured use of exit() #include <stdio. h> #include <stdlib. h> void main(void) { int option = 0; printf("1. Find length"); printf("n 2. Find volume"); printf("n 3. Find area"); printf("n. Please select an option: "); scanf("%d", &option); if (option == 1) { /*Code to find length goes here*/ } else if (option == 2) { /*Code to find volume goes here*/ } else if (option == 3) { /*Code to find area goes here*/ } else exit(0); //More code here }

atexit ( ) function • Specifies one (or more) functions that are automatically executed at termination time • Up to 32 functions can be registered in this way • Executed in reverse order

atexit( ) function continued #include <stdio. h> #include <stdlib. h> void cleanup(); void cleanup. Last(); void main ( ) { char i; atexit(cleanup. Last); atexit(cleanup); printf("Enter a character"); scanf("%c", &i); }

#include <stdio. h> #include <stdlib. h> Sample Program 8. 1 void message 2(); void message 1(); int main(void) { int number = 0; atexit(message 2); atexit(message 1); printf("n. Please enter the number 2: "); scanf("%d", &number); if (number != 2) exit(0); else printf("You entered the correct number!"); printf("n. You can follow directions!"); return 0; } void message 2() { printf("n. Please remember to logoff when you are done. "); } void message 1() { printf("nn. Thank you for using this program. "); }

system ( ) function • Executes operating system commands • Example: system(“dir c: \*. exe /s”); • Can execute any command line system(“c: \winnt\system 32\notepad. exe”);

#include <stdio. h> #include <stdlib. h> #include <ctype. h> Sample Program 8. 2 void main(void) { int option = 0; char keep. Going = 'Y'; do { printf("1. Run the Notepad application"); printf("n 2. Run the Calculator application"); printf("n 3. Display all executable files on the C drive"); printf("nn. Please select an option: "); scanf("%d", &option); if (option == 1) system("c: \windows\system 32\notepad. exe"); else if (option == 2) system("c: \windows\system 32\calc. exe"); else system("dir c: \*. exe /s"); printf("n. Do you want to run the program again (Y/N)? "); scanf(" %c", &keep. Going); } while (toupper(keep. Going) == 'Y');

Preprocessor • Part of all C compiler packages • First component that processes source code • Source code changed based on directives – all preprocessor directives begin with # • Output - modified source code file – used in next step of compilation – deleted automatically by system

#include • Imports other files into source code – maybe library functions (stdio. h) • use < > – may be user defined functions • use “ ” • may have more than one function in file

Advantages of #include • Structure • Portability • Conventions: – similar functions grouped in one file – file given descriptive name

main. c • Main. c is sometimes called the driver – ‘drives’ the flow of logic – often does not contain any function definitions – includes functions with #include preprocessor directive – #include “calculate. h”

#define • Used for substitution macros – substituting values for variables • Used for function macros – defining a function ‘on the fly’

#define - substitution macro • Creates substitution macro • #define PI 3. 14 area = radius * PI circumference = 2 * radius * PI • Changes in source code after precompiling area = radius * 3. 14 circumference = 2 * radius * 3. 14 • Space after constant indicates substitution macro

#define - function macro • Shorthand for a more complicated operation • Arguments not type sensitive • Ex: #define HALFOF(value) ((value)/2) printf(“%f”, HALFOF(x + y)); • Changes in source code after precompiling printf(“%f”, ((x+y)/2)); • No space after function name indicates function macro

Other function macro examples #define AVG 3(a, b, c) (((a) + (b) + (c)) / 3) #define SMALLER(x, y) ((x) < (y) ? (x) : (y)) #define SUM (x, y, z) ((x) + (y) + (z))

Common Errors-function macros • Spaces after function macro name – #define SUM (x, y, z) ((x) + (y) + (z)) • Forgetting parenthesis – #define AREA(x, y) x*y • All parameters must be used – #define SUM(x, y, z) ((x) + (y))

Sample Program 8. 3 #include <stdio. h> #define PRODUCT(x, y) x*y void main(void) { int a = 1, b = 2, c = 3, d = 4, answer = 0; answer = PRODUCT(a, b); printf("The product of %d and %d is: %d", a, b, answer); answer = PRODUCT(a+c, b+d); printf("nn. The product of %d and %d is: %d", a+c, b+d, answer); }

Macros vs. Functions • Macros can be used for simple functions • Size of program – Functions exist as a single copy – Macro expanded in code every time it is called • Execution efficiency – no overhead to use a macro – overhead required for functions

#if and #endif • Preprocessor directives controlling conditional compilation – if statement determines if statements executed – #if statement determines if statements compiled • #elif, #else work as else if, else

Where would #if be used • For purposes of CSCI 171 – building function libraries When including files – If file included more than once, code is imported for each time – out of memory – Use #if and ‘defined’ keywords to conditionally include statements for compilation – We will place all prototypes inside the appropriate. h file, and use these keywords – See ‘Process to Create. h and. c function files’ link

Function Libraries • Files associated with libraries – Header files (filename. h) • Include all function prototypes • Include all necessary preprocessor directives • Are included in the appropriate files via the #include directive – Source code files (filename. c) • Include all function definitions • Include the corresponding header file via the #include directive • Are added to the project via the IDE (instead of being included) • Detailed instructions are posted on CSCI 171 web page

Example #if defined mathfn_h #else #define mathfn_h //Note: no periods can be used here int area. Of. Square(int); int area. Of. Rectangle(int, int); #endif

Not (!) is allowed #if !defined mathfn_h #define mathfn_h int area. Of. Square(int); int area. Of. Rectangle(int, int); #endif

#ifndef directive Most common and structured approach #ifndef mathfn_h #define mathfn_h int area. Of. Square(int); int area. Of. Rectangle(int, int); #endif

#undef • Opposite effect of #define PI 3. 14. . . #undef PI

Sample Program 8. 4 • View sample program 8. 4 • Structure the files according to the comments at the top of Program 8. 4