OS Libraries

Motivation

• By using libraries, you can achieve shorter development time and better reliability, maintainability, portability.
• Before writing a C function, check for a library function that already accomplishes the task.

Character handling

• Often you need to classify characters as numeric, alphabetic, etc. This classification depends on details of the underlying representation of the characters. For example, in ASCII:
  • 'a' is represented as 97
  • 'z' is represented as 122
  • 'A' is represented as 65
  • 'Z' is represented as 90
  This representation may change.
• So, writing code that uses the representation directly is
  • Hard to read
  • Non-portable
• Character handling functions allow you to classify characters in a readable and portable way. There are many functions available. See /usr/include/ctype.h for all the function prototypes.
• Example: classifying characters using the ASCII representation directly

  #include <stdio.h>
  #include <ctype.h>
  
  int main()
  {
  	int c;
  
  	c = getchar();
  	while (c != EOF) {
  		if (c >= 97 && c <= 122 || c >= 65 && c <= 90)
  			printf("alphabetic: %c\n",c);
  		else
  			printf("not alphabetic: %c\n",c);
  		c = getchar();
  	}
  		return 0;
  }
  

• Example: classifying characters using the ctype.h functions. Same as above except that the if condition is replaced by isalpha(c).
• Run both examples on a file containing:
  • Input file:

    	aA1!@
    	

  • Output:

    	alphabetic: a
    	alphabetic: A
    	not alphabetic: 1
    	not alphabetic: !
    	not alphabetic: @
    	not alphabetic: 
    	

String handling

• Programmers must often manipulate strings in a variety of ways.
• Writing correct and efficient string handling routines is surprizingly difficult.
• As a result, string handling routines have long been provided in a standard C library. Dozens of routines are available. See /usr/include/string.h for all the function prototypes.
• Example: copying strings safely

  	#include <stdio.h>
  	#include <string.h>
  
  	int main()
  	{
  		char s0[5],s1[100];
  		char *small = "abcd";
  		char *large = "abcdefghijklmnopqrstuvwxyz";
  
  		strcpy(s1,large);
  		strcpy(s0,small);
  		printf("s0:%s! s1:%s!\n",s0,s1);
  
  		strcpy(s0,large);
  		printf("s0:%s! s1:%s!\n",s0,s1);
  
  		strcpy(s1,large);
  
  		strncpy(s0,large,4);
  		s0[4] = '\0';
  		printf("s0:%s! s1:%s!\n",s0,s1);
  
  		return 0;
  	}
  	

• Output:

  	s0:abcd! s1:abcdefghijklmnopqrstuvwxyz!
  	s0:abcdefghijklmnopqrstuvwxyz! s1:ijklmnopqrstuvwxyz!
  	s0:abcd! s1:abcdefghijklmnopqrstuvwxyz!
  	

Date handling

• Dates and times are widely used in computing.
  • Pervasive in the financial industry:
    Age calculations for insurance
    Time stamps for electronic transfer
    Time differences for interest calculations
  • Heavily used in software development:
    Computer backups
    Make and make-like programs
  • Global positioning system
  • MANY other examples (time is money, literally)
• Typical questions
  1. Given a date, what day of the week is it?
  2. What is the date 90 days from now?
  3. Given two dates, what is the difference between them?
• The ANSI standard date/time library
  #include &time.h>
• Two kinds of time
  • Low-level time
    • Number of clock ticks since Jan. 1, 1900
    • Data type: time_t
  • High-level time
    • Conventional day/month/year
    • Data type: struct tm with fields
      tm_sec, tm_min, tm_hour, tm_mday, tm_mod, tm_year, etc.
    • The Perl localtime function uses struct tminternally.
• Functions
  • time_t mktime(struct tm*)
    Converts high-level time to low-level time.
    Calculates day of week from DD/MM/YY.
    Normalizes high-level dates in a useful way.
  • char *asctime(struct tm*)
    Converts a struct tm to string form.
  • double difftime(time_t,time_t)
    Calculates the difference, in seconds, between two low-level dates.

Example: What day of the week is Nov. 24 1995?

#include <stdio.h>
#include <time.h>

int main()
{
	struct tm high; /* "high-level" time: month, day, year, etc. */

	/* given a date, what day of the week is it? */
	high.tm_sec = 0;
	high.tm_min = 30;
	high.tm_hour = 13;
	high.tm_mday = 24;
	high.tm_mon = 10;
	high.tm_year = 95;
	mktime(&high); /* to compute day of week */
	printf("day of week: %d\n",high.tm_wday);
	printf("%s",asctime(&high)); /* human-readable form */

	return 0;
}

Example: What is the date 90 days from Nov. 24 1995?

#include <stdio.h>
#include <time.h>

int main()
{
	struct tm high; /* high-level" time: month, day, year, etc. */

	/* today's date */
	high.tm_sec = 0;
	high.tm_min = 30;
	high.tm_hour = 13;
	high.tm_mday = 24;
	high.tm_mon = 10;
	high.tm_year = 95;

	/* increment */
	high.tm_mday += 90;

	/* to normalize */
	mktime(&high);

	/* display */
	printf("%s",asctime(&high));

	return 0;
}

Example: How many days in the F96 term?

#include <stdio.h>
#include <time.h>

int main()
{
	struct tm high0,high1; /* high-level" time: month, day, year, etc. */
	time_t low0,low1; /* low-level time: ticks since Jan. 1, 1900 */
	int diff;

	/* 13:30 on first day of class F96 */
	high0.tm_sec = 0;
	high0.tm_min = 30;
	high0.tm_hour = 13;
	high0.tm_mday = 4;
	high0.tm_mon = 8;
	high0.tm_year = 96;

	/* 13:30 on last day of class F96 */
	high1.tm_sec = 0;
	high1.tm_min = 30;
	high1.tm_hour = 13;
	high1.tm_mday = 4;
	high1.tm_mon = 11;
	high1.tm_year = 96;

	/* convert to low-level times */
	low0 = mktime(&high0);
	low1 = mktime(&high1);

	/* calculate the difference in seconds */
	diff = difftime(low1,low0);

	/* convert to days */
	diff = diff / (24*60*60);

	/* display */
	printf("%d\n",diff);

	return 0;
}

Bottom line

• Learn to use the character, string, and date library functions.
• Learn to write demonstration programs like the examples above. It is the most efficient way to test your understanding.