A Little C Primer/C Standard Utility Library & Time Library

From Wikibooks, open books for an open world
Jump to: navigation, search

The utility functions library features a grab-bag of functions. It requires the declaration:

   #include <stdlib.h>

Useful functions include:

   atof( <string> )     Convert numeric string to double value.
   atoi( <string> )     Convert numeric string to int value.
   atol( <string> )     Convert numeric string to long value.
   rand()               Generates pseudorandom integer.
   srand( <seed> )      Seed random-number generator -- "seed" is an "int".
   exit( <status> )     Exits program -- "status" is an "int".
   system( <string> )   Tells system to execute program given by "string".
   abs( n )             Absolute value of "int" argument.
   labs( n )            Absolute value of long-int argument.

The functions "atof()", "atoi()", and "atol()" will return 0 if they can't convert the string given them into a value.

The time and date library includes a wide variety of functions, some of them obscure and nonstandard. This library requires the declaration:

   #include <time.h>

The most essential function is "time()", which returns the number of seconds since midnight proleptic Coordinated Universal Time (UTC) of January 1, 1970, not counting leap seconds. It returns a value as "time_t" (a "long") as defined in the header file.

The following function uses "time()" to implement a program delay with resolution in seconds:

   /delay.c */

   #include <stdio.h>

   #include <time.h>

   void sleep( time_t delay );

   void main()
   {
     puts( "Delaying for 3 seconds." );
     sleep( 3 );
     puts( "Done!" );
   }

   void sleep( time_t delay )
   {
     time_t t0, t1;
     time( &t0 );
     do
     {
       time( &t1 );
     }
     while (( t1 - t0 ) < delay );
   }

The "ctime()" function converts the time value returned by "time()" into a time-and-date string. The following little program prints the current time and date:

   /time.c */

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

   void main()
   {
     time_t *t;
     time( t );
     puts( ctime( t ));
   }

This program prints a string of the form:

   Tue Dec 27 15:18:16 1994

further reading[edit]