Sams Teach Yourself C in 21 Days

Exploring the C Function Library 541

19

%z The locale’s time zone or abbreviation. If the time zone isn’t known, then this

would be blank.

%Z The time zone name if the information is available or blank if not.

%% A single percent sign %.

Calculating Time Differences

You can calculate the difference, in seconds, between two times with thedifftime()

macro, which subtracts two time_tvalues and returns the difference. The prototype is

double difftime(time_t later, time_t earlier);

This function subtracts earlierfromlaterand returns the difference, the number of

seconds between the two times. A common use of difftime()is to calculate elapsed

time, as demonstrated (along with other time operations) in Listing 19.2.

You can determine duration of a different sort using theclock()function, which returns

the amount of time that has passed since the program started execution, in 1/100-second

units. The prototype is

clock_t clock(void);

To determine the duration of some portion of a program, call clock()twice—before and

after the process occurs—and subtract the two return values.

Using the Time Functions ............................................................................

Listing 19.2 demonstrates how to use the C library time functions.

LISTING19.2 times.c. Using the C library time functions

1: /* Demonstrates the time functions. */

2:

3: #include <stdio.h>

4: #include <time.h>

5:

6: int main( void )

7: {

8: time_t start, finish, now;

9: struct tm *ptr;

10: char *c, buf1[80];

11: double duration;

12:

TABLE19.1 continued

Specifier What It’s Replaced By

INPUT

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