Overview #

The difftime() function calculates the difference between two calendar times.

The result is returned as a value of type double representing the elapsed time in seconds.

difftime() is commonly used to determine the duration between two events.

Header File #

#include <time.h>

Man Pages #

man 3 difftime

Function Prototype #

double difftime(time_t end, time_t beginning);

Parameters #

Return Value #

Returns the difference between end and beginning in seconds as a value of type double.

The returned value can be:

• Positive if end is later than beginning.
• Zero if both times are equal.
• Negative if end is earlier than beginning.

Examples #

Example 1 – Calculate Elapsed Time #

main.c #

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

int main(void)
{
    time_t start, end;
    double elapsed_time;

    start = time(NULL);

    sleep(5);

    end = time(NULL);

    elapsed_time = difftime(end, start);

    printf("Elapsed Time = %.0f seconds\n", elapsed_time);

    return 0;
}

Output #

Elapsed Time = 5 seconds

The output may vary slightly depending on your system.

Explanation #

start = time(NULL);

Stores the starting calendar time.

sleep(5);

Pauses the program for five seconds.

end = time(NULL);

Stores the ending calendar time.

elapsed_time = difftime(end, start);

Calculates the difference between the two calendar times.

The returned value is expressed in seconds.

Example 2 – Calculate the Difference Between Two Time Values #

main.c #

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

int main(void)
{
    time_t start = 100;
    time_t end = 250;

    printf("Difference = %.0f seconds\n",
           difftime(end, start));

    return 0;
}

Output #

Difference = 150 seconds

Explanation #

time_t start = 100;
time_t end = 250;

Creates two calendar time values.

difftime(end, start);

Returns the difference between the two values.

Since 250 - 100 = 150, the function returns 150.

Notes #

• difftime() returns the difference between two calendar times.
• The returned value is expressed in seconds.
• The return type is double.
• end should normally represent a later time than beginning.
• difftime() should be preferred over manually subtracting two time_t values because it provides a portable implementation regardless of how time_t is represented.

Related Functions #

• time()
• clock()
• clock_gettime()
• ctime()
• localtime()
• gmtime()