C difftime() Function – time.h

difftime() Function

The difftime() function in C time.h computes the difference in seconds between two time points. It calculates the elapsed time between an earlier and a later moment and returns the result as a floating-point number.

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Syntax of difftime()

double difftime(time_t end, time_t beginning);

Parameters

Parameter	Description
end	The later time point in the interval whose length is calculated.
beginning	The earlier time point in the interval. If this time is later than end, the result will be negative.

The difftime() function calculates the difference (end – beginning) in seconds and returns this value as a double. The type time_t is an arithmetic type that represents time values returned by the time() function.

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Return Value

The function returns the difference between the two time points in seconds as a double. A negative value indicates that the beginning time is later than the end time.

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Examples for difftime()

Example 1: Measuring Elapsed Time Using difftime()

This example demonstrates how to measure elapsed time by capturing the current time before and after a delay, then calculating the difference.

Program

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

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

    // Capture the start time
    start = time(NULL);

    // Simulate a delay using a loop
    for (volatile long i = 0; i < 100000000; i++);

    // Capture the end time after the delay
    end = time(NULL);

    // Calculate the elapsed time in seconds
    elapsed = difftime(end, start);

    printf("Elapsed time: %.0f seconds\n", elapsed);
    return 0;
}

Explanation:

1. The program captures the current time before a simulated delay.
2. A loop is used to create a time delay.
3. The current time is captured again after the delay.
4. difftime() computes the difference between the two captured times.
5. The elapsed time is printed to the console.

Program Output:

Elapsed time: 1 seconds

Example 2: Calculating a Fixed Time Interval

This example uses fixed Unix timestamp values to calculate the difference between two specific time points.

Program

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

int main(void) {
    // Fixed time points represented as Unix timestamps
    time_t beginning = 1609459200; // Jan 1, 2021 00:00:00 UTC
    time_t end = 1609545600;       // Jan 2, 2021 00:00:00 UTC

    double diff = difftime(end, beginning);

    printf("Difference: %.0f seconds\n", diff);
    return 0;
}

Explanation:

1. Two time points are defined using Unix timestamps.
2. difftime() calculates the difference between these two time points.
3. The result, which represents one day (86400 seconds), is printed.

Program Output:

Difference: 86400 seconds

Example 3: Handling Negative Time Differences

This example shows the behavior of difftime() when the beginning time is later than the end time, resulting in a negative value.

Program

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

int main(void) {
    // Fixed time points where the beginning is later than the end
    time_t beginning = 1609545600; // Jan 2, 2021 00:00:00 UTC
    time_t end = 1609459200;       // Jan 1, 2021 00:00:00 UTC

    double diff = difftime(end, beginning);

    printf("Difference: %.0f seconds\n", diff);
    return 0;
}

Explanation:

1. The time points are set such that the beginning time is later than the end time.
2. difftime() computes the difference, resulting in a negative value.
3. The negative result is printed, indicating an inverse time interval.

Program Output:

Difference: -86400 seconds