C asctime() Function – time.h

asctime() Function

The asctime() function in C time.h converts a broken-down time, represented by a struct tm, into a human-readable string that displays the date and time in a standardized format.

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Syntax of asctime()

char* asctime(const struct tm *timeptr);

Parameters

Parameter	Description
timeptr	A pointer to a struct tm containing the calendar time information.

The function interprets the contents of the provided time structure and converts it into a string of the format "Www Mmm dd hh:mm:ss yyyy". The resulting string is stored in an internal static buffer and is terminated with a newline character and a null-terminator. Keep in mind that subsequent calls to asctime() may overwrite the content of this buffer.

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Return Value

The function returns a pointer to a C-string containing the formatted date and time. This string is stored in a static internal buffer that may be altered by later calls to asctime() or related functions.

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Examples for asctime()

Example 1: Displaying the Current Local Time

This example shows how to obtain the current local time, convert it to a struct tm using localtime(), and then display the formatted string using asctime().

Program

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

int main() {
    time_t currentTime = time(NULL);
    struct tm *localTime = localtime(&currentTime);

    printf("Current local time: %s", asctime(localTime));
    return 0;
}

Explanation:

1. The time() function retrieves the current time.
2. localtime() converts the time to a struct tm representing local time.
3. asctime() converts this struct tm to a formatted string.
4. The resulting string is printed to display the current local time.

Program Output:

Current local time: Sun Feb 23 04:59:21 2025

Example 2: Converting a Custom Time Structure

This example demonstrates how to manually set a struct tm with specific date and time values and then convert it to a human-readable string using asctime().

Program

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

int main() {
    struct tm customTime;
    customTime.tm_year = 2023 - 1900;  /* Years since 1900 */
    customTime.tm_mon  = 7;            /* August (0-based index: 7 means August) */
    customTime.tm_mday = 15;
    customTime.tm_hour = 14;
    customTime.tm_min  = 30;
    customTime.tm_sec  = 45;
    customTime.tm_wday = 2;            /* Tuesday (0 = Sunday, 1 = Monday, 2 = Tuesday) */
    customTime.tm_yday = 0;            /* Not used by asctime() */
    customTime.tm_isdst = -1;          /* Not used by asctime() */

    printf("Custom time: %s", asctime(&customTime));
    return 0;
}

Explanation:

1. A struct tm named customTime is manually initialized with specific date and time values.
2. The year is set by subtracting 1900, as required by the struct tm structure.
3. asctime() converts the structure into a formatted string.
4. The resulting string is printed to display the custom date and time.

Program Output:

Custom time: Tue Aug 15 14:30:45 2023

Example 3: Demonstrating the Static Buffer Behavior

This example illustrates that the string returned by asctime() is stored in a static buffer, which may be overwritten by subsequent calls to the function.

Program

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

int main() {
    time_t currentTime = time(NULL);
    struct tm *tmInfo = localtime(&currentTime);

    char *firstCall = asctime(tmInfo);
    char *secondCall = asctime(tmInfo);

    printf("First call: %s", firstCall);
    printf("Second call: %s", secondCall);
    return 0;
}

Explanation:

1. The current local time is obtained and converted to a struct tm using localtime().
2. asctime() is called twice with the same struct tm, returning pointers to the same internal static buffer.
3. The output of both calls is identical, demonstrating that the buffer is reused and may be overwritten by subsequent calls.

Program Output:

First call: Sun Feb 23 04:59:38 2025
Second call: Sun Feb 23 04:59:38 2025