The mktime() function is a part of the standard C library and is declared in the <time.h> header file. It is used to convert a struct tm representation of a time to a time_t representation, which represents the number of seconds since the Epoch.

The function signature is:

time_t mktime(struct tm *timeptr);

The mktime() function takes a pointer to a struct tm that contains the broken-down time representation. It returns a time_t value that represents the number of seconds since the Epoch, calculated from the given struct tm.

Here is an example usage of mktime():

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

int main() {
    struct tm time_info = {
        .tm_sec = 0,
        .tm_min = 0,
        .tm_hour = 12,
        .tm_mday = 1,
        .tm_mon = 0,
        .tm_year = 121,
        .tm_isdst = -1
    };
    time_t time_value;

    // Convert the struct tm to a time_t
    time_value = mktime(&time_info);

    // Print the resulting time value
    printf("The time value is %ld.\n", (long)time_value);

    return 0;
}

In this example, a struct tm is initialized with a date and time of January 1st, 2021 at 12:00:00 PM. mktime() is then used to convert the struct tm to a time_t value. The resulting time_t value is printed to the console.

Note that mktime() performs normalization on the struct tm input, adjusting fields such as tm_hour, tm_min, and tm_sec if necessary to ensure that the resulting time_t value is within the range of values representable by time_t. The tm_isdst field can be set to -1 to indicate that the function should determine whether Daylight Saving Time is in effect for the given time.