PROGRAMMING - Hacking: The Art of Exploitation - page 97

A simple example will suffice for now. When dealing with many time functions,
these functions use a time struct called tm , which is defined in /usr/include/
time.h. The struct's definition is as follows.
struct tm {
int tm_sec; /* seconds */
int tm_min; /* minutes */
int tm_hour; /* hours */
int tm_mday; /* day of the month */
int tm_mon; /* month */
int tm_year; /* year */
int tm_wday; /* day of the week */
int tm_yday; /* day in the year */
int tm_isdst; /* daylight saving time */
};
After this struct is defined, struct tm becomes a usable variable type, which
can be used to declare variables and pointers with the data type of the tm struct.
The time_example.c program demonstrates this. When time.h is included,
the tm struct is defined, which is later used to declare the current_time and
time_ptr variables.
time_example.c
#include <stdio.h>
#include <time.h>
int main() {
long int seconds_since_epoch;
struct tm current_time, *time_ptr;
int hour, minute, second, day, month, year;
seconds_since_epoch = time(0); // Pass time a null pointer as argument.
printf("time() - seconds since epoch: %ld\n", seconds_since_epoch);
time_ptr = &current_time; // Set time_ptr to the address of
// the current_time struct.
localtime_r(&seconds_since_epoch, time_ptr);
// Three different ways to access struct elements:
hour = current_time.tm_hour; // Direct access
minute = time_ptr->tm_min; // Access via pointer
second = *((int *) time_ptr); // Hacky pointer access
printf("Current time is: %02d:%02d:%02d\n", hour, minute, second);
}
The time() function will return the number of seconds since January 1,
1970. Time on Unix systems is kept relative to this rather arbitrary point in
time, which is also known as the epoch . The localtime_r() function expects two
pointers as arguments: one to the number of seconds since epoch and the
other to a tm struct. The pointer time_ptr has already been set to the address