How to get the current time using time(NULL) function and localtime() in Linux

time(); function

Function prototype: time_t time(time_t *timer)
Function purpose: Get the machine's calendar time or set the calendar time Header file: time.h
Input parameters: When timer=NULL, the machine calendar time is obtained; when time=time value, it is used to set the calendar time;
time_t is a long type

/* time - Get the current calendar time of the computer system
 * Functions that process date and time are calculated based on the return value of this function*
 * Function prototype:
 * #include <time.h>
 * 
 * time_t time(time_t *calptr);
 *
 * Return value:
 * Success: Number of seconds since 1970-1-1, 00:00:00
 *
 * use:
 * time_t now;
 * 
 * time(&now); // == now = time(NULL);
 */

localtime(); function

Function prototype: struct tm *localtime(const time_t *timer)
Function purpose: Returns machine time information expressed in a tm structure Header file: time.h
Input parameters: timer: machine time obtained using the time() function;

/*
 * localtime - converts a time value to local time, taking into account the local time zone and daylight saving time flags*
 * Function declaration:
 * #include <time.h>
 *
 * struct tm * localtime(const time_t *timer);
 *
 */

//The definition of structure tm is: 
 struct tm 
 { 
   int tm_sec; /* Seconds: 0-59 (K&R says 0-61?) */ 
   int tm_min; /* Minutes: 0-59 */ 
   int tm_hour; /* Hours since midnight: 0-23 */ 
   int tm_mday; /* Day of the month: 1-31 */ 
   int tm_mon; /* Months *since* january: 0-11 */ 
   int tm_year; /* Years since 1900 */ 
   int tm_wday; /* Days since Sunday (0-6) */ 
   int tm_yday; /* Days since Jan. 1: 0-365 */ 
   int tm_isdst; /* +1 Daylight Savings Time, 0 No DST, 
    * -1 don't know */ 
 };

Since time_t is actually a long integer, what should we do if the number of seconds from a time point (usually 00:00:00 on January 1, 1970) to that time (i.e. calendar time) exceeds the range of numbers that can be represented by a long integer? For the value of the time_t data type, the time it represents cannot be later than 19:14:07 on January 18, 2038. In order to represent longer time, some compiler manufacturers introduced 64-bit or even longer integers to save calendar time. For example, Microsoft uses the __time64_t data type to save calendar time in Visual C++, and obtains calendar time through the _time64() function (instead of using the 32-bit word time() function). In this way, the data type can be used to save the time before 00:00:00 on January 1, 3001 (excluding this time point).

/*
* time();
* @author 李政<[email protected]>
*/

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

int main(int argc, char* argv[])
{ 
  struct tm *tp; 
  time_t t = time(NULL); 
  tp = localtime(&t);

  printf("%d/%d/%d\n",tp->tm_mon+1,tp->tm_mday,tp->tm_year+1900); 
  printf("%d:%d:%d\n",tp->tm_hour,tp->tm_min,tp->tm_sec); 

  return 0;
}

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