C Time Functions

1. Obtain the time:
Use the time () function to obtain the calendar time. Its prototype is time_t time (time_t * timer );

# Include "stdafx. H"
# Include "time. H"
# Include "stdio. H"
# Include "stdlib. H"

Int main (void)
{
Struct TM t; // defines the TM time structure, used to store data information in time format
Time_t t_of_day; // defines the time_t time structure.
T. tm_year = 2006-1900; // The calculation time is based on January 1, 1900.
T. tm_mon = 6; // assign values to struct members
T. tm_mday = 1;
T. tm_hour = 0;
T. tm_min = 0;
T. tm_sec = 1;
T. tm_isdst = 0;
T_of_day = mktime (& T );
// Use the mktime () function to convert the time expressed in the TM structure to the calendar time: time_t variable. The function prototype is as follows: time_t mktime (struct TM * timeptr); The ctime () function (the parameter is in the time_t structure) displays the time in a fixed format, the returned value is a char * string.
Return 0;
}

2. Storage of time is stored through two predefined structures:
1. The calendar time is represented by the time_t data type. The time expressed by time_t (calendar time) is from a time point (for example, January 1, 1970 00:00:00) the number of seconds. In time. H, we can also see that time_t is a long integer:

# Ifndef _ time_t_defined
Typedef long time_t;/* Time Value */
# DEFINE _ time_t_defined/* avoid repeatedly defining time_t */
# Endif
2. In Standard C/C ++, we can obtain the date and time through the TM structure. The definition of the TM structure in time. H is as follows:

Struct TM {
Int tm_sec;/* Second-value range: [0, 59] */
Int tm_min;/* minute-value range: [0, 59] */
Int tm_hour;/* hour-value range: [0, 23] */
Int tm_mday;/* date in a month-value range: [] */
Int tm_mon;/* month (from January 1, 0 represents January 1,)-value range: [] */
Int tm_year;/* year, whose value is equal to the actual year minus 1900 */
Int tm_wday;/* day-value range: [0, 6], where 0 indicates Sunday, 1 indicates Monday, and so on */
Int tm_yday;/* Number of days from January 1, 0,365 each year-value range: [1,]. 0 indicates January 1, and so on */
Int tm_isdst;/* identifier of the sequence. When the sequence is executed, the sequence is positive. When the sequence is not implemented, the tm_isdst value is 0. If the sequence is unknown, the tm_isdst () value is negative. */
};

3. display of time:
Time. the H header file provides the asctime () function (the parameter is a TM structure pointer) and ctime () function (the parameter is a time_t structure) to display the time in a fixed format, both return values are char * strings. The returned time format is: day of the week, month, hour, minute, second, year, N, 0, time. H also provides two different functions to convert the calendar time (an integer expressed in time_t) to the time format that we usually see separately displaying year, month, day, hour, minute, and second. TM:
Struct TM * gmtime (const time_t * timer );
The gmtime () function converts the calendar time to the world standard time (Greenwich Mean Time), and returns a TM struct to save the time.
Struct TM * localtime (const time_t * timer); The localtime () function converts the calendar time to the local time.
# Include <stdafx. h>
# Include <time. h>
# Include <stdio. h>
# Include <stdlib. h>

Int main (void)
{
Struct TM * local, * PTR; // defines the storage time information of the TM structure pointer.
Time_t t; // time structure or object
T = time (null); // obtain the calendar time of the Current System
// Use the time () function to obtain the calendar time ),
// Its prototype is time_t time (time_t * timer );
Local = localtime (& T); // The localtime () function converts the calendar time to the local time.
Printf ("Local hour is: % d/N", local-> tm_hour); // time member of the output TM struct
Printf ("UTC hour is: % d/N", local-> tm_hour );
// Local = gmtime (& T );
// The gmtime () function converts the calendar time to the world standard time (Greenwich Mean Time ),
// Return a TM struct to save the time
PTR = gmtime (& T); // converts the calendar time to the world standard time.
Printf ("the UTC time is % s/n", asctime (PTR); // format the output World Standard Time
Printf ("The local time is % s/n", ctime (& T); // output local time
/* The asctime () function (the parameter is a TM structure pointer) and the ctime () function (the parameter is a time_t structure) display the time in a fixed format, both return values are char * strings. The returned time format is: day of the week, month, hour, minute, second, year, N, 0 */
Return 0;
}

4. Time difference calculation:
Function used: the timing function in C/C ++ is clock (), and the related data type is clock_t. The clock function is defined as follows in msdn:
Clock_t clock (void); the number of CPU clock units (clock tick) returned by the function from "enabling this program process" to "calling the clock () function in the program, clock_t is a data type with a long integer and storage time. In the time. h file, a constant clocks_per_sec is also defined to indicate the number of clock units in one second. Its definition is as follows:
# Define clocks_per_sec (clock_t) 1000)
Every 1‰ seconds (1 millisecond), the value returned by calling the clock () function is increased by 1, and the length of the clock timing unit is 1 millisecond, so the timing accuracy is also 1 millisecond, can we change the definition of clocks_per_sec to a greater value, so that the timing accuracy is higher? This is not acceptable. In standard C/C ++, the minimum unit of time is one millisecond. Double difftime (time_t time1, time_t time0); this function is used to calculate the time difference.

# Include "stdafx. H"
# Include "time. H"
# Include "stdio. H"
# Include "stdlib. H"
Int main (void)
{
Time_t c_start, t_start, c_end, t_end;
C_start = clock ();
T_start = Time (null );
System ("pause ");
C_end = clock ();
T_end = Time (null );
Printf ("the pause used % f ms by time ()./N", difftime (c_end, c_start ));
Printf ("the pause used % f s by clock ()./N", difftime (t_end, t_start ));
System ("pause ");
Return 0;
}
5. Other uses of time
Used as the seed of a random number. Since time is actually a long integer of the double type, it is obtained through the time (null) function as srand (Time (null )) is better.

# Include "stdafx. H"
# Include "time. H"
# Include "stdio. H"
# Include "stdlib. H"
Int main (void)
{
Srand (Time (null ));
// Set the seed. If this function is commented out, the random number 10 is the same for each running program.
For (INT I = 0; I <100; I ++)
{
Printf ("% d/T", Rand ());
}
System ("pause ");
Return 0;
}