Major operating system experiment progress (4) ----- how to obtain system time in C/C ++

// Solution-advantage: only the C standard library is used. disadvantage: The value can only be accurate to seconds. # include <time. h> # include <stdio. h> int main (void) {time_t = time (0); char tmp [64]; strftime (tmp, sizeof (tmp ), "% Y/% m/% d % X % A % j day % z this year", localtime (& t); puts (tmp); return 0 ;} size_t strftime (char * strDest, size_t maxsize, const char * format, const struct tm * timeptr); generate a string based on the format string. Struct tm * localtime (const time_t * timer); get the local time. the result obtained by localtime is returned by the structure tm. The returned string can be in the following format: % a abbreviation of the day of the week. Eg: Full name of Tue % A day of the week. Eg: Abbreviation of Tuesday % B month name. % B full name of the month name. % C the local date time is better than the string. % D indicates the day of the month (range: 00 to 31) with a number ). The date % H represents the hour in the 24-hour format (range: 00 to 23 ). % I represents the hour in 12-hour format (range: 01 to 12 ). % J indicates the day of the year (range: 001 to 366 ). The number of % m months (ranging from 1 to 12 ). % M minutes. % P uses ''am'' or ''pm ''to indicate the local time. % S seconds. % U indicates the week of the current year. The first week starts from the first Sunday. % W indicates the week of the current year. The first week starts from the first Monday. % W indicates the day of the week by number (0 indicates Sunday ). % X date representation without time. % X does not include the time representation of the date. Eg: 15:26:30% y two digits indicate the year (range: 00 to 99 ). % Y indicates the complete year number, that is, the four-digit number. Eg: 2008% Z (% z) Time Zone or abbreviation. Eg: Chinese Standard Time % characters. // Solution 2 advantages: accurate to milliseconds; disadvantage: windows API # include <windows. h> # include <stdio. h> int main (void) {SYSTEMTIME sys; GetLocalTime (& sys); printf ("% 4d/% 02d/% 02d % 02d: % 02d: % 02d. % 03d week % 1d/n ", sys. wYear, sys. wMonth, sys. wDay, sys. wHour, sys. wMinute, sys. wSecond, sys. wMilliseconds, sys. wDayOfWeek); return 0;} // method 3. Advantages: the system function can also be used to modify the system time. // the file must be a c ++ file # include <stdlib. h> # include <iostream> using namespace std; void m Ain () {system ("time") ;}// solution 4: Convert the current time to seconds, you can use the corresponding time conversion. // This file must be a c ++ file # include <iostream> # include <ctime> using namespace std; int main () {time_t now_time; now_time = time (NULL); cout <now_time; return 0;} 1. Obtain 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 the time Grid Format: time_t t_of_day; // defines the time_t time structure t. tm_year = 2006-1900; // calculate the time t based on January 1, 1900. tm_mon = 6; // assign a value to the struct 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 represented in the tm structure to a 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, which is stored through two predefined structures: 1. The Calendar Time is represented by the time_t data type, the time (calendar time) represented by time_t is the number of seconds from a time point (for example, January 1, 1970 00:00:00) to this time point. In time. h, we can also see that time_t is a long integer: # ifndef _ TIME_T_DEFINEDtypedef long time_t; /* Time Value */# define _ TIME_T_DEFINED/* avoid repeatedly defining time_t */# endif2. In Standard C/C ++, we can use the tm structure to obtain the date and time. The tm structure is in time. the definition in h is as follows: struct tm {int tm_sec;/* Second-value range: [0, 59] */int tm_min; /* minute-value range: [] */int tm_hour;/* Time-value range: [] */int tm_mday; /* date in a month-value range: [1, 31] */int tm_mon;/* month (starting from January 1, January, 0 indicates January 1, January) -The value range is [0, 11] */int tm_year;/* year The value is equal to the actual year minus 1900 */int tm_wday;/* the week-value range is [], where 0 represents Sunday, 1 represents Monday, and so on */int tm_yday; /* The number of days starting from January 1, 0,365-the value range is []. 0 indicates January 1, and so on. */int tm_isdst;/* indicates the identifier of the hour, when the sequence is executed, tm_isdst 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. 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's Standard Time (GMT ), 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 tm structure pointer to store the time information time_t ;// Time structure or object t = time (NULL); // obtain the current system Calendar Time // use the time () function to obtain the Calendar time ), // Its prototype is time_t time (time_t * timer); local = localtime (& t); // localtime () the function is to convert the calendar time to the Local time printf ("local hour is: % d/n", Local-> tm_hour ); // 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). // it returns a tm struct to save the time ptr = gmtime (& t ); // convert The calendar time to The world standard time printf ("The UTC time is % s/n", asctim E (ptr); // format and output The world standard time printf ("The local time is % s/n", ctime (& t )); // output the local time/* 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 Date Hour: minute: Second year/n/0 */return 0;} 4. Time Difference Calculation: function used: the timing function in C/C ++ is clock (), and the data type associated with it is clock_t. The clock function is defined as follows in MSDN: clock_t clock (void); the function returns from "enable this program process" to "program call clock () the number of clock units (clock tick) between functions. clock_t is a long integer and the Data Type of the storage time. In time. in the H file, a constant CLOCKS_PER_SEC is also defined to indicate the number of clock units in a 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 The 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 use of time As the seed of a random number, because the time is actually a long integer of the double type, 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); // sets the seed. If you comment out this function, for (int I = 0; I <100; I ++) {printf ("% d/t", rand ());} system ("pause"); return 0;