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  [Heiß] Dating c date 2025 (7 อ่าน)

22 ก.ค. 2568 17:59

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This tutorial uses C99. C Date Time. In this tutorial, we show how to work with date and time in C.


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In the tutorial, we use C99. If you are compiling programs on Windows, we highly recommend Pelles C compiler. Windows API tutorial has a chapter dedicated to date and time. C date time definitions. We start with a few definitions. A calendar time , also called an absolute time, is a point in the time continuum, for example February 17, 2016 at 13:02:5 CET. A time interval is a continuous part of the time continuum between two calendar times, for example the hour between 13:00 and 14:00 on February 20, 2000. An elapsed time is the length of an interval, for example, 28 minutes. An amount of time is a sum of elapsed times. The elapsed times do not need to be successive. When the work took us eleven hours, we might be working on different days. A period is the elapsed time of an interval between two events. CPU time is the amount of time for which a central processing unit (CPU) was used for processing instructions of a computer program or operating system. It is measured in clock ticks or seconds. An epoch is an instant in time chosen as the origin of a particular era. The Unix epoch is the time 00:00:00 UTC on 1 January 1970 (or 1970-01-01T00:00:00Z ISO 8601). Simple time is a compact representation of a calendar time, it is the number of seconds of elapsed time since Unix epoch. Simple time uses the time_t data type. Broken-down time represents a human-readable calendar time. It is divided into a set of components specifying the year, month, day, and so on in the Gregorian calendar, for a specific time zone. The broken-down time uses the struct tm data type. Wall time , also called real-world time or wall-clock time, refers to elapsed time as determined by a chronometer such as a wristwatch or wall clock. (The reference to a wall clock is how the term originally got its name.) Wall time differs from time as measured by counting microprocessor clock pulses or cycles. The time.h contains the following function prototypes: char *asctime(const struct tm *) — converts date and time to a string (obsolete) clock_t clock(void) — returns an approximation of the CPU time used by a process int clock_getres(clockid_t, struct timespec *) — returns the resolution of a clock int clock_gettime(clockid_t, struct timespec *) — retrieves the current time of the clock int clock_settime(clockid_t, const struct timespec *) — sets the current time of a clock specified char *ctime(const time_t *) — converts a time value to date and time string (obsolete) double difftime(time_t, time_t) — computes the difference between two calendar time values struct tm *getdate(const char *) — converts a string representation of a date or time into a broken-down time struct tm *gmtime(const time_t *) — converts a time value to a broken-down UTC time struct tm *localtime(const time_t *) — converts a time value to a broken-down local time time_t mktime(struct tm *) — converts a broken-down time into Unix time size_t strftime(char *, size_t, const char *, const struct tm *) — convert date and time to a string char *strptime(const char *, const char *, struct tm *) — converts a time string to a broken-down time time_t time(time_t *) — returns the Unix time, the number of seconds since the Unix epoch void tzset(void) — sets time zone conversion information. In addition, the time.h file defines the CLOCKS_PER_SEC macro, which holds the number of processor clock ticks per second. The clock_t is the process running time type. Unix time. The Unix time is the number of seconds since the Unix epoch. The time function returns the value of time in seconds since 0 hours, 0 minutes, 0 seconds, January 1, 1970, Coordinated Universal Time. If an error occurs, it returns -1. If t is not NULL , the return value is also stored in the memory pointed to by t . The example prints the Unix time. At this moment, 1674029496 have passed since Unix epoch. Broken-down time. Broken-down time is a human-readable version of the calendar time. The struct tm data type is used for broken-down time. The localtime functions converts a simple calendar time into a broken-down time. It takes the current timezone into account. It stores a tm structure and returns a pointer to that structure. The following description of the struct tm was taken from FreeBSD manual. The members of the tm structure are: tm_sec — the number of seconds after the minute, normally in the range 0 to 59, but can be up to 60 to allow for leap seconds. tm_min — the number of minutes after the hour, in the range 0 to 59. tm_hour — the number of hours past midnight, in the range 0 to 23. tm_mday — the day of the month, in the range 1 to 31. tm_mon — the number of months since January, in the range 0 to 11. tm_year — the number of years since 1900. tm_wday — the number of days since Sunday, in the range 0 to 6. tm_yday — the number of days since January 1, in the range 0 to 365. tm_isdst — a flag that indicates whether daylight saving time is in effect at the time described. The value is positive if daylight saving time is in effect, zero if it is not, and negative if the information is not available. In the example, we get the Unix time and convert it into the local broken-down time. We get the raw Unix time. The raw time, expressed in seconds, is converted into the brokend-down time. The functions fills a struct tm type and returns a pointer to it. The structure is statically allocated, so we do not have to free it. We use the tm_hour , tm_min , and tm_sec members to express a human-readable time format. Converting broken-down time to Unix time. The mktime function converts the broken-down time, expressed as local time, in the structure pointed to by tm into the Unix time (simple calendar time). The mktime function receives a pointer to the struct tm value and returns a time_t value. In the example, we calculate a Unix time for a specific time in the future. This is a time value for the beginning of a XMas in 2016. With the mktime function, we convert the broken-down time into the Unix time. The 1482534000 is the Unix time for the beginning of the XMas in 2016. UTC time. Our planet is a sphere, it revolves round its axis. The Earth rotates towards the east, so the Sun rises at different times in different locations. The Earth rotates once in about 24 hours. Therefore, the world was divided into 24 time zones. In each time zone, there is a different local time. This local time is often further modified by the daylight saving. There is a pragmatic need for one global time. One global time helps to avoid confusion about time zones and daylight saving time. The UTC (Universal Coordinated time) was chosen to be the primary time standard. UTC is used in aviation, weather forecasts, flight plans, air traffic control clearances, and maps. Unlike local time, UTC does not change with a change of seasons. The gmtime converts the time_t value into the broken-down time, not adjusting to the timezone. The example prints the current UTC time. We get the UTC time with the gmtime function. We print the UTC time. The asctime function is obsolete, we should use strftime , which is covered next, instead. For the CET time zone, there is one hour difference in time. Formatting time. The strftime function formats date and time. The strftime function formats the broken-down time tm according to the format specification format and places the result in the character array s of size max . List of date and time format specifiers Specifier Meaning Example %% The % character.













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