получить время (get time)
Пролог (Prolog)
This manual page is part of the POSIX Programmer's Manual. The
Linux implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
Имя (Name)
time — get time
Синопсис (Synopsis)
#include <time.h>
time_t time(time_t *tloc);
Описание (Description)
The functionality described on this reference page is aligned
with the ISO C standard. Any conflict between the requirements
described here and the ISO C standard is unintentional. This
volume of POSIX.1‐2017 defers to the ISO C standard.
The time() function shall return the value of time in seconds
since the Epoch.
The tloc argument points to an area where the return value is
also stored. If tloc is a null pointer, no value is stored.
Возвращаемое значение (Return value)
Upon successful completion, time() shall return the value of
time. Otherwise, (time_t)-1 shall be returned.
Ошибки (Error)
The time() function may fail if:
EOVERFLOW
The number of seconds since the Epoch will not fit in an
object of type time_t.
The following sections are informative.
Примеры (Examples)
Getting the Current Time
The following example uses the time() function to calculate the
time elapsed, in seconds, since the Epoch, localtime() to convert
that value to a broken-down time, and asctime() to convert the
broken-down time values into a printable string.
#include <stdio.h>
#include <time.h>
int main(void)
{
time_t result;
result = time(NULL);
printf("%s%ju secs since the Epoch\n",
asctime(localtime(&result)),
(uintmax_t)result);
return(0);
}
This example writes the current time to stdout in a form like
this:
Wed Jun 26 10:32:15 1996
835810335 secs since the Epoch
Timing an Event
The following example gets the current time, prints it out in the
user's format, and prints the number of minutes to an event being
timed.
#include <time.h>
#include <stdio.h>
...
time_t now;
int minutes_to_event;
...
time(&now);
minutes_to_event = ...;
printf("The time is ");
puts(asctime(localtime(&now)));
printf("There are %d minutes to the event.\n",
minutes_to_event);
...
Использование в приложениях (Application usage)
None.
Обоснование (Rationale)
The time() function returns a value in seconds while
clock_gettime() and gettimeofday() return a struct timespec
(seconds and nanoseconds) and struct timeval (seconds and
microseconds), respectively, and are therefore capable of
returning more precise times. The times() function is also
capable of more precision than time() as it returns a value in
clock ticks, although it returns the elapsed time since an
arbitrary point such as system boot time, not since the epoch.
Implementations in which time_t is a 32-bit signed integer (many
historical implementations) fail in the year 2038. POSIX.1‐2008
does not address this problem. However, the use of the time_t
type is mandated in order to ease the eventual fix.
On some systems the time() function is implemented using a system
call that does not return an error condition in addition to the
return value. On these systems it is impossible to differentiate
between valid and invalid return values and hence overflow
conditions cannot be reliably detected.
The use of the <time.h> header instead of <sys/types.h> allows
compatibility with the ISO C standard.
Many historical implementations (including Version 7) and the
1984 /usr/group standard use long instead of time_t. This volume
of POSIX.1‐2017 uses the latter type in order to agree with the
ISO C standard.
Будущие направления (Future directions)
In a future version of this volume of POSIX.1‐2017, time_t is
likely to be required to be capable of representing times far in
the future. Whether this will be mandated as a 64-bit type or a
requirement that a specific date in the future be representable
(for example, 10000 AD) is not yet determined. Systems purchased
after the approval of this volume of POSIX.1‐2017 should be
evaluated to determine whether their lifetime will extend past
2038.
Смотри также (See also)
asctime(3p), clock(3p), clock_getres(3p), ctime(3p),
difftime(3p), futimens(3p), gettimeofday(3p), gmtime(3p),
localtime(3p), mktime(3p), strftime(3p), strptime(3p), times(3p),
utime(3p)
The Base Definitions volume of POSIX.1‐2017, time.h(0p)
