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* manual/sysinfo.texi: Fix typo. * manual/time.texi: Document timelocal, timegm, adjtimex, and stime. Replace "high precision time" with "high accuracy clock". Fix language. Patches by Bryan Henderson <bryanh@giraffe-data.com>.
2402 lines
86 KiB
Plaintext
2402 lines
86 KiB
Plaintext
@node Date and Time, Resource Usage And Limitation, Arithmetic, Top
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@c %MENU% Functions for getting the date and time and formatting them nicely
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@chapter Date and Time
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This chapter describes functions for manipulating dates and times,
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including functions for determining what the current time is and
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conversion between different time representations.
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The time functions fall into three main categories:
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@itemize @bullet
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@item
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Functions for measuring elapsed CPU time are discussed in @ref{Processor
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Time}.
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@item
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Functions for measuring absolute clock or calendar time are discussed in
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@ref{Calendar Time}.
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@item
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Functions for setting alarms and timers are discussed in @ref{Setting
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an Alarm}.
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@end itemize
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@menu
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* Processor Time:: Measures processor time used by a program.
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* Calendar Time:: Manipulation of ``real'' dates and times.
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* Setting an Alarm:: Sending a signal after a specified time.
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* Sleeping:: Waiting for a period of time.
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@end menu
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For functions to examine and control a process' CPU time, see
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@xref{Resource Usage And Limitation}.
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@node Processor Time
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@section Processor Time
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If you're trying to optimize your program or measure its efficiency, it's
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very useful to be able to know how much @dfn{processor time} or @dfn{CPU
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time} it has used at any given point. Processor time is different from
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actual wall clock time because it doesn't include any time spent waiting
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for I/O or when some other process is running. Processor time is
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represented by the data type @code{clock_t}, and is given as a number of
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@dfn{clock ticks} relative to an arbitrary base time marking the beginning
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of a single program invocation.
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@cindex CPU time
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@cindex processor time
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@cindex clock ticks
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@cindex ticks, clock
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@cindex time, elapsed CPU
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@menu
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* Basic CPU Time:: The @code{clock} function.
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* Detailed CPU Time:: The @code{times} function.
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@end menu
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@node Basic CPU Time
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@subsection Basic CPU Time Inquiry
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To get the elapsed CPU time used by a process, you can use the
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@code{clock} function. This facility is declared in the header file
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@file{time.h}.
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@pindex time.h
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In typical usage, you call the @code{clock} function at the beginning and
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end of the interval you want to time, subtract the values, and then divide
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by @code{CLOCKS_PER_SEC} (the number of clock ticks per second), like this:
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@smallexample
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@group
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#include <time.h>
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clock_t start, end;
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double elapsed;
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start = clock();
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@dots{} /* @r{Do the work.} */
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end = clock();
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elapsed = ((double) (end - start)) / CLOCKS_PER_SEC;
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@end group
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@end smallexample
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Different computers and operating systems vary wildly in how they keep
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track of processor time. It's common for the internal processor clock
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to have a resolution somewhere between hundredth and millionth of a
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second.
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In the GNU system, @code{clock_t} is equivalent to @code{long int} and
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@code{CLOCKS_PER_SEC} is an integer value. But in other systems, both
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@code{clock_t} and the type of the macro @code{CLOCKS_PER_SEC} can be
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either integer or floating-point types. Casting processor time values
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to @code{double}, as in the example above, makes sure that operations
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such as arithmetic and printing work properly and consistently no matter
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what the underlying representation is.
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Note that the clock can wrap around. On a 32bit system with
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@code{CLOCKS_PER_SEC} set to one million this function will return the
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same value approximately every 72 minutes.
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@comment time.h
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@comment ISO
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@deftypevr Macro int CLOCKS_PER_SEC
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The value of this macro is the number of clock ticks per second measured
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by the @code{clock} function. POSIX requires that this value is one
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million independent of the actual resolution.
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@end deftypevr
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@comment time.h
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@comment POSIX.1
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@deftypevr Macro int CLK_TCK
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This is an obsolete name for @code{CLOCKS_PER_SEC}.
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@end deftypevr
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@comment time.h
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@comment ISO
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@deftp {Data Type} clock_t
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This is the type of the value returned by the @code{clock} function.
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Values of type @code{clock_t} are in units of clock ticks.
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@end deftp
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@comment time.h
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@comment ISO
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@deftypefun clock_t clock (void)
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This function returns the elapsed processor time. The base time is
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arbitrary but doesn't change within a single process. If the processor
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time is not available or cannot be represented, @code{clock} returns the
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value @code{(clock_t)(-1)}.
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@end deftypefun
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@node Detailed CPU Time
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@subsection Detailed Elapsed CPU Time Inquiry
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The @code{times} function returns more detailed information about
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elapsed processor time in a @w{@code{struct tms}} object. You should
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include the header file @file{sys/times.h} to use this facility.
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@pindex sys/times.h
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@comment sys/times.h
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@comment POSIX.1
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@deftp {Data Type} {struct tms}
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The @code{tms} structure is used to return information about process
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times. It contains at least the following members:
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@table @code
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@item clock_t tms_utime
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This is the CPU time used in executing the instructions of the calling
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process.
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@item clock_t tms_stime
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This is the CPU time used by the system on behalf of the calling process.
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@item clock_t tms_cutime
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This is the sum of the @code{tms_utime} values and the @code{tms_cutime}
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values of all terminated child processes of the calling process, whose
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status has been reported to the parent process by @code{wait} or
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@code{waitpid}; see @ref{Process Completion}. In other words, it
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represents the total CPU time used in executing the instructions of all
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the terminated child processes of the calling process, excluding child
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processes which have not yet been reported by @code{wait} or
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@code{waitpid}.
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@item clock_t tms_cstime
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This is similar to @code{tms_cutime}, but represents the total CPU time
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used by the system on behalf of all the terminated child processes of the
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calling process.
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@end table
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All of the times are given in clock ticks. These are absolute values; in a
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newly created process, they are all zero. @xref{Creating a Process}.
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@end deftp
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@comment sys/times.h
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@comment POSIX.1
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@deftypefun clock_t times (struct tms *@var{buffer})
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The @code{times} function stores the processor time information for
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the calling process in @var{buffer}.
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The return value is the same as the value of @code{clock()}: the elapsed
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real time relative to an arbitrary base. The base is a constant within a
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particular process, and typically represents the time since system
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start-up. A value of @code{(clock_t)(-1)} is returned to indicate failure.
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@end deftypefun
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@strong{Portability Note:} The @code{clock} function described in
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@ref{Basic CPU Time}, is specified by the @w{ISO C} standard. The
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@code{times} function is a feature of POSIX.1. In the GNU system, the
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value returned by the @code{clock} function is equivalent to the sum of
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the @code{tms_utime} and @code{tms_stime} fields returned by
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@code{times}.
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@node Calendar Time
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@section Calendar Time
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This section describes facilities for keeping track of points in time.
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This is called calendar time and sometimes absolute time.
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@cindex time, absolute
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@cindex time, calendar
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@cindex date and time
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The GNU C library represents calendar time three ways:
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@itemize @bullet
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@item
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@dfn{Simple time} (the @code{time_t} data type) is a compact
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representation, typically giving the number of seconds elapsed since
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some implementation-specific base time.
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@cindex simple time
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@item
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There is also a @dfn{high-resolution time} representation (the @code{struct
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timeval} data type) that includes fractions of a second. Use this time
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representation instead of simple time when you need greater
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precision.
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@cindex high-resolution time
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@item
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@dfn{Local time} or @dfn{broken-down time} (the @code{struct tm} data
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type) represents the date and time as a set of components specifying the
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year, month, and so on in the Gregorian calendar, for a specific time
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zone. This time representation is usually used only to communicate with
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people.
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@cindex local time
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@cindex broken-down time
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@cindex Gregorian calendar
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@cindex calendar, Gregorian
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@end itemize
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@menu
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* Simple Calendar Time:: Facilities for manipulating calendar time.
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* High-Resolution Calendar:: A time representation with greater precision.
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* Broken-down Time:: Facilities for manipulating local time.
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* High Accuracy Clock:: Maintaining a high accuracy system clock.
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* Formatting Date and Time:: Converting times to strings.
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* Parsing Date and Time:: Convert textual time and date information back
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into broken-down time values.
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* TZ Variable:: How users specify the time zone.
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* Time Zone Functions:: Functions to examine or specify the time zone.
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* Time Functions Example:: An example program showing use of some of
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the time functions.
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@end menu
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@node Simple Calendar Time
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@subsection Simple Calendar Time
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This section describes the @code{time_t} data type for representing calendar
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time as simple time, and the functions which operate on simple time objects.
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These facilities are declared in the header file @file{time.h}.
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@pindex time.h
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@cindex epoch
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@comment time.h
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@comment ISO
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@deftp {Data Type} time_t
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This is the data type used to represent simple time. Sometimes, it also
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represents an elapsed time.
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When interpreted as an absolute time
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value, it represents the number of seconds elapsed since 00:00:00 on
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January 1, 1970, Coordinated Universal Time. (This time is sometimes
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referred to as the @dfn{epoch}.) POSIX requires that this count
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not include leap seconds, but on some hosts this count includes leap seconds
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if you set @code{TZ} to certain values (@pxref{TZ Variable}).
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Note that a simple time has no concept of local time zone. Time @var{N}
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is the same instant in time regardless of where on the globe the
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computer is.
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In the GNU C library, @code{time_t} is equivalent to @code{long int}.
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In other systems, @code{time_t} might be either an integer or
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floating-point type.
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@end deftp
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@comment time.h
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@comment ISO
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@deftypefun double difftime (time_t @var{time1}, time_t @var{time0})
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The @code{difftime} function returns the number of seconds elapsed
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between time @var{time1} and time @var{time0}, as a value of type
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@code{double}. The difference ignores leap seconds unless leap
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second support is enabled.
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In the GNU system, you can simply subtract @code{time_t} values. But on
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other systems, the @code{time_t} data type might use some other encoding
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where subtraction doesn't work directly.
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@end deftypefun
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@comment time.h
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@comment ISO
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@deftypefun time_t time (time_t *@var{result})
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The @code{time} function returns the current time as a value of type
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@code{time_t}. If the argument @var{result} is not a null pointer, the
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time value is also stored in @code{*@var{result}}. If the calendar
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time is not available, the value @w{@code{(time_t)(-1)}} is returned.
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@end deftypefun
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@c The GNU C library implements stime() with a call to settimeofday() on
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@c Linux.
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@comment time.h
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@comment SVID, XPG
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@deftypefun int stime (time_t *@var{newtime})
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@code{stime} sets the system clock, i.e. it tells the system that the
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present absolute time is @var{newtime}, where @code{newtime} is
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interpreted as described in the above definition of @code{time_t}.
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@code{settimeofday} is a newer function which sets the system clock to
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better than one second precision. @code{settimeofday} is generally a
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better choice than @code{stime}. @xref{High-Resolution Calendar}.
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Only the superuser can set the system clock.
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If the function succeeds, the return value is zero. Otherwise, it is
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@code{-1} and @code{errno} is set accordingly:
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@table @code
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@item EPERM
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The process is not superuser.
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@end table
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@end deftypefun
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@node High-Resolution Calendar
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@subsection High-Resolution Calendar
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The @code{time_t} data type used to represent simple times has a
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resolution of only one second. Some applications need more precision.
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So, the GNU C library also contains functions which are capable of
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representing calendar times to a higher resolution than one second. The
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functions and the associated data types described in this section are
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declared in @file{sys/time.h}.
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@pindex sys/time.h
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@comment sys/time.h
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@comment BSD
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@deftp {Data Type} {struct timeval}
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The @code{struct timeval} structure represents a calendar time. It
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has the following members:
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@table @code
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@item long int tv_sec
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This represents the number of seconds since the epoch. It is equivalent
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to a normal @code{time_t} value.
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@item long int tv_usec
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This is the fractional second value, represented as the number of
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microseconds.
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Some times struct timeval values are used for time intervals. Then the
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@code{tv_sec} member is the number of seconds in the interval, and
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@code{tv_usec} is the number of additional microseconds.
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@end table
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@end deftp
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@comment sys/time.h
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@comment BSD
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@deftp {Data Type} {struct timezone}
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The @code{struct timezone} structure is used to hold minimal information
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about the local time zone. It has the following members:
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@table @code
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@item int tz_minuteswest
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This is the number of minutes west of UTC.
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@item int tz_dsttime
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If nonzero, Daylight Saving Time applies during some part of the year.
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@end table
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The @code{struct timezone} type is obsolete and should never be used.
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Instead, use the facilities described in @ref{Time Zone Functions}.
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@end deftp
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It is often necessary to subtract two values of type @w{@code{struct
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timeval}}. Here is the best way to do this. It works even on some
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peculiar operating systems where the @code{tv_sec} member has an
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unsigned type.
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@smallexample
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/* @r{Subtract the `struct timeval' values X and Y,}
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@r{storing the result in RESULT.}
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@r{Return 1 if the difference is negative, otherwise 0.} */
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int
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timeval_subtract (result, x, y)
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struct timeval *result, *x, *y;
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@{
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/* @r{Perform the carry for the later subtraction by updating @var{y}.} */
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if (x->tv_usec < y->tv_usec) @{
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int nsec = (y->tv_usec - x->tv_usec) / 1000000 + 1;
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y->tv_usec -= 1000000 * nsec;
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y->tv_sec += nsec;
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@}
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if (x->tv_usec - y->tv_usec > 1000000) @{
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int nsec = (x->tv_usec - y->tv_usec) / 1000000;
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y->tv_usec += 1000000 * nsec;
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y->tv_sec -= nsec;
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@}
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/* @r{Compute the time remaining to wait.}
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@r{@code{tv_usec} is certainly positive.} */
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result->tv_sec = x->tv_sec - y->tv_sec;
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result->tv_usec = x->tv_usec - y->tv_usec;
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/* @r{Return 1 if result is negative.} */
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return x->tv_sec < y->tv_sec;
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@}
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@end smallexample
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@comment sys/time.h
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@comment BSD
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@deftypefun int gettimeofday (struct timeval *@var{tp}, struct timezone *@var{tzp})
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The @code{gettimeofday} function returns the current date and time in the
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@code{struct timeval} structure indicated by @var{tp}. Information about the
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time zone is returned in the structure pointed at @var{tzp}. If the @var{tzp}
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argument is a null pointer, time zone information is ignored.
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The return value is @code{0} on success and @code{-1} on failure. The
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following @code{errno} error condition is defined for this function:
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@table @code
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@item ENOSYS
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The operating system does not support getting time zone information, and
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@var{tzp} is not a null pointer. The GNU operating system does not
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support using @w{@code{struct timezone}} to represent time zone
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information; that is an obsolete feature of 4.3 BSD.
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Instead, use the facilities described in @ref{Time Zone Functions}.
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@end table
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@end deftypefun
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@comment sys/time.h
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@comment BSD
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@deftypefun int settimeofday (const struct timeval *@var{tp}, const struct timezone *@var{tzp})
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The @code{settimeofday} function sets the current date and time
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according to the arguments. As for @code{gettimeofday}, time zone
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information is ignored if @var{tzp} is a null pointer.
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You must be a privileged user in order to use @code{settimeofday}.
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Some kernels automatically set the system clock from some source such as
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a hardware clock when they start up. Others, including Linux, place the
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system clock in an ``invalid'' state (in which attempts to read the time
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fail). A call of @code{stime} removes the system clock from an invalid
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state, and system startup scripts typically run a program that calls
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@code{stime}.
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@code{settimeofday} causes a sudden jump forwards or backwards, which
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can cause a variety of problems in a system. Use @code{adjtime} (below)
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to make a smooth transition from one time to another by temporarily
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speeding up or slowing down the clock.
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With a Linux kernel, @code{adjtimex} does the same thing and can also
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make permanent changes to the speed of the system clock so it doesn't
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need to be corrected as often.
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The return value is @code{0} on success and @code{-1} on failure. The
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following @code{errno} error conditions are defined for this function:
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@table @code
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@item EPERM
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This process cannot set the time because it is not privileged.
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@item ENOSYS
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The operating system does not support setting time zone information, and
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@var{tzp} is not a null pointer.
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@end table
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@end deftypefun
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@c On Linux, GNU libc implements adjtime() as a call to adjtimex().
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@comment sys/time.h
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@comment BSD
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@deftypefun int adjtime (const struct timeval *@var{delta}, struct timeval *@var{olddelta})
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This function speeds up or slows down the system clock in order to make
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gradual adjustments in the current time. This ensures that the time
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reported by the system clock is always monotonically increasing, which
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might not happen if you simply set the current time.
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The @var{delta} argument specifies a relative adjustment to be made to
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the current time. If negative, the system clock is slowed down for a
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while until it has lost this much time. If positive, the system clock
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is speeded up for a while.
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|
|
If the @var{olddelta} argument is not a null pointer, the @code{adjtime}
|
|
function returns information about any previous time adjustment that
|
|
has not yet completed.
|
|
|
|
This function is typically used to synchronize the clocks of computers
|
|
in a local network. You must be a privileged user to use it.
|
|
|
|
With a Linux kernel, you can use the @code{adjtimex} function to
|
|
permanently change the clock speed.
|
|
|
|
The return value is @code{0} on success and @code{-1} on failure. The
|
|
following @code{errno} error condition is defined for this function:
|
|
|
|
@table @code
|
|
@item EPERM
|
|
You do not have privilege to set the time.
|
|
@end table
|
|
@end deftypefun
|
|
|
|
@strong{Portability Note:} The @code{gettimeofday}, @code{settimeofday},
|
|
and @code{adjtime} functions are derived from BSD.
|
|
|
|
|
|
Symbols for the following function are declared in @file{sys/timex.h}.
|
|
|
|
@comment sys/timex.h
|
|
@comment GNU
|
|
@deftypefun int adjtimex (struct timex *@var{timex})
|
|
|
|
@code{adjtimex} is functionally identical to @code{ntp_adjtime}.
|
|
@xref{High Accuracy Clock}.
|
|
|
|
This function is present only with a Linux kernel.
|
|
|
|
@end deftypefun
|
|
|
|
@node Broken-down Time
|
|
@subsection Broken-down Time
|
|
@cindex broken-down time
|
|
@cindex calendar time and broken-down time
|
|
|
|
Calendar time is represented as an amount of time since a fixed base
|
|
time. This is convenient for calculation, but has no relation to the
|
|
way people normally think of dates and times. By contrast,
|
|
@dfn{broken-down time} is a binary representation separated into year,
|
|
month, day, and so on. Broken-down time values are not useful for
|
|
calculations, but they are useful for printing human readable time.
|
|
|
|
A broken-down time value is always relative to a choice of time
|
|
zone, and it also indicates which time zone was used.
|
|
|
|
The symbols in this section are declared in the header file @file{time.h}.
|
|
|
|
@comment time.h
|
|
@comment ISO
|
|
@deftp {Data Type} {struct tm}
|
|
This is the data type used to represent a broken-down time. The structure
|
|
contains at least the following members, which can appear in any order:
|
|
|
|
@table @code
|
|
@item int tm_sec
|
|
This is the number of seconds after the minute, normally in the range
|
|
@code{0} through @code{59}. (The actual upper limit is @code{60}, to allow
|
|
for leap seconds if leap second support is available.)
|
|
@cindex leap second
|
|
|
|
@item int tm_min
|
|
This is the number of minutes after the hour, in the range @code{0} through
|
|
@code{59}.
|
|
|
|
@item int tm_hour
|
|
This is the number of hours past midnight, in the range @code{0} through
|
|
@code{23}.
|
|
|
|
@item int tm_mday
|
|
This is the day of the month, in the range @code{1} through @code{31}.
|
|
|
|
@item int tm_mon
|
|
This is the number of months since January, in the range @code{0} through
|
|
@code{11}.
|
|
|
|
@item int tm_year
|
|
This is the number of years since @code{1900}.
|
|
|
|
@item int tm_wday
|
|
This is the number of days since Sunday, in the range @code{0} through
|
|
@code{6}.
|
|
|
|
@item int tm_yday
|
|
This is the number of days since January 1, in the range @code{0} through
|
|
@code{365}.
|
|
|
|
@item int tm_isdst
|
|
@cindex Daylight Saving Time
|
|
@cindex summer time
|
|
This is a flag that indicates whether Daylight Saving Time is (or was, or
|
|
will be) 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.
|
|
|
|
@item long int tm_gmtoff
|
|
This field describes the time zone that was used to compute this
|
|
broken-down time value, including any adjustment for daylight saving; it
|
|
is the number of seconds that you must add to UTC to get local time.
|
|
You can also think of this as the number of seconds east of UTC. For
|
|
example, for U.S. Eastern Standard Time, the value is @code{-5*60*60}.
|
|
The @code{tm_gmtoff} field is derived from BSD and is a GNU library
|
|
extension; it is not visible in a strict @w{ISO C} environment.
|
|
|
|
@item const char *tm_zone
|
|
This field is the name for the time zone that was used to compute this
|
|
broken-down time value. Like @code{tm_gmtoff}, this field is a BSD and
|
|
GNU extension, and is not visible in a strict @w{ISO C} environment.
|
|
@end table
|
|
@end deftp
|
|
|
|
@comment time.h
|
|
@comment ISO
|
|
@deftypefun {struct tm *} localtime (const time_t *@var{time})
|
|
The @code{localtime} function converts the simple time pointed to by
|
|
@var{time} to broken-down time representation, expressed relative to the
|
|
user's specified time zone.
|
|
|
|
The return value is a pointer to a static broken-down time structure, which
|
|
might be overwritten by subsequent calls to @code{ctime}, @code{gmtime},
|
|
or @code{localtime}. (But no other library function overwrites the contents
|
|
of this object.)
|
|
|
|
The return value is the null pointer if @var{time} cannot be represented
|
|
as a broken-down time; typically this is because the year cannot fit into
|
|
an @code{int}.
|
|
|
|
Calling @code{localtime} has one other effect: it sets the variable
|
|
@code{tzname} with information about the current time zone. @xref{Time
|
|
Zone Functions}.
|
|
@end deftypefun
|
|
|
|
Using the @code{localtime} function is a big problem in multi-threaded
|
|
programs. The result is returned in a static buffer and this is used in
|
|
all threads. POSIX.1c introduced a variant of this function.
|
|
|
|
@comment time.h
|
|
@comment POSIX.1c
|
|
@deftypefun {struct tm *} localtime_r (const time_t *@var{time}, struct tm *@var{resultp})
|
|
The @code{localtime_r} function works just like the @code{localtime}
|
|
function. It takes a pointer to a variable containing a simple time
|
|
and converts it to the broken-down time format.
|
|
|
|
But the result is not placed in a static buffer. Instead it is placed
|
|
in the object of type @code{struct tm} to which the parameter
|
|
@var{resultp} points.
|
|
|
|
If the conversion is successful the function returns a pointer to the
|
|
object the result was written into, i.e., it returns @var{resultp}.
|
|
@end deftypefun
|
|
|
|
|
|
@comment time.h
|
|
@comment ISO
|
|
@deftypefun {struct tm *} gmtime (const time_t *@var{time})
|
|
This function is similar to @code{localtime}, except that the broken-down
|
|
time is expressed as Coordinated Universal Time (UTC) (formerly called
|
|
Greenwich Mean Time (GMT)) rather than relative to a local time zone.
|
|
|
|
@end deftypefun
|
|
|
|
As for the @code{localtime} function we have the problem that the result
|
|
is placed in a static variable. POSIX.1c also provides a replacement for
|
|
@code{gmtime}.
|
|
|
|
@comment time.h
|
|
@comment POSIX.1c
|
|
@deftypefun {struct tm *} gmtime_r (const time_t *@var{time}, struct tm *@var{resultp})
|
|
This function is similar to @code{localtime_r}, except that it converts
|
|
just like @code{gmtime} the given time as Coordinated Universal Time.
|
|
|
|
If the conversion is successful the function returns a pointer to the
|
|
object the result was written into, i.e., it returns @var{resultp}.
|
|
@end deftypefun
|
|
|
|
|
|
@comment time.h
|
|
@comment ISO
|
|
@deftypefun time_t mktime (struct tm *@var{brokentime})
|
|
The @code{mktime} function is used to convert a broken-down time structure
|
|
to a simple time representation. It also ``normalizes'' the contents of
|
|
the broken-down time structure, by filling in the day of week and day of
|
|
year based on the other date and time components.
|
|
|
|
The @code{mktime} function ignores the specified contents of the
|
|
@code{tm_wday} and @code{tm_yday} members of the broken-down time
|
|
structure. It uses the values of the other components to determine the
|
|
calendar time; it's permissible for these components to have
|
|
unnormalized values outside their normal ranges. The last thing that
|
|
@code{mktime} does is adjust the components of the @var{brokentime}
|
|
structure (including the @code{tm_wday} and @code{tm_yday}).
|
|
|
|
If the specified broken-down time cannot be represented as a simple time,
|
|
@code{mktime} returns a value of @code{(time_t)(-1)} and does not modify
|
|
the contents of @var{brokentime}.
|
|
|
|
Calling @code{mktime} also sets the variable @code{tzname} with
|
|
information about the current time zone. @xref{Time Zone Functions}.
|
|
@end deftypefun
|
|
|
|
@comment time.h
|
|
@comment ???
|
|
@deftypefun time_t timelocal (struct tm *@var{brokentime})
|
|
|
|
@code{timelocal} is functionally identical to @code{mktime}, but more
|
|
mnemonically named. Note that it is the inverse of the @code{localtime}
|
|
function.
|
|
|
|
@strong{Portability note:} @code{mktime} is essentially universally
|
|
available. @code{timelocal} is rather rare.
|
|
|
|
@end deftypefun
|
|
|
|
@comment time.h
|
|
@comment ???
|
|
@deftypefun time_t timegm (struct tm *@var{brokentime})
|
|
|
|
@code{timegm} is functionally identical to @code{mktime} except it
|
|
always takes the input values to be Coordinated Universal Time (UTC)
|
|
regardless of any local time zone setting.
|
|
|
|
Note that @code{timegm} is the inverse of @code{gmtime}.
|
|
|
|
@strong{Portability note:} @code{mktime} is essentially universally
|
|
available. @code{timegm} is rather rare. For the most portable
|
|
conversion from a UTC broken-down time to a simple time, set
|
|
the @code{TZ} environment variable to UTC, call @code{mktime}, then set
|
|
@code{TZ} back.
|
|
|
|
@end deftypefun
|
|
|
|
|
|
|
|
@node High Accuracy Clock
|
|
@subsection High Accuracy Clock
|
|
|
|
@cindex time, high precision
|
|
@cindex clock, high accuracy
|
|
@pindex sys/timex.h
|
|
@c On Linux, GNU libc implements ntp_gettime() and npt_adjtime() as calls
|
|
@c to adjtimex().
|
|
The @code{ntp_gettime} and @code{ntp_adjtime} functions provide an
|
|
interface to monitor and manipulate the system clock to maintain high
|
|
accuracy time. For example, you can fine tune the speed of the clock
|
|
or synchronize it with another time source.
|
|
|
|
A typical use of these functions is by a server implementing the Network
|
|
Time Protocol to synchronize the clocks of multiple systems and high
|
|
precision clocks.
|
|
|
|
These functions are declared in @file{sys/timex.h}.
|
|
|
|
@tindex struct ntptimeval
|
|
@deftp {Data Type} {struct ntptimeval}
|
|
This structure is used to monitor kernel time. It contains the
|
|
following members:
|
|
@table @code
|
|
@item struct timeval time
|
|
This is the current time. The @code{struct timeval} data type is
|
|
described in @ref{High-Resolution Calendar}.
|
|
|
|
@item long int maxerror
|
|
This is the maximum error, measured in microseconds. Unless updated
|
|
via @code{ntp_adjtime} periodically, this value will reach some
|
|
platform-specific maximum value.
|
|
|
|
@item long int esterror
|
|
This is the estimated error, measured in microseconds. This value can
|
|
be set by @code{ntp_adjtime} to indicate the estimated offset of the
|
|
local clock against the true time.
|
|
@end table
|
|
@end deftp
|
|
|
|
@comment sys/timex.h
|
|
@comment GNU
|
|
@deftypefun int ntp_gettime (struct ntptimeval *@var{tptr})
|
|
The @code{ntp_gettime} function sets the structure pointed to by
|
|
@var{tptr} to current values. The elements of the structure afterwards
|
|
contain the values the timer implementation in the kernel assumes. They
|
|
might or might not be correct. If they are not a @code{ntp_adjtime}
|
|
call is necessary.
|
|
|
|
The return value is @code{0} on success and other values on failure. The
|
|
following @code{errno} error conditions are defined for this function:
|
|
|
|
@table @code
|
|
@item TIME_ERROR
|
|
The precision clock model is not properly set up at the moment, thus the
|
|
clock must be considered unsynchronized, and the values should be
|
|
treated with care.
|
|
@end table
|
|
@end deftypefun
|
|
|
|
@tindex struct timex
|
|
@deftp {Data Type} {struct timex}
|
|
This structure is used to control and monitor the system clock. It
|
|
contains the following members:
|
|
@table @code
|
|
@item unsigned int modes
|
|
This variable controls whether and which values are set. Several
|
|
symbolic constants have to be combined with @emph{binary or} to specify
|
|
the effective mode. These constants start with @code{MOD_}.
|
|
|
|
@item long int offset
|
|
This value indicates the current offset of the local clock from the true
|
|
time. The value is given in microseconds. If bit @code{MOD_OFFSET} is
|
|
set in @code{modes}, the offset (and possibly other dependent values) can
|
|
be set. The offset's absolute value must not exceed @code{MAXPHASE}.
|
|
|
|
|
|
@item long int frequency
|
|
This value indicates the difference in frequency between the true time
|
|
and the local clock. The value is expressed as scaled PPM (parts per
|
|
million, 0.0001%). The scaling is @code{1 << SHIFT_USEC}. The value
|
|
can be set with bit @code{MOD_FREQUENCY}, but the absolute value must
|
|
not exceed @code{MAXFREQ}.
|
|
|
|
@item long int maxerror
|
|
This is the maximum error, measured in microseconds. A new value can be
|
|
set using bit @code{MOD_MAXERROR}. Unless updated via
|
|
@code{ntp_adjtime} periodically, this value will increase steadily
|
|
and reach some platform-specific maximum value.
|
|
|
|
@item long int esterror
|
|
This is the estimated error, measured in microseconds. This value can
|
|
be set using bit @code{MOD_ESTERROR}.
|
|
|
|
@item int status
|
|
This variable reflects the various states of the clock machinery. There
|
|
are symbolic constants for the significant bits, starting with
|
|
@code{STA_}. Some of these flags can be updated using the
|
|
@code{MOD_STATUS} bit.
|
|
|
|
@item long int constant
|
|
This value represents the bandwidth or stiffness of the PLL (phase
|
|
locked loop) implemented in the kernel. The value can be changed using
|
|
bit @code{MOD_TIMECONST}.
|
|
|
|
@item long int precision
|
|
This value represents the accuracy or the maximum error when reading the
|
|
system clock. The value is expressed in microseconds.
|
|
|
|
@item long int tolerance
|
|
This value represents the maximum frequency error of the system clock in
|
|
scaled PPM. This value is used to increase the @code{maxerror} every
|
|
second.
|
|
|
|
@item struct timeval time
|
|
The current time.
|
|
|
|
@item long int tick
|
|
The time between clock ticks in microseconds. A clock tick is a
|
|
periodic timer interrupt on which the system clock is based.
|
|
|
|
@item long int ppsfreq
|
|
This is the first of a few optional variables that are present only if
|
|
the system clock can use a PPS (pulse per second) signal to discipline
|
|
the local clock. The value is expressed in scaled PPM and it denotes
|
|
the difference in frequency between the local clock and the PPS signal.
|
|
|
|
@item long int jitter
|
|
This value expresses a median filtered average of the PPS signal's
|
|
dispersion in microseconds.
|
|
|
|
@item int shift
|
|
This value is a binary exponent for the duration of the PPS calibration
|
|
interval, ranging from @code{PPS_SHIFT} to @code{PPS_SHIFTMAX}.
|
|
|
|
@item long int stabil
|
|
This value represents the median filtered dispersion of the PPS
|
|
frequency in scaled PPM.
|
|
|
|
@item long int jitcnt
|
|
This counter represents the number of pulses where the jitter exceeded
|
|
the allowed maximum @code{MAXTIME}.
|
|
|
|
@item long int calcnt
|
|
This counter reflects the number of successful calibration intervals.
|
|
|
|
@item long int errcnt
|
|
This counter represents the number of calibration errors (caused by
|
|
large offsets or jitter).
|
|
|
|
@item long int stbcnt
|
|
This counter denotes the number of of calibrations where the stability
|
|
exceeded the threshold.
|
|
@end table
|
|
@end deftp
|
|
|
|
@comment sys/timex.h
|
|
@comment GNU
|
|
@deftypefun int ntp_adjtime (struct timex *@var{tptr})
|
|
The @code{ntp_adjtime} function sets the structure specified by
|
|
@var{tptr} to current values.
|
|
|
|
In addition, @code{ntp_adjtime} updates some settings to match what you
|
|
pass to it in *@var{tptr}. Use the @code{modes} element of *@var{tptr}
|
|
to select what settings to update. You can set @code{offset},
|
|
@code{freq}, @code{maxerror}, @code{esterror}, @code{status},
|
|
@code{constant}, and @code{tick}.
|
|
|
|
@code{modes} = zero means set nothing.
|
|
|
|
Only the superuser can update settings.
|
|
|
|
@c On Linux, ntp_adjtime() also does the adjtime() function if you set
|
|
@c modes = ADJ_OFFSET_SINGLESHOT (in fact, that is how GNU libc implements
|
|
@c adjtime()). But this should be considered an internal function because
|
|
@c it's so inconsistent with the rest of what ntp_adjtime() does and is
|
|
@c forced in an ugly way into the struct timex. So we don't document it
|
|
@c and instead document adjtime() as the way to achieve the function.
|
|
|
|
The return value is @code{0} on success and other values on failure. The
|
|
following @code{errno} error conditions are defined for this function:
|
|
|
|
@table @code
|
|
@item TIME_ERROR
|
|
The high accuracy clock model is not properly set up at the moment, thus the
|
|
clock must be considered unsynchronized, and the values should be
|
|
treated with care. Another reason could be that the specified new values
|
|
are not allowed.
|
|
|
|
@item EPERM
|
|
The process specified a settings update, but is not superuser.
|
|
|
|
@end table
|
|
|
|
For more details see RFC1305 (Network Time Protocol, Version 3) and
|
|
related documents.
|
|
|
|
@strong{Portability note:} Early versions of the GNU C library did not
|
|
have this function but did have the synonymous @code{adjtimex}.
|
|
|
|
@end deftypefun
|
|
|
|
|
|
@node Formatting Date and Time
|
|
@subsection Formatting Date and Time
|
|
|
|
The functions described in this section format time values as strings.
|
|
These functions are declared in the header file @file{time.h}.
|
|
@pindex time.h
|
|
|
|
@comment time.h
|
|
@comment ISO
|
|
@deftypefun {char *} asctime (const struct tm *@var{brokentime})
|
|
The @code{asctime} function converts the broken-down time value that
|
|
@var{brokentime} points to into a string in a standard format:
|
|
|
|
@smallexample
|
|
"Tue May 21 13:46:22 1991\n"
|
|
@end smallexample
|
|
|
|
The abbreviations for the days of week are: @samp{Sun}, @samp{Mon},
|
|
@samp{Tue}, @samp{Wed}, @samp{Thu}, @samp{Fri}, and @samp{Sat}.
|
|
|
|
The abbreviations for the months are: @samp{Jan}, @samp{Feb},
|
|
@samp{Mar}, @samp{Apr}, @samp{May}, @samp{Jun}, @samp{Jul}, @samp{Aug},
|
|
@samp{Sep}, @samp{Oct}, @samp{Nov}, and @samp{Dec}.
|
|
|
|
The return value points to a statically allocated string, which might be
|
|
overwritten by subsequent calls to @code{asctime} or @code{ctime}.
|
|
(But no other library function overwrites the contents of this
|
|
string.)
|
|
@end deftypefun
|
|
|
|
@comment time.h
|
|
@comment POSIX.1c
|
|
@deftypefun {char *} asctime_r (const struct tm *@var{brokentime}, char *@var{buffer})
|
|
This function is similar to @code{asctime} but instead of placing the
|
|
result in a static buffer it writes the string in the buffer pointed to
|
|
by the parameter @var{buffer}. This buffer should have room
|
|
for at least 26 bytes, including the terminating null.
|
|
|
|
If no error occurred the function returns a pointer to the string the
|
|
result was written into, i.e., it returns @var{buffer}. Otherwise
|
|
return @code{NULL}.
|
|
@end deftypefun
|
|
|
|
|
|
@comment time.h
|
|
@comment ISO
|
|
@deftypefun {char *} ctime (const time_t *@var{time})
|
|
The @code{ctime} function is similar to @code{asctime}, except that the
|
|
time value is specified as a @code{time_t} simple time value rather
|
|
than in broken-down local time format. It is equivalent to
|
|
|
|
@smallexample
|
|
asctime (localtime (@var{time}))
|
|
@end smallexample
|
|
|
|
@code{ctime} sets the variable @code{tzname}, because @code{localtime}
|
|
does so. @xref{Time Zone Functions}.
|
|
@end deftypefun
|
|
|
|
@comment time.h
|
|
@comment POSIX.1c
|
|
@deftypefun {char *} ctime_r (const time_t *@var{time}, char *@var{buffer})
|
|
This function is similar to @code{ctime}, only that it places the result
|
|
in the string pointed to by @var{buffer}. It is equivalent to (written
|
|
using gcc extensions, @pxref{Statement Exprs,,,gcc,Porting and Using gcc}):
|
|
|
|
@smallexample
|
|
(@{ struct tm tm; asctime_r (localtime_r (time, &tm), buf); @})
|
|
@end smallexample
|
|
|
|
If no error occurred the function returns a pointer to the string the
|
|
result was written into, i.e., it returns @var{buffer}. Otherwise
|
|
return @code{NULL}.
|
|
@end deftypefun
|
|
|
|
|
|
@comment time.h
|
|
@comment ISO
|
|
@deftypefun size_t strftime (char *@var{s}, size_t @var{size}, const char *@var{template}, const struct tm *@var{brokentime})
|
|
This function is similar to the @code{sprintf} function (@pxref{Formatted
|
|
Input}), but the conversion specifications that can appear in the format
|
|
template @var{template} are specialized for printing components of the date
|
|
and time @var{brokentime} according to the locale currently specified for
|
|
time conversion (@pxref{Locales}).
|
|
|
|
Ordinary characters appearing in the @var{template} are copied to the
|
|
output string @var{s}; this can include multibyte character sequences.
|
|
Conversion specifiers are introduced by a @samp{%} character, followed
|
|
by an optional flag which can be one of the following. These flags
|
|
are all GNU extensions. The first three affect only the output of
|
|
numbers:
|
|
|
|
@table @code
|
|
@item _
|
|
The number is padded with spaces.
|
|
|
|
@item -
|
|
The number is not padded at all.
|
|
|
|
@item 0
|
|
The number is padded with zeros even if the format specifies padding
|
|
with spaces.
|
|
|
|
@item ^
|
|
The output uses uppercase characters, but only if this is possible
|
|
(@pxref{Case Conversion}).
|
|
@end table
|
|
|
|
The default action is to pad the number with zeros to keep it a constant
|
|
width. Numbers that do not have a range indicated below are never
|
|
padded, since there is no natural width for them.
|
|
|
|
Following the flag an optional specification of the width is possible.
|
|
This is specified in decimal notation. If the natural size of the
|
|
output is of the field has less than the specified number of characters,
|
|
the result is written right adjusted and space padded to the given
|
|
size.
|
|
|
|
An optional modifier can follow the optional flag and width
|
|
specification. The modifiers, which are POSIX.2 extensions, are:
|
|
|
|
@table @code
|
|
@item E
|
|
Use the locale's alternate representation for date and time. This
|
|
modifier applies to the @code{%c}, @code{%C}, @code{%x}, @code{%X},
|
|
@code{%y} and @code{%Y} format specifiers. In a Japanese locale, for
|
|
example, @code{%Ex} might yield a date format based on the Japanese
|
|
Emperors' reigns.
|
|
|
|
@item O
|
|
Use the locale's alternate numeric symbols for numbers. This modifier
|
|
applies only to numeric format specifiers.
|
|
@end table
|
|
|
|
If the format supports the modifier but no alternate representation
|
|
is available, it is ignored.
|
|
|
|
The conversion specifier ends with a format specifier taken from the
|
|
following list. The whole @samp{%} sequence is replaced in the output
|
|
string as follows:
|
|
|
|
@table @code
|
|
@item %a
|
|
The abbreviated weekday name according to the current locale.
|
|
|
|
@item %A
|
|
The full weekday name according to the current locale.
|
|
|
|
@item %b
|
|
The abbreviated month name according to the current locale.
|
|
|
|
@item %B
|
|
The full month name according to the current locale.
|
|
|
|
@item %c
|
|
The preferred date and time representation for the current locale.
|
|
|
|
@item %C
|
|
The century of the year. This is equivalent to the greatest integer not
|
|
greater than the year divided by 100.
|
|
|
|
This format is a POSIX.2 extension and also appears in @w{ISO C99}.
|
|
|
|
@item %d
|
|
The day of the month as a decimal number (range @code{01} through @code{31}).
|
|
|
|
@item %D
|
|
The date using the format @code{%m/%d/%y}.
|
|
|
|
This format is a POSIX.2 extension and also appears in @w{ISO C99}.
|
|
|
|
@item %e
|
|
The day of the month like with @code{%d}, but padded with blank (range
|
|
@code{ 1} through @code{31}).
|
|
|
|
This format is a POSIX.2 extension and also appears in @w{ISO C99}.
|
|
|
|
@item %F
|
|
The date using the format @code{%Y-%m-%d}. This is the form specified
|
|
in the @w{ISO 8601} standard and is the preferred form for all uses.
|
|
|
|
This format is a @w{ISO C99} extension.
|
|
|
|
@item %g
|
|
The year corresponding to the ISO week number, but without the century
|
|
(range @code{00} through @code{99}). This has the same format and value
|
|
as @code{%y}, except that if the ISO week number (see @code{%V}) belongs
|
|
to the previous or next year, that year is used instead.
|
|
|
|
This format was introduced in @w{ISO C99}.
|
|
|
|
@item %G
|
|
The year corresponding to the ISO week number. This has the same format
|
|
and value as @code{%Y}, except that if the ISO week number (see
|
|
@code{%V}) belongs to the previous or next year, that year is used
|
|
instead.
|
|
|
|
This format was introduced in @w{ISO C99} but was previously available
|
|
as a GNU extension.
|
|
|
|
@item %h
|
|
The abbreviated month name according to the current locale. The action
|
|
is the same as for @code{%b}.
|
|
|
|
This format is a POSIX.2 extension and also appears in @w{ISO C99}.
|
|
|
|
@item %H
|
|
The hour as a decimal number, using a 24-hour clock (range @code{00} through
|
|
@code{23}).
|
|
|
|
@item %I
|
|
The hour as a decimal number, using a 12-hour clock (range @code{01} through
|
|
@code{12}).
|
|
|
|
@item %j
|
|
The day of the year as a decimal number (range @code{001} through @code{366}).
|
|
|
|
@item %k
|
|
The hour as a decimal number, using a 24-hour clock like @code{%H}, but
|
|
padded with blank (range @code{ 0} through @code{23}).
|
|
|
|
This format is a GNU extension.
|
|
|
|
@item %l
|
|
The hour as a decimal number, using a 12-hour clock like @code{%I}, but
|
|
padded with blank (range @code{ 1} through @code{12}).
|
|
|
|
This format is a GNU extension.
|
|
|
|
@item %m
|
|
The month as a decimal number (range @code{01} through @code{12}).
|
|
|
|
@item %M
|
|
The minute as a decimal number (range @code{00} through @code{59}).
|
|
|
|
@item %n
|
|
A single @samp{\n} (newline) character.
|
|
|
|
This format is a POSIX.2 extension and also appears in @w{ISO C99}.
|
|
|
|
@item %p
|
|
Either @samp{AM} or @samp{PM}, according to the given time value; or the
|
|
corresponding strings for the current locale. Noon is treated as
|
|
@samp{PM} and midnight as @samp{AM}.
|
|
|
|
@ignore
|
|
We currently have a problem with makeinfo. Write @samp{AM} and @samp{am}
|
|
both results in `am'. I.e., the difference in case is not visible anymore.
|
|
@end ignore
|
|
@item %P
|
|
Either @samp{am} or @samp{pm}, according to the given time value; or the
|
|
corresponding strings for the current locale, printed in lowercase
|
|
characters. Noon is treated as @samp{pm} and midnight as @samp{am}.
|
|
|
|
This format was introduced in @w{ISO C99} but was previously available
|
|
as a GNU extension.
|
|
|
|
@item %r
|
|
The complete time using the AM/PM format of the current locale.
|
|
|
|
This format is a POSIX.2 extension and also appears in @w{ISO C99}.
|
|
|
|
@item %R
|
|
The hour and minute in decimal numbers using the format @code{%H:%M}.
|
|
|
|
This format was introduced in @w{ISO C99} but was previously available
|
|
as a GNU extension.
|
|
|
|
@item %s
|
|
The number of seconds since the epoch, i.e., since 1970-01-01 00:00:00 UTC.
|
|
Leap seconds are not counted unless leap second support is available.
|
|
|
|
This format is a GNU extension.
|
|
|
|
@item %S
|
|
The seconds as a decimal number (range @code{00} through @code{60}).
|
|
|
|
@item %t
|
|
A single @samp{\t} (tabulator) character.
|
|
|
|
This format is a POSIX.2 extension and also appears in @w{ISO C99}.
|
|
|
|
@item %T
|
|
The time using decimal numbers using the format @code{%H:%M:%S}.
|
|
|
|
This format is a POSIX.2 extension.
|
|
|
|
@item %u
|
|
The day of the week as a decimal number (range @code{1} through
|
|
@code{7}), Monday being @code{1}.
|
|
|
|
This format is a POSIX.2 extension and also appears in @w{ISO C99}.
|
|
|
|
@item %U
|
|
The week number of the current year as a decimal number (range @code{00}
|
|
through @code{53}), starting with the first Sunday as the first day of
|
|
the first week. Days preceding the first Sunday in the year are
|
|
considered to be in week @code{00}.
|
|
|
|
@item %V
|
|
The @w{ISO 8601:1988} week number as a decimal number (range @code{01}
|
|
through @code{53}). ISO weeks start with Monday and end with Sunday.
|
|
Week @code{01} of a year is the first week which has the majority of its
|
|
days in that year; this is equivalent to the week containing the year's
|
|
first Thursday, and it is also equivalent to the week containing January
|
|
4. Week @code{01} of a year can contain days from the previous year.
|
|
The week before week @code{01} of a year is the last week (@code{52} or
|
|
@code{53}) of the previous year even if it contains days from the new
|
|
year.
|
|
|
|
This format is a POSIX.2 extension and also appears in @w{ISO C99}.
|
|
|
|
@item %w
|
|
The day of the week as a decimal number (range @code{0} through
|
|
@code{6}), Sunday being @code{0}.
|
|
|
|
@item %W
|
|
The week number of the current year as a decimal number (range @code{00}
|
|
through @code{53}), starting with the first Monday as the first day of
|
|
the first week. All days preceding the first Monday in the year are
|
|
considered to be in week @code{00}.
|
|
|
|
@item %x
|
|
The preferred date representation for the current locale, but without the
|
|
time.
|
|
|
|
@item %X
|
|
The preferred time representation for the current locale, but with no date.
|
|
|
|
@item %y
|
|
The year without a century as a decimal number (range @code{00} through
|
|
@code{99}). This is equivalent to the year modulo 100.
|
|
|
|
@item %Y
|
|
The year as a decimal number, using the Gregorian calendar. Years
|
|
before the year @code{1} are numbered @code{0}, @code{-1}, and so on.
|
|
|
|
@item %z
|
|
@w{RFC 822}/@w{ISO 8601:1988} style numeric time zone (e.g.,
|
|
@code{-0600} or @code{+0100}), or nothing if no time zone is
|
|
determinable.
|
|
|
|
This format was introduced in @w{ISO C99} but was previously available
|
|
as a GNU extension.
|
|
|
|
A full @w{RFC 822} timestamp is generated by the format
|
|
@w{@samp{"%a, %d %b %Y %H:%M:%S %z"}} (or the equivalent
|
|
@w{@samp{"%a, %d %b %Y %T %z"}}).
|
|
|
|
@item %Z
|
|
The time zone abbreviation (empty if the time zone can't be determined).
|
|
|
|
@item %%
|
|
A literal @samp{%} character.
|
|
@end table
|
|
|
|
The @var{size} parameter can be used to specify the maximum number of
|
|
characters to be stored in the array @var{s}, including the terminating
|
|
null character. If the formatted time requires more than @var{size}
|
|
characters, @code{strftime} returns zero and the contents of the array
|
|
@var{s} are undefined. Otherwise the return value indicates the
|
|
number of characters placed in the array @var{s}, not including the
|
|
terminating null character.
|
|
|
|
@emph{Warning:} This convention for the return value which is prescribed
|
|
in @w{ISO C} can lead to problems in some situations. For certain
|
|
format strings and certain locales the output really can be the empty
|
|
string and this cannot be discovered by testing the return value only.
|
|
E.g., in most locales the AM/PM time format is not supported (most of
|
|
the world uses the 24 hour time representation). In such locales
|
|
@code{"%p"} will return the empty string, i.e., the return value is
|
|
zero. To detect situations like this something similar to the following
|
|
code should be used:
|
|
|
|
@smallexample
|
|
buf[0] = '\1';
|
|
len = strftime (buf, bufsize, format, tp);
|
|
if (len == 0 && buf[0] != '\0')
|
|
@{
|
|
/* Something went wrong in the strftime call. */
|
|
@dots{}
|
|
@}
|
|
@end smallexample
|
|
|
|
If @var{s} is a null pointer, @code{strftime} does not actually write
|
|
anything, but instead returns the number of characters it would have written.
|
|
|
|
According to POSIX.1 every call to @code{strftime} implies a call to
|
|
@code{tzset}. So the contents of the environment variable @code{TZ}
|
|
is examined before any output is produced.
|
|
|
|
For an example of @code{strftime}, see @ref{Time Functions Example}.
|
|
@end deftypefun
|
|
|
|
@comment time.h
|
|
@comment ISO/Amend1
|
|
@deftypefun size_t wcsftime (wchar_t *@var{s}, size_t @var{size}, const wchar_t *@var{template}, const struct tm *@var{brokentime})
|
|
The @code{wcsftime} function is equivalent to the @code{strftime}
|
|
function with the difference that it operates on wide character
|
|
strings. The buffer where the result is stored, pointed to by @var{s},
|
|
must be an array of wide characters. The parameter @var{size} which
|
|
specifies the size of the output buffer gives the number of wide
|
|
character, not the number of bytes.
|
|
|
|
Also the format string @var{template} is a wide character string. Since
|
|
all characters needed to specify the format string are in the basic
|
|
character set it is portably possible to write format strings in the C
|
|
source code using the @code{L"..."} notation. The parameter
|
|
@var{brokentime} has the same meaning as in the @code{strftime} call.
|
|
|
|
The @code{wcsftime} function supports the same flags, modifiers, and
|
|
format specifiers as the @code{strftime} function.
|
|
|
|
The return value of @code{wcsftime} is the number of wide characters
|
|
stored in @code{s}. When more characters would have to be written than
|
|
can be placed in the buffer @var{s} the return value is zero, with the
|
|
same problems indicated in the @code{strftime} documentation.
|
|
@end deftypefun
|
|
|
|
@node Parsing Date and Time
|
|
@subsection Convert textual time and date information back
|
|
|
|
The @w{ISO C} standard does not specify any functions which can convert
|
|
the output of the @code{strftime} function back into a binary format.
|
|
This led to a variety of more-or-less successful implementations with
|
|
different interfaces over the years. Then the Unix standard was
|
|
extended by the addition of two functions: @code{strptime} and
|
|
@code{getdate}. Both have strange interfaces but at least they are
|
|
widely available.
|
|
|
|
@menu
|
|
* Low-Level Time String Parsing:: Interpret string according to given format.
|
|
* General Time String Parsing:: User-friendly function to parse data and
|
|
time strings.
|
|
@end menu
|
|
|
|
@node Low-Level Time String Parsing
|
|
@subsubsection Interpret string according to given format
|
|
|
|
he first function is rather low-level. It is nevertheless frequently
|
|
used in software since it is better known. Its interface and
|
|
implementation are heavily influenced by the @code{getdate} function,
|
|
which is defined and implemented in terms of calls to @code{strptime}.
|
|
|
|
@comment time.h
|
|
@comment XPG4
|
|
@deftypefun {char *} strptime (const char *@var{s}, const char *@var{fmt}, struct tm *@var{tp})
|
|
The @code{strptime} function parses the input string @var{s} according
|
|
to the format string @var{fmt} and stores its results in the
|
|
structure @var{tp}.
|
|
|
|
The input string could be generated by a @code{strftime} call or
|
|
obtained any other way. It does not need to be in a human-recognizable
|
|
format; e.g. a date passed as @code{"02:1999:9"} is acceptable, even
|
|
though it is ambiguous without context. As long as the format string
|
|
@var{fmt} matches the input string the function will succeed.
|
|
|
|
The format string consists of the same components as the format string
|
|
of the @code{strftime} function. The only difference is that the flags
|
|
@code{_}, @code{-}, @code{0}, and @code{^} are not allowed.
|
|
@comment Is this really the intention? --drepper
|
|
Several of the distinct formats of @code{strftime} do the same work in
|
|
@code{strptime} since differences like case of the input do not matter.
|
|
For reasons of symmetry all formats are supported, though.
|
|
|
|
The modifiers @code{E} and @code{O} are also allowed everywhere the
|
|
@code{strftime} function allows them.
|
|
|
|
The formats are:
|
|
|
|
@table @code
|
|
@item %a
|
|
@itemx %A
|
|
The weekday name according to the current locale, in abbreviated form or
|
|
the full name.
|
|
|
|
@item %b
|
|
@itemx %B
|
|
@itemx %h
|
|
The month name according to the current locale, in abbreviated form or
|
|
the full name.
|
|
|
|
@item %c
|
|
The date and time representation for the current locale.
|
|
|
|
@item %Ec
|
|
Like @code{%c} but the locale's alternative date and time format is used.
|
|
|
|
@item %C
|
|
The century of the year.
|
|
|
|
It makes sense to use this format only if the format string also
|
|
contains the @code{%y} format.
|
|
|
|
@item %EC
|
|
The locale's representation of the period.
|
|
|
|
Unlike @code{%C} it sometimes makes sense to use this format since some
|
|
cultures represent years relative to the beginning of eras instead of
|
|
using the Gregorian years.
|
|
|
|
@item %d
|
|
@item %e
|
|
The day of the month as a decimal number (range @code{1} through @code{31}).
|
|
Leading zeroes are permitted but not required.
|
|
|
|
@item %Od
|
|
@itemx %Oe
|
|
Same as @code{%d} but using the locale's alternative numeric symbols.
|
|
|
|
Leading zeroes are permitted but not required.
|
|
|
|
@item %D
|
|
Equivalent to @code{%m/%d/%y}.
|
|
|
|
@item %F
|
|
Equivalent to @code{%Y-%m-%d}, which is the @w{ISO 8601} date
|
|
format.
|
|
|
|
This is a GNU extension following an @w{ISO C99} extension to
|
|
@code{strftime}.
|
|
|
|
@item %g
|
|
The year corresponding to the ISO week number, but without the century
|
|
(range @code{00} through @code{99}).
|
|
|
|
@emph{Note:} Currently, this is not fully implemented. The format is
|
|
recognized, input is consumed but no field in @var{tm} is set.
|
|
|
|
This format is a GNU extension following a GNU extension of @code{strftime}.
|
|
|
|
@item %G
|
|
The year corresponding to the ISO week number.
|
|
|
|
@emph{Note:} Currently, this is not fully implemented. The format is
|
|
recognized, input is consumed but no field in @var{tm} is set.
|
|
|
|
This format is a GNU extension following a GNU extension of @code{strftime}.
|
|
|
|
@item %H
|
|
@itemx %k
|
|
The hour as a decimal number, using a 24-hour clock (range @code{00} through
|
|
@code{23}).
|
|
|
|
@code{%k} is a GNU extension following a GNU extension of @code{strftime}.
|
|
|
|
@item %OH
|
|
Same as @code{%H} but using the locale's alternative numeric symbols.
|
|
|
|
@item %I
|
|
@itemx %l
|
|
The hour as a decimal number, using a 12-hour clock (range @code{01} through
|
|
@code{12}).
|
|
|
|
@code{%l} is a GNU extension following a GNU extension of @code{strftime}.
|
|
|
|
@item %OI
|
|
Same as @code{%I} but using the locale's alternative numeric symbols.
|
|
|
|
@item %j
|
|
The day of the year as a decimal number (range @code{1} through @code{366}).
|
|
|
|
Leading zeroes are permitted but not required.
|
|
|
|
@item %m
|
|
The month as a decimal number (range @code{1} through @code{12}).
|
|
|
|
Leading zeroes are permitted but not required.
|
|
|
|
@item %Om
|
|
Same as @code{%m} but using the locale's alternative numeric symbols.
|
|
|
|
@item %M
|
|
The minute as a decimal number (range @code{0} through @code{59}).
|
|
|
|
Leading zeroes are permitted but not required.
|
|
|
|
@item %OM
|
|
Same as @code{%M} but using the locale's alternative numeric symbols.
|
|
|
|
@item %n
|
|
@itemx %t
|
|
Matches any white space.
|
|
|
|
@item %p
|
|
@item %P
|
|
The locale-dependent equivalent to @samp{AM} or @samp{PM}.
|
|
|
|
This format is not useful unless @code{%I} or @code{%l} is also used.
|
|
Another complication is that the locale might not define these values at
|
|
all and therefore the conversion fails.
|
|
|
|
@code{%P} is a GNU extension following a GNU extension to @code{strftime}.
|
|
|
|
@item %r
|
|
The complete time using the AM/PM format of the current locale.
|
|
|
|
A complication is that the locale might not define this format at all
|
|
and therefore the conversion fails.
|
|
|
|
@item %R
|
|
The hour and minute in decimal numbers using the format @code{%H:%M}.
|
|
|
|
@code{%R} is a GNU extension following a GNU extension to @code{strftime}.
|
|
|
|
@item %s
|
|
The number of seconds since the epoch, i.e., since 1970-01-01 00:00:00 UTC.
|
|
Leap seconds are not counted unless leap second support is available.
|
|
|
|
@code{%s} is a GNU extension following a GNU extension to @code{strftime}.
|
|
|
|
@item %S
|
|
The seconds as a decimal number (range @code{0} through @code{61}).
|
|
|
|
Leading zeroes are permitted but not required.
|
|
|
|
Note the nonsense with @code{61}, as given in the Unix specification.
|
|
This is a result of a decision to allow double leap seconds. These do
|
|
not in fact exist but the myth persists.
|
|
|
|
@item %OS
|
|
Same as @code{%S} but using the locale's alternative numeric symbols.
|
|
|
|
@item %T
|
|
Equivalent to the use of @code{%H:%M:%S} in this place.
|
|
|
|
@item %u
|
|
The day of the week as a decimal number (range @code{1} through
|
|
@code{7}), Monday being @code{1}.
|
|
|
|
Leading zeroes are permitted but not required.
|
|
|
|
@emph{Note:} Currently, this is not fully implemented. The format is
|
|
recognized, input is consumed but no field in @var{tm} is set.
|
|
|
|
@item %U
|
|
The week number of the current year as a decimal number (range @code{0}
|
|
through @code{53}).
|
|
|
|
Leading zeroes are permitted but not required.
|
|
|
|
@item %OU
|
|
Same as @code{%U} but using the locale's alternative numeric symbols.
|
|
|
|
@item %V
|
|
The @w{ISO 8601:1988} week number as a decimal number (range @code{1}
|
|
through @code{53}).
|
|
|
|
Leading zeroes are permitted but not required.
|
|
|
|
@emph{Note:} Currently, this is not fully implemented. The format is
|
|
recognized, input is consumed but no field in @var{tm} is set.
|
|
|
|
@item %w
|
|
The day of the week as a decimal number (range @code{0} through
|
|
@code{6}), Sunday being @code{0}.
|
|
|
|
Leading zeroes are permitted but not required.
|
|
|
|
@emph{Note:} Currently, this is not fully implemented. The format is
|
|
recognized, input is consumed but no field in @var{tm} is set.
|
|
|
|
@item %Ow
|
|
Same as @code{%w} but using the locale's alternative numeric symbols.
|
|
|
|
@item %W
|
|
The week number of the current year as a decimal number (range @code{0}
|
|
through @code{53}).
|
|
|
|
Leading zeroes are permitted but not required.
|
|
|
|
@emph{Note:} Currently, this is not fully implemented. The format is
|
|
recognized, input is consumed but no field in @var{tm} is set.
|
|
|
|
@item %OW
|
|
Same as @code{%W} but using the locale's alternative numeric symbols.
|
|
|
|
@item %x
|
|
The date using the locale's date format.
|
|
|
|
@item %Ex
|
|
Like @code{%x} but the locale's alternative data representation is used.
|
|
|
|
@item %X
|
|
The time using the locale's time format.
|
|
|
|
@item %EX
|
|
Like @code{%X} but the locale's alternative time representation is used.
|
|
|
|
@item %y
|
|
The year without a century as a decimal number (range @code{0} through
|
|
@code{99}).
|
|
|
|
Leading zeroes are permitted but not required.
|
|
|
|
Note that it is questionable to use this format without
|
|
the @code{%C} format. The @code{strptime} function does regard input
|
|
values in the range @math{68} to @math{99} as the years @math{1969} to
|
|
@math{1999} and the values @math{0} to @math{68} as the years
|
|
@math{2000} to @math{2068}. But maybe this heuristic fails for some
|
|
input data.
|
|
|
|
Therefore it is best to avoid @code{%y} completely and use @code{%Y}
|
|
instead.
|
|
|
|
@item %Ey
|
|
The offset from @code{%EC} in the locale's alternative representation.
|
|
|
|
@item %Oy
|
|
The offset of the year (from @code{%C}) using the locale's alternative
|
|
numeric symbols.
|
|
|
|
@item %Y
|
|
The year as a decimal number, using the Gregorian calendar.
|
|
|
|
@item %EY
|
|
The full alternative year representation.
|
|
|
|
@item %z
|
|
Equivalent to the use of @code{%a, %d %b %Y %H:%M:%S %z} in this place.
|
|
This is the full @w{ISO 8601} date and time format.
|
|
|
|
@item %Z
|
|
The timezone name.
|
|
|
|
@emph{Note:} Currently, this is not fully implemented. The format is
|
|
recognized, input is consumed but no field in @var{tm} is set.
|
|
|
|
@item %%
|
|
A literal @samp{%} character.
|
|
@end table
|
|
|
|
All other characters in the format string must have a matching character
|
|
in the input string. Exceptions are white spaces in the input string
|
|
which can match zero or more white space characters in the format string.
|
|
|
|
The @code{strptime} function processes the input string from right to
|
|
left. Each of the three possible input elements (white space, literal,
|
|
or format) are handled one after the other. If the input cannot be
|
|
matched to the format string the function stops. The remainder of the
|
|
format and input strings are not processed.
|
|
|
|
The function returns a pointer to the first character it was unable to
|
|
process. If the input string contains more characters than required by
|
|
the format string the return value points right after the last consumed
|
|
input character. If the whole input string is consumed the return value
|
|
points to the @code{NULL} byte at the end of the string. If an error
|
|
occurs, i.e. @code{strptime} fails to match all of the format string,
|
|
the function returns @code{NULL}.
|
|
@end deftypefun
|
|
|
|
The specification of the function in the XPG standard is rather vague,
|
|
leaving out a few important pieces of information. Most importantly, it
|
|
does not specify what happens to those elements of @var{tm} which are
|
|
not directly initialized by the different formats. The
|
|
implementations on different Unix systems vary here.
|
|
|
|
The GNU libc implementation does not touch those fields which are not
|
|
directly initialized. Exceptions are the @code{tm_wday} and
|
|
@code{tm_yday} elements, which are recomputed if any of the year, month,
|
|
or date elements changed. This has two implications:
|
|
|
|
@itemize @bullet
|
|
@item
|
|
Before calling the @code{strptime} function for a new input string, you
|
|
should prepare the @var{tm} structure you pass. Normally this will mean
|
|
initializing all values are to zero. Alternatively, you can set all
|
|
fields to values like @code{INT_MAX}, allowing you to determine which
|
|
elements were set by the function call. Zero does not work here since
|
|
it is a valid value for many of the fields.
|
|
|
|
Careful initialization is necessary if you want to find out whether a
|
|
certain field in @var{tm} was initialized by the function call.
|
|
|
|
@item
|
|
You can construct a @code{struct tm} value with several consecutive
|
|
@code{strptime} calls. A useful application of this is e.g. the parsing
|
|
of two separate strings, one containing date information and the other
|
|
time information. By parsing one after the other without clearing the
|
|
structure in-between, you can construct a complete broken-down time.
|
|
@end itemize
|
|
|
|
The following example shows a function which parses a string which is
|
|
contains the date information in either US style or @w{ISO 8601} form:
|
|
|
|
@smallexample
|
|
const char *
|
|
parse_date (const char *input, struct tm *tm)
|
|
@{
|
|
const char *cp;
|
|
|
|
/* @r{First clear the result structure.} */
|
|
memset (tm, '\0', sizeof (*tm));
|
|
|
|
/* @r{Try the ISO format first.} */
|
|
cp = strptime (input, "%F", tm);
|
|
if (cp == NULL)
|
|
@{
|
|
/* @r{Does not match. Try the US form.} */
|
|
cp = strptime (input, "%D", tm);
|
|
@}
|
|
|
|
return cp;
|
|
@}
|
|
@end smallexample
|
|
|
|
@node General Time String Parsing
|
|
@subsubsection A More User-friendly Way to Parse Times and Dates
|
|
|
|
The Unix standard defines another function for parsing date strings.
|
|
The interface is weird, but if the function happens to suit your
|
|
application it is just fine. It is problematic to use this function
|
|
in multi-threaded programs or libraries, since it returns a pointer to
|
|
a static variable, and uses a global variable and global state (an
|
|
environment variable).
|
|
|
|
@comment time.h
|
|
@comment Unix98
|
|
@defvar getdate_err
|
|
This variable of type @code{int} contains the error code of the last
|
|
unsuccessful call to @code{getdate}. Defined values are:
|
|
|
|
@table @math
|
|
@item 1
|
|
The environment variable @code{DATEMSK} is not defined or null.
|
|
@item 2
|
|
The template file denoted by the @code{DATEMSK} environment variable
|
|
cannot be opened.
|
|
@item 3
|
|
Information about the template file cannot retrieved.
|
|
@item 4
|
|
The template file is not a regular file.
|
|
@item 5
|
|
An I/O error occurred while reading the template file.
|
|
@item 6
|
|
Not enough memory available to execute the function.
|
|
@item 7
|
|
The template file contains no matching template.
|
|
@item 8
|
|
The input date is invalid, but would match a template otherwise. This
|
|
includes dates like February 31st, and dates which cannot be represented
|
|
in a @code{time_t} variable.
|
|
@end table
|
|
@end defvar
|
|
|
|
@comment time.h
|
|
@comment Unix98
|
|
@deftypefun {struct tm *} getdate (const char *@var{string})
|
|
The interface to @code{getdate} is the simplest possible for a function
|
|
to parse a string and return the value. @var{string} is the input
|
|
string and the result is returned in a statically-allocated variable.
|
|
|
|
The details about how the string is processed are hidden from the user.
|
|
In fact, they can be outside the control of the program. Which formats
|
|
are recognized is controlled by the file named by the environment
|
|
variable @code{DATEMSK}. This file should contain
|
|
lines of valid format strings which could be passed to @code{strptime}.
|
|
|
|
The @code{getdate} function reads these format strings one after the
|
|
other and tries to match the input string. The first line which
|
|
completely matches the input string is used.
|
|
|
|
Elements not initialized through the format string retain the values
|
|
present at the time of the @code{getdate} function call.
|
|
|
|
The formats recognized by @code{getdate} are the same as for
|
|
@code{strptime}. See above for an explanation. There are only a few
|
|
extensions to the @code{strptime} behavior:
|
|
|
|
@itemize @bullet
|
|
@item
|
|
If the @code{%Z} format is given the broken-down time is based on the
|
|
current time of the timezone matched, not of the current timezone of the
|
|
runtime environment.
|
|
|
|
@emph{Note}: This is not implemented (currently). The problem is that
|
|
timezone names are not unique. If a fixed timezone is assumed for a
|
|
given string (say @code{EST} meaning US East Coast time), then uses for
|
|
countries other than the USA will fail. So far we have found no good
|
|
solution to this.
|
|
|
|
@item
|
|
If only the weekday is specified the selected day depends on the current
|
|
date. If the current weekday is greater or equal to the @code{tm_wday}
|
|
value the current week's day is chosen, otherwise the day next week is chosen.
|
|
|
|
@item
|
|
A similar heuristic is used when only the month is given and not the
|
|
year. If the month is greater than or equal to the current month, then
|
|
the current year is used. Otherwise it wraps to next year. The first
|
|
day of the month is assumed if one is not explicitly specified.
|
|
|
|
@item
|
|
The current hour, minute, and second are used if the appropriate value is
|
|
not set through the format.
|
|
|
|
@item
|
|
If no date is given tomorrow's date is used if the time is
|
|
smaller than the current time. Otherwise today's date is taken.
|
|
@end itemize
|
|
|
|
It should be noted that the format in the template file need not only
|
|
contain format elements. The following is a list of possible format
|
|
strings (taken from the Unix standard):
|
|
|
|
@smallexample
|
|
%m
|
|
%A %B %d, %Y %H:%M:%S
|
|
%A
|
|
%B
|
|
%m/%d/%y %I %p
|
|
%d,%m,%Y %H:%M
|
|
at %A the %dst of %B in %Y
|
|
run job at %I %p,%B %dnd
|
|
%A den %d. %B %Y %H.%M Uhr
|
|
@end smallexample
|
|
|
|
As you can see, the template list can contain very specific strings like
|
|
@code{run job at %I %p,%B %dnd}. Using the above list of templates and
|
|
assuming the current time is Mon Sep 22 12:19:47 EDT 1986 we can obtain the
|
|
following results for the given input.
|
|
|
|
@multitable {xxxxxxxxxxxx} {xxxxxxxxxx} {xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx}
|
|
@item Input @tab Match @tab Result
|
|
@item Mon @tab %a @tab Mon Sep 22 12:19:47 EDT 1986
|
|
@item Sun @tab %a @tab Sun Sep 28 12:19:47 EDT 1986
|
|
@item Fri @tab %a @tab Fri Sep 26 12:19:47 EDT 1986
|
|
@item September @tab %B @tab Mon Sep 1 12:19:47 EDT 1986
|
|
@item January @tab %B @tab Thu Jan 1 12:19:47 EST 1987
|
|
@item December @tab %B @tab Mon Dec 1 12:19:47 EST 1986
|
|
@item Sep Mon @tab %b %a @tab Mon Sep 1 12:19:47 EDT 1986
|
|
@item Jan Fri @tab %b %a @tab Fri Jan 2 12:19:47 EST 1987
|
|
@item Dec Mon @tab %b %a @tab Mon Dec 1 12:19:47 EST 1986
|
|
@item Jan Wed 1989 @tab %b %a %Y @tab Wed Jan 4 12:19:47 EST 1989
|
|
@item Fri 9 @tab %a %H @tab Fri Sep 26 09:00:00 EDT 1986
|
|
@item Feb 10:30 @tab %b %H:%S @tab Sun Feb 1 10:00:30 EST 1987
|
|
@item 10:30 @tab %H:%M @tab Tue Sep 23 10:30:00 EDT 1986
|
|
@item 13:30 @tab %H:%M @tab Mon Sep 22 13:30:00 EDT 1986
|
|
@end multitable
|
|
|
|
The return value of the function is a pointer to a static variable of
|
|
type @w{@code{struct tm}}, or a null pointer if an error occurred. The
|
|
result is only valid until the next @code{getdate} call, making this
|
|
function unusable in multi-threaded applications.
|
|
|
|
The @code{errno} variable is @emph{not} changed. Error conditions are
|
|
stored in the global variable @code{getdate_err}. See the
|
|
description above for a list of the possible error values.
|
|
|
|
@emph{Warning:} The @code{getdate} function should @emph{never} be
|
|
used in SUID-programs. The reason is obvious: using the
|
|
@code{DATEMSK} environment variable you can get the function to open
|
|
any arbitrary file and chances are high that with some bogus input
|
|
(such as a binary file) the program will crash.
|
|
@end deftypefun
|
|
|
|
@comment time.h
|
|
@comment GNU
|
|
@deftypefun int getdate_r (const char *@var{string}, struct tm *@var{tp})
|
|
The @code{getdate_r} function is the reentrant counterpart of
|
|
@code{getdate}. It does not use the global variable @code{getdate_err}
|
|
to signal an error, but instead returns an error code. The same error
|
|
codes as described in the @code{getdate_err} documentation above are
|
|
used, with 0 meaning success.
|
|
|
|
Moreover, @code{getdate_r} stores the broken-down time in the variable
|
|
of type @code{struct tm} pointed to by the second argument, rather than
|
|
in a static variable.
|
|
|
|
This function is not defined in the Unix standard. Nevertheless it is
|
|
available on some other Unix systems as well.
|
|
|
|
The warning against using @code{getdate} in SUID-programs applies to
|
|
@code{getdate_r} as well.
|
|
@end deftypefun
|
|
|
|
@node TZ Variable
|
|
@subsection Specifying the Time Zone with @code{TZ}
|
|
|
|
In POSIX systems, a user can specify the time zone by means of the
|
|
@code{TZ} environment variable. For information about how to set
|
|
environment variables, see @ref{Environment Variables}. The functions
|
|
for accessing the time zone are declared in @file{time.h}.
|
|
@pindex time.h
|
|
@cindex time zone
|
|
|
|
You should not normally need to set @code{TZ}. If the system is
|
|
configured properly, the default time zone will be correct. You might
|
|
set @code{TZ} if you are using a computer over a network from a
|
|
different time zone, and would like times reported to you in the time
|
|
zone local to you, rather than what is local to the computer.
|
|
|
|
In POSIX.1 systems the value of the @code{TZ} variable can be in one of
|
|
three formats. With the GNU C library, the most common format is the
|
|
last one, which can specify a selection from a large database of time
|
|
zone information for many regions of the world. The first two formats
|
|
are used to describe the time zone information directly, which is both
|
|
more cumbersome and less precise. But the POSIX.1 standard only
|
|
specifies the details of the first two formats, so it is good to be
|
|
familiar with them in case you come across a POSIX.1 system that doesn't
|
|
support a time zone information database.
|
|
|
|
The first format is used when there is no Daylight Saving Time (or
|
|
summer time) in the local time zone:
|
|
|
|
@smallexample
|
|
@r{@var{std} @var{offset}}
|
|
@end smallexample
|
|
|
|
The @var{std} string specifies the name of the time zone. It must be
|
|
three or more characters long and must not contain a leading colon,
|
|
embedded digits, commas, nor plus and minus signs. There is no space
|
|
character separating the time zone name from the @var{offset}, so these
|
|
restrictions are necessary to parse the specification correctly.
|
|
|
|
The @var{offset} specifies the time value you must add to the local time
|
|
to get a Coordinated Universal Time value. It has syntax like
|
|
[@code{+}|@code{-}]@var{hh}[@code{:}@var{mm}[@code{:}@var{ss}]]. This
|
|
is positive if the local time zone is west of the Prime Meridian and
|
|
negative if it is east. The hour must be between @code{0} and
|
|
@code{23}, and the minute and seconds between @code{0} and @code{59}.
|
|
|
|
For example, here is how we would specify Eastern Standard Time, but
|
|
without any Daylight Saving Time alternative:
|
|
|
|
@smallexample
|
|
EST+5
|
|
@end smallexample
|
|
|
|
The second format is used when there is Daylight Saving Time:
|
|
|
|
@smallexample
|
|
@r{@var{std} @var{offset} @var{dst} [@var{offset}]@code{,}@var{start}[@code{/}@var{time}]@code{,}@var{end}[@code{/}@var{time}]}
|
|
@end smallexample
|
|
|
|
The initial @var{std} and @var{offset} specify the standard time zone, as
|
|
described above. The @var{dst} string and @var{offset} specify the name
|
|
and offset for the corresponding Daylight Saving Time zone; if the
|
|
@var{offset} is omitted, it defaults to one hour ahead of standard time.
|
|
|
|
The remainder of the specification describes when Daylight Saving Time is
|
|
in effect. The @var{start} field is when Daylight Saving Time goes into
|
|
effect and the @var{end} field is when the change is made back to standard
|
|
time. The following formats are recognized for these fields:
|
|
|
|
@table @code
|
|
@item J@var{n}
|
|
This specifies the Julian day, with @var{n} between @code{1} and @code{365}.
|
|
February 29 is never counted, even in leap years.
|
|
|
|
@item @var{n}
|
|
This specifies the Julian day, with @var{n} between @code{0} and @code{365}.
|
|
February 29 is counted in leap years.
|
|
|
|
@item M@var{m}.@var{w}.@var{d}
|
|
This specifies day @var{d} of week @var{w} of month @var{m}. The day
|
|
@var{d} must be between @code{0} (Sunday) and @code{6}. The week
|
|
@var{w} must be between @code{1} and @code{5}; week @code{1} is the
|
|
first week in which day @var{d} occurs, and week @code{5} specifies the
|
|
@emph{last} @var{d} day in the month. The month @var{m} should be
|
|
between @code{1} and @code{12}.
|
|
@end table
|
|
|
|
The @var{time} fields specify when, in the local time currently in
|
|
effect, the change to the other time occurs. If omitted, the default is
|
|
@code{02:00:00}.
|
|
|
|
For example, here is how you would specify the Eastern time zone in the
|
|
United States, including the appropriate Daylight Saving Time and its dates
|
|
of applicability. The normal offset from UTC is 5 hours; since this is
|
|
west of the prime meridian, the sign is positive. Summer time begins on
|
|
the first Sunday in April at 2:00am, and ends on the last Sunday in October
|
|
at 2:00am.
|
|
|
|
@smallexample
|
|
EST+5EDT,M4.1.0/2,M10.5.0/2
|
|
@end smallexample
|
|
|
|
The schedule of Daylight Saving Time in any particular jurisdiction has
|
|
changed over the years. To be strictly correct, the conversion of dates
|
|
and times in the past should be based on the schedule that was in effect
|
|
then. However, this format has no facilities to let you specify how the
|
|
schedule has changed from year to year. The most you can do is specify
|
|
one particular schedule---usually the present day schedule---and this is
|
|
used to convert any date, no matter when. For precise time zone
|
|
specifications, it is best to use the time zone information database
|
|
(see below).
|
|
|
|
The third format looks like this:
|
|
|
|
@smallexample
|
|
:@var{characters}
|
|
@end smallexample
|
|
|
|
Each operating system interprets this format differently; in the GNU C
|
|
library, @var{characters} is the name of a file which describes the time
|
|
zone.
|
|
|
|
@pindex /etc/localtime
|
|
@pindex localtime
|
|
If the @code{TZ} environment variable does not have a value, the
|
|
operation chooses a time zone by default. In the GNU C library, the
|
|
default time zone is like the specification @samp{TZ=:/etc/localtime}
|
|
(or @samp{TZ=:/usr/local/etc/localtime}, depending on how GNU C library
|
|
was configured; @pxref{Installation}). Other C libraries use their own
|
|
rule for choosing the default time zone, so there is little we can say
|
|
about them.
|
|
|
|
@cindex time zone database
|
|
@pindex /share/lib/zoneinfo
|
|
@pindex zoneinfo
|
|
If @var{characters} begins with a slash, it is an absolute file name;
|
|
otherwise the library looks for the file
|
|
@w{@file{/share/lib/zoneinfo/@var{characters}}}. The @file{zoneinfo}
|
|
directory contains data files describing local time zones in many
|
|
different parts of the world. The names represent major cities, with
|
|
subdirectories for geographical areas; for example,
|
|
@file{America/New_York}, @file{Europe/London}, @file{Asia/Hong_Kong}.
|
|
These data files are installed by the system administrator, who also
|
|
sets @file{/etc/localtime} to point to the data file for the local time
|
|
zone. The GNU C library comes with a large database of time zone
|
|
information for most regions of the world, which is maintained by a
|
|
community of volunteers and put in the public domain.
|
|
|
|
@node Time Zone Functions
|
|
@subsection Functions and Variables for Time Zones
|
|
|
|
@comment time.h
|
|
@comment POSIX.1
|
|
@deftypevar {char *} tzname [2]
|
|
The array @code{tzname} contains two strings, which are the standard
|
|
names of the pair of time zones (standard and Daylight
|
|
Saving) that the user has selected. @code{tzname[0]} is the name of
|
|
the standard time zone (for example, @code{"EST"}), and @code{tzname[1]}
|
|
is the name for the time zone when Daylight Saving Time is in use (for
|
|
example, @code{"EDT"}). These correspond to the @var{std} and @var{dst}
|
|
strings (respectively) from the @code{TZ} environment variable. If
|
|
Daylight Saving Time is never used, @code{tzname[1]} is the empty string.
|
|
|
|
The @code{tzname} array is initialized from the @code{TZ} environment
|
|
variable whenever @code{tzset}, @code{ctime}, @code{strftime},
|
|
@code{mktime}, or @code{localtime} is called. If multiple abbreviations
|
|
have been used (e.g. @code{"EWT"} and @code{"EDT"} for U.S. Eastern War
|
|
Time and Eastern Daylight Time), the array contains the most recent
|
|
abbreviation.
|
|
|
|
The @code{tzname} array is required for POSIX.1 compatibility, but in
|
|
GNU programs it is better to use the @code{tm_zone} member of the
|
|
broken-down time structure, since @code{tm_zone} reports the correct
|
|
abbreviation even when it is not the latest one.
|
|
|
|
Though the strings are declared as @code{char *} the user must refrain
|
|
from modifying these strings. Modifying the strings will almost certainly
|
|
lead to trouble.
|
|
|
|
@end deftypevar
|
|
|
|
@comment time.h
|
|
@comment POSIX.1
|
|
@deftypefun void tzset (void)
|
|
The @code{tzset} function initializes the @code{tzname} variable from
|
|
the value of the @code{TZ} environment variable. It is not usually
|
|
necessary for your program to call this function, because it is called
|
|
automatically when you use the other time conversion functions that
|
|
depend on the time zone.
|
|
@end deftypefun
|
|
|
|
The following variables are defined for compatibility with System V
|
|
Unix. Like @code{tzname}, these variables are set by calling
|
|
@code{tzset} or the other time conversion functions.
|
|
|
|
@comment time.h
|
|
@comment SVID
|
|
@deftypevar {long int} timezone
|
|
This contains the difference between UTC and the latest local standard
|
|
time, in seconds west of UTC. For example, in the U.S. Eastern time
|
|
zone, the value is @code{5*60*60}. Unlike the @code{tm_gmtoff} member
|
|
of the broken-down time structure, this value is not adjusted for
|
|
daylight saving, and its sign is reversed. In GNU programs it is better
|
|
to use @code{tm_gmtoff}, since it contains the correct offset even when
|
|
it is not the latest one.
|
|
@end deftypevar
|
|
|
|
@comment time.h
|
|
@comment SVID
|
|
@deftypevar int daylight
|
|
This variable has a nonzero value if Daylight Saving Time rules apply.
|
|
A nonzero value does not necessarily mean that Daylight Saving Time is
|
|
now in effect; it means only that Daylight Saving Time is sometimes in
|
|
effect.
|
|
@end deftypevar
|
|
|
|
@node Time Functions Example
|
|
@subsection Time Functions Example
|
|
|
|
Here is an example program showing the use of some of the calendar time
|
|
functions.
|
|
|
|
@smallexample
|
|
@include strftim.c.texi
|
|
@end smallexample
|
|
|
|
It produces output like this:
|
|
|
|
@smallexample
|
|
Wed Jul 31 13:02:36 1991
|
|
Today is Wednesday, July 31.
|
|
The time is 01:02 PM.
|
|
@end smallexample
|
|
|
|
|
|
@node Setting an Alarm
|
|
@section Setting an Alarm
|
|
|
|
The @code{alarm} and @code{setitimer} functions provide a mechanism for a
|
|
process to interrupt itself at some future time. They do this by setting a
|
|
timer; when the timer expires, the process receives a signal.
|
|
|
|
@cindex setting an alarm
|
|
@cindex interval timer, setting
|
|
@cindex alarms, setting
|
|
@cindex timers, setting
|
|
Each process has three independent interval timers available:
|
|
|
|
@itemize @bullet
|
|
@item
|
|
A real-time timer that counts clock time. This timer sends a
|
|
@code{SIGALRM} signal to the process when it expires.
|
|
@cindex real-time timer
|
|
@cindex timer, real-time
|
|
|
|
@item
|
|
A virtual timer that counts CPU time used by the process. This timer
|
|
sends a @code{SIGVTALRM} signal to the process when it expires.
|
|
@cindex virtual timer
|
|
@cindex timer, virtual
|
|
|
|
@item
|
|
A profiling timer that counts both CPU time used by the process, and CPU
|
|
time spent in system calls on behalf of the process. This timer sends a
|
|
@code{SIGPROF} signal to the process when it expires.
|
|
@cindex profiling timer
|
|
@cindex timer, profiling
|
|
|
|
This timer is useful for profiling in interpreters. The interval timer
|
|
mechanism does not have the fine granularity necessary for profiling
|
|
native code.
|
|
@c @xref{profil} !!!
|
|
@end itemize
|
|
|
|
You can only have one timer of each kind set at any given time. If you
|
|
set a timer that has not yet expired, that timer is simply reset to the
|
|
new value.
|
|
|
|
You should establish a handler for the appropriate alarm signal using
|
|
@code{signal} or @code{sigaction} before issuing a call to
|
|
@code{setitimer} or @code{alarm}. Otherwise, an unusual chain of events
|
|
could cause the timer to expire before your program establishes the
|
|
handler. In this case it would be terminated, since termination is the
|
|
default action for the alarm signals. @xref{Signal Handling}.
|
|
|
|
The @code{setitimer} function is the primary means for setting an alarm.
|
|
This facility is declared in the header file @file{sys/time.h}. The
|
|
@code{alarm} function, declared in @file{unistd.h}, provides a somewhat
|
|
simpler interface for setting the real-time timer.
|
|
@pindex unistd.h
|
|
@pindex sys/time.h
|
|
|
|
@comment sys/time.h
|
|
@comment BSD
|
|
@deftp {Data Type} {struct itimerval}
|
|
This structure is used to specify when a timer should expire. It contains
|
|
the following members:
|
|
@table @code
|
|
@item struct timeval it_interval
|
|
This is the interval between successive timer interrupts. If zero, the
|
|
alarm will only be sent once.
|
|
|
|
@item struct timeval it_value
|
|
This is the interval to the first timer interrupt. If zero, the alarm is
|
|
disabled.
|
|
@end table
|
|
|
|
The @code{struct timeval} data type is described in @ref{High-Resolution
|
|
Calendar}.
|
|
@end deftp
|
|
|
|
@comment sys/time.h
|
|
@comment BSD
|
|
@deftypefun int setitimer (int @var{which}, struct itimerval *@var{new}, struct itimerval *@var{old})
|
|
The @code{setitimer} function sets the timer specified by @var{which}
|
|
according to @var{new}. The @var{which} argument can have a value of
|
|
@code{ITIMER_REAL}, @code{ITIMER_VIRTUAL}, or @code{ITIMER_PROF}.
|
|
|
|
If @var{old} is not a null pointer, @code{setitimer} returns information
|
|
about any previous unexpired timer of the same kind in the structure it
|
|
points to.
|
|
|
|
The return value is @code{0} on success and @code{-1} on failure. The
|
|
following @code{errno} error conditions are defined for this function:
|
|
|
|
@table @code
|
|
@item EINVAL
|
|
The timer interval was too large.
|
|
@end table
|
|
@end deftypefun
|
|
|
|
@comment sys/time.h
|
|
@comment BSD
|
|
@deftypefun int getitimer (int @var{which}, struct itimerval *@var{old})
|
|
The @code{getitimer} function stores information about the timer specified
|
|
by @var{which} in the structure pointed at by @var{old}.
|
|
|
|
The return value and error conditions are the same as for @code{setitimer}.
|
|
@end deftypefun
|
|
|
|
@comment sys/time.h
|
|
@comment BSD
|
|
@table @code
|
|
@item ITIMER_REAL
|
|
@findex ITIMER_REAL
|
|
This constant can be used as the @var{which} argument to the
|
|
@code{setitimer} and @code{getitimer} functions to specify the real-time
|
|
timer.
|
|
|
|
@comment sys/time.h
|
|
@comment BSD
|
|
@item ITIMER_VIRTUAL
|
|
@findex ITIMER_VIRTUAL
|
|
This constant can be used as the @var{which} argument to the
|
|
@code{setitimer} and @code{getitimer} functions to specify the virtual
|
|
timer.
|
|
|
|
@comment sys/time.h
|
|
@comment BSD
|
|
@item ITIMER_PROF
|
|
@findex ITIMER_PROF
|
|
This constant can be used as the @var{which} argument to the
|
|
@code{setitimer} and @code{getitimer} functions to specify the profiling
|
|
timer.
|
|
@end table
|
|
|
|
@comment unistd.h
|
|
@comment POSIX.1
|
|
@deftypefun {unsigned int} alarm (unsigned int @var{seconds})
|
|
The @code{alarm} function sets the real-time timer to expire in
|
|
@var{seconds} seconds. If you want to cancel any existing alarm, you
|
|
can do this by calling @code{alarm} with a @var{seconds} argument of
|
|
zero.
|
|
|
|
The return value indicates how many seconds remain before the previous
|
|
alarm would have been sent. If there is no previous alarm, @code{alarm}
|
|
returns zero.
|
|
@end deftypefun
|
|
|
|
The @code{alarm} function could be defined in terms of @code{setitimer}
|
|
like this:
|
|
|
|
@smallexample
|
|
unsigned int
|
|
alarm (unsigned int seconds)
|
|
@{
|
|
struct itimerval old, new;
|
|
new.it_interval.tv_usec = 0;
|
|
new.it_interval.tv_sec = 0;
|
|
new.it_value.tv_usec = 0;
|
|
new.it_value.tv_sec = (long int) seconds;
|
|
if (setitimer (ITIMER_REAL, &new, &old) < 0)
|
|
return 0;
|
|
else
|
|
return old.it_value.tv_sec;
|
|
@}
|
|
@end smallexample
|
|
|
|
There is an example showing the use of the @code{alarm} function in
|
|
@ref{Handler Returns}.
|
|
|
|
If you simply want your process to wait for a given number of seconds,
|
|
you should use the @code{sleep} function. @xref{Sleeping}.
|
|
|
|
You shouldn't count on the signal arriving precisely when the timer
|
|
expires. In a multiprocessing environment there is typically some
|
|
amount of delay involved.
|
|
|
|
@strong{Portability Note:} The @code{setitimer} and @code{getitimer}
|
|
functions are derived from BSD Unix, while the @code{alarm} function is
|
|
specified by the POSIX.1 standard. @code{setitimer} is more powerful than
|
|
@code{alarm}, but @code{alarm} is more widely used.
|
|
|
|
@node Sleeping
|
|
@section Sleeping
|
|
|
|
The function @code{sleep} gives a simple way to make the program wait
|
|
for short periods of time. If your program doesn't use signals (except
|
|
to terminate), then you can expect @code{sleep} to wait reliably for
|
|
the specified amount of time. Otherwise, @code{sleep} can return sooner
|
|
if a signal arrives; if you want to wait for a given period regardless
|
|
of signals, use @code{select} (@pxref{Waiting for I/O}) and don't
|
|
specify any descriptors to wait for.
|
|
@c !!! select can get EINTR; using SA_RESTART makes sleep win too.
|
|
|
|
@comment unistd.h
|
|
@comment POSIX.1
|
|
@deftypefun {unsigned int} sleep (unsigned int @var{seconds})
|
|
The @code{sleep} function waits for @var{seconds} or until a signal
|
|
is delivered, whichever happens first.
|
|
|
|
If @code{sleep} function returns because the requested time has
|
|
elapsed, it returns a value of zero. If it returns because of delivery
|
|
of a signal, its return value is the remaining time in the sleep period.
|
|
|
|
The @code{sleep} function is declared in @file{unistd.h}.
|
|
@end deftypefun
|
|
|
|
Resist the temptation to implement a sleep for a fixed amount of time by
|
|
using the return value of @code{sleep}, when nonzero, to call
|
|
@code{sleep} again. This will work with a certain amount of accuracy as
|
|
long as signals arrive infrequently. But each signal can cause the
|
|
eventual wakeup time to be off by an additional second or so. Suppose a
|
|
few signals happen to arrive in rapid succession by bad luck---there is
|
|
no limit on how much this could shorten or lengthen the wait.
|
|
|
|
Instead, compute the time at which the program should stop waiting, and
|
|
keep trying to wait until that time. This won't be off by more than a
|
|
second. With just a little more work, you can use @code{select} and
|
|
make the waiting period quite accurate. (Of course, heavy system load
|
|
can cause additional unavoidable delays---unless the machine is
|
|
dedicated to one application, there is no way you can avoid this.)
|
|
|
|
On some systems, @code{sleep} can do strange things if your program uses
|
|
@code{SIGALRM} explicitly. Even if @code{SIGALRM} signals are being
|
|
ignored or blocked when @code{sleep} is called, @code{sleep} might
|
|
return prematurely on delivery of a @code{SIGALRM} signal. If you have
|
|
established a handler for @code{SIGALRM} signals and a @code{SIGALRM}
|
|
signal is delivered while the process is sleeping, the action taken
|
|
might be just to cause @code{sleep} to return instead of invoking your
|
|
handler. And, if @code{sleep} is interrupted by delivery of a signal
|
|
whose handler requests an alarm or alters the handling of @code{SIGALRM},
|
|
this handler and @code{sleep} will interfere.
|
|
|
|
On the GNU system, it is safe to use @code{sleep} and @code{SIGALRM} in
|
|
the same program, because @code{sleep} does not work by means of
|
|
@code{SIGALRM}.
|
|
|
|
@comment time.h
|
|
@comment POSIX.1
|
|
@deftypefun int nanosleep (const struct timespec *@var{requested_time}, struct timespec *@var{remaining})
|
|
If resolution to seconds is not enough the @code{nanosleep} function
|
|
can be used. As the name suggests the sleeping period can be specified
|
|
in nanoseconds. The actual period of waiting time might be longer since
|
|
the requested time in the @var{requested_time} parameter is rounded up
|
|
to the next integer multiple of the actual resolution of the system.
|
|
|
|
If the function returns because the time has elapsed the return value is
|
|
zero. If the function returns @math{-1} the global variable @var{errno}
|
|
is set to the following values:
|
|
|
|
@table @code
|
|
@item EINTR
|
|
The call was interrupted because a signal was delivered to the thread.
|
|
If the @var{remaining} parameter is not the null pointer the structure
|
|
pointed to by @var{remaining} is updated to contain the remaining time.
|
|
|
|
@item EINVAL
|
|
The nanosecond value in the @var{requested_time} parameter contains an
|
|
illegal value. Either the value is negative or greater than or equal to
|
|
1000 million.
|
|
@end table
|
|
|
|
This function is a cancellation point in multi-threaded programs. This
|
|
is a problem if the thread allocates some resources (like memory, file
|
|
descriptors, semaphores or whatever) at the time @code{nanosleep} is
|
|
called. If the thread gets canceled these resources stay allocated
|
|
until the program ends. To avoid this calls to @code{nanosleep} should
|
|
be protected using cancellation handlers.
|
|
@c ref pthread_cleanup_push / pthread_cleanup_pop
|
|
|
|
The @code{nanosleep} function is declared in @file{time.h}.
|
|
@end deftypefun
|