mirror of
https://sourceware.org/git/glibc.git
synced 2024-11-10 15:20:10 +00:00
4cfd80263a
[BZ #2636] * manual/time.texi (Processor Time): Return type of times is elapsed real time since an arbitrary point in the past. (CPU Time): Move CLK_TCK from here... (Processor Time): ...to here. Correct description. * manual/conf.texi (Constants for Sysconf): Correct description of _SC_CLK_TCK.
2515 lines
92 KiB
Plaintext
2515 lines
92 KiB
Plaintext
@node Date and Time, Resource Usage And Limitation, Arithmetic, Top
|
|
@c %MENU% Functions for getting the date and time and formatting them nicely
|
|
@chapter Date and Time
|
|
|
|
This chapter describes functions for manipulating dates and times,
|
|
including functions for determining what time it is and conversion
|
|
between different time representations.
|
|
|
|
@menu
|
|
* Time Basics:: Concepts and definitions.
|
|
* Elapsed Time:: Data types to represent elapsed times
|
|
* Processor And CPU Time:: Time a program has spent executing.
|
|
* Calendar Time:: Manipulation of ``real'' dates and times.
|
|
* Setting an Alarm:: Sending a signal after a specified time.
|
|
* Sleeping:: Waiting for a period of time.
|
|
@end menu
|
|
|
|
|
|
@node Time Basics
|
|
@section Time Basics
|
|
@cindex time
|
|
|
|
Discussing time in a technical manual can be difficult because the word
|
|
``time'' in English refers to lots of different things. In this manual,
|
|
we use a rigorous terminology to avoid confusion, and the only thing we
|
|
use the simple word ``time'' for is to talk about the abstract concept.
|
|
|
|
A @dfn{calendar time} is a point in the time continuum, for example
|
|
November 4, 1990 at 18:02.5 UTC. Sometimes this is called ``absolute
|
|
time''.
|
|
@cindex calendar time
|
|
|
|
We don't speak of a ``date'', because that is inherent in a calendar
|
|
time.
|
|
@cindex date
|
|
|
|
An @dfn{interval} is a contiguous part of the time continuum between two
|
|
calendar times, for example the hour between 9:00 and 10:00 on July 4,
|
|
1980.
|
|
@cindex interval
|
|
|
|
An @dfn{elapsed time} is the length of an interval, for example, 35
|
|
minutes. People sometimes sloppily use the word ``interval'' to refer
|
|
to the elapsed time of some interval.
|
|
@cindex elapsed time
|
|
@cindex time, elapsed
|
|
|
|
An @dfn{amount of time} is a sum of elapsed times, which need not be of
|
|
any specific intervals. For example, the amount of time it takes to
|
|
read a book might be 9 hours, independently of when and in how many
|
|
sittings it is read.
|
|
|
|
A @dfn{period} is the elapsed time of an interval between two events,
|
|
especially when they are part of a sequence of regularly repeating
|
|
events.
|
|
@cindex period of time
|
|
|
|
@dfn{CPU time} is like calendar time, except that it is based on the
|
|
subset of the time continuum when a particular process is actively
|
|
using a CPU. CPU time is, therefore, relative to a process.
|
|
@cindex CPU time
|
|
|
|
@dfn{Processor time} is an amount of time that a CPU is in use. In
|
|
fact, it's a basic system resource, since there's a limit to how much
|
|
can exist in any given interval (that limit is the elapsed time of the
|
|
interval times the number of CPUs in the processor). People often call
|
|
this CPU time, but we reserve the latter term in this manual for the
|
|
definition above.
|
|
@cindex processor time
|
|
|
|
@node Elapsed Time
|
|
@section Elapsed Time
|
|
@cindex elapsed time
|
|
|
|
One way to represent an elapsed time is with a simple arithmetic data
|
|
type, as with the following function to compute the elapsed time between
|
|
two calendar times. This function is declared in @file{time.h}.
|
|
|
|
@comment time.h
|
|
@comment ISO
|
|
@deftypefun double difftime (time_t @var{time1}, time_t @var{time0})
|
|
The @code{difftime} function returns the number of seconds of elapsed
|
|
time between calendar time @var{time1} and calendar time @var{time0}, as
|
|
a value of type @code{double}. The difference ignores leap seconds
|
|
unless leap second support is enabled.
|
|
|
|
In @theglibc{}, you can simply subtract @code{time_t} values. But on
|
|
other systems, the @code{time_t} data type might use some other encoding
|
|
where subtraction doesn't work directly.
|
|
@end deftypefun
|
|
|
|
@Theglibc{} provides two data types specifically for representing
|
|
an elapsed time. They are used by various @glibcadj{} functions, and
|
|
you can use them for your own purposes too. They're exactly the same
|
|
except that one has a resolution in microseconds, and the other, newer
|
|
one, is in nanoseconds.
|
|
|
|
@comment sys/time.h
|
|
@comment BSD
|
|
@deftp {Data Type} {struct timeval}
|
|
@cindex timeval
|
|
The @code{struct timeval} structure represents an elapsed time. It is
|
|
declared in @file{sys/time.h} and has the following members:
|
|
|
|
@table @code
|
|
@item long int tv_sec
|
|
This represents the number of whole seconds of elapsed time.
|
|
|
|
@item long int tv_usec
|
|
This is the rest of the elapsed time (a fraction of a second),
|
|
represented as the number of microseconds. It is always less than one
|
|
million.
|
|
|
|
@end table
|
|
@end deftp
|
|
|
|
@comment sys/time.h
|
|
@comment POSIX.1
|
|
@deftp {Data Type} {struct timespec}
|
|
@cindex timespec
|
|
The @code{struct timespec} structure represents an elapsed time. It is
|
|
declared in @file{time.h} and has the following members:
|
|
|
|
@table @code
|
|
@item long int tv_sec
|
|
This represents the number of whole seconds of elapsed time.
|
|
|
|
@item long int tv_nsec
|
|
This is the rest of the elapsed time (a fraction of a second),
|
|
represented as the number of nanoseconds. It is always less than one
|
|
billion.
|
|
|
|
@end table
|
|
@end deftp
|
|
|
|
It is often necessary to subtract two values of type @w{@code{struct
|
|
timeval}} or @w{@code{struct timespec}}. Here is the best way to do
|
|
this. It works even on some peculiar operating systems where the
|
|
@code{tv_sec} member has an unsigned type.
|
|
|
|
@smallexample
|
|
@include timeval_subtract.c.texi
|
|
@end smallexample
|
|
|
|
Common functions that use @code{struct timeval} are @code{gettimeofday}
|
|
and @code{settimeofday}.
|
|
|
|
|
|
There are no @glibcadj{} functions specifically oriented toward
|
|
dealing with elapsed times, but the calendar time, processor time, and
|
|
alarm and sleeping functions have a lot to do with them.
|
|
|
|
|
|
@node Processor And CPU Time
|
|
@section Processor And CPU Time
|
|
|
|
If you're trying to optimize your program or measure its efficiency,
|
|
it's very useful to know how much processor time it uses. For that,
|
|
calendar time and elapsed times are useless because a process may spend
|
|
time waiting for I/O or for other processes to use the CPU. However,
|
|
you can get the information with the functions in this section.
|
|
|
|
CPU time (@pxref{Time Basics}) is represented by the data type
|
|
@code{clock_t}, which is a number of @dfn{clock ticks}. It gives the
|
|
total amount of time a process has actively used a CPU since some
|
|
arbitrary event. On @gnusystems{}, that event is the creation of the
|
|
process. While arbitrary in general, the event is always the same event
|
|
for any particular process, so you can always measure how much time on
|
|
the CPU a particular computation takes by examining the process' CPU
|
|
time before and after the computation.
|
|
@cindex CPU time
|
|
@cindex clock ticks
|
|
@cindex ticks, clock
|
|
|
|
On @gnulinuxhurdsystems{}, @code{clock_t} is equivalent to @code{long int} and
|
|
@code{CLOCKS_PER_SEC} is an integer value. But in other systems, both
|
|
@code{clock_t} and the macro @code{CLOCKS_PER_SEC} can be either integer
|
|
or floating-point types. Casting CPU time values to @code{double}, as
|
|
in the example above, makes sure that operations such as arithmetic and
|
|
printing work properly and consistently no matter what the underlying
|
|
representation is.
|
|
|
|
Note that the clock can wrap around. On a 32bit system with
|
|
@code{CLOCKS_PER_SEC} set to one million this function will return the
|
|
same value approximately every 72 minutes.
|
|
|
|
For additional functions to examine a process' use of processor time,
|
|
and to control it, see @ref{Resource Usage And Limitation}.
|
|
|
|
|
|
@menu
|
|
* CPU Time:: The @code{clock} function.
|
|
* Processor Time:: The @code{times} function.
|
|
@end menu
|
|
|
|
@node CPU Time
|
|
@subsection CPU Time Inquiry
|
|
|
|
To get a process' CPU time, you can use the @code{clock} function. This
|
|
facility is declared in the header file @file{time.h}.
|
|
@pindex time.h
|
|
|
|
In typical usage, you call the @code{clock} function at the beginning
|
|
and end of the interval you want to time, subtract the values, and then
|
|
divide by @code{CLOCKS_PER_SEC} (the number of clock ticks per second)
|
|
to get processor time, like this:
|
|
|
|
@smallexample
|
|
@group
|
|
#include <time.h>
|
|
|
|
clock_t start, end;
|
|
double cpu_time_used;
|
|
|
|
start = clock();
|
|
@dots{} /* @r{Do the work.} */
|
|
end = clock();
|
|
cpu_time_used = ((double) (end - start)) / CLOCKS_PER_SEC;
|
|
@end group
|
|
@end smallexample
|
|
|
|
Do not use a single CPU time as an amount of time; it doesn't work that
|
|
way. Either do a subtraction as shown above or query processor time
|
|
directly. @xref{Processor Time}.
|
|
|
|
Different computers and operating systems vary wildly in how they keep
|
|
track of CPU time. It's common for the internal processor clock
|
|
to have a resolution somewhere between a hundredth and millionth of a
|
|
second.
|
|
|
|
@comment time.h
|
|
@comment ISO
|
|
@deftypevr Macro int CLOCKS_PER_SEC
|
|
The value of this macro is the number of clock ticks per second measured
|
|
by the @code{clock} function. POSIX requires that this value be one
|
|
million independent of the actual resolution.
|
|
@end deftypevr
|
|
|
|
@comment time.h
|
|
@comment ISO
|
|
@deftp {Data Type} clock_t
|
|
This is the type of the value returned by the @code{clock} function.
|
|
Values of type @code{clock_t} are numbers of clock ticks.
|
|
@end deftp
|
|
|
|
@comment time.h
|
|
@comment ISO
|
|
@deftypefun clock_t clock (void)
|
|
This function returns the calling process' current CPU time. If the CPU
|
|
time is not available or cannot be represented, @code{clock} returns the
|
|
value @code{(clock_t)(-1)}.
|
|
@end deftypefun
|
|
|
|
|
|
@node Processor Time
|
|
@subsection Processor Time Inquiry
|
|
|
|
The @code{times} function returns information about a process'
|
|
consumption of processor time in a @w{@code{struct tms}} object, in
|
|
addition to the process' CPU time. @xref{Time Basics}. You should
|
|
include the header file @file{sys/times.h} to use this facility.
|
|
@cindex processor time
|
|
@cindex CPU time
|
|
@pindex sys/times.h
|
|
|
|
@comment sys/times.h
|
|
@comment POSIX.1
|
|
@deftp {Data Type} {struct tms}
|
|
The @code{tms} structure is used to return information about process
|
|
times. It contains at least the following members:
|
|
|
|
@table @code
|
|
@item clock_t tms_utime
|
|
This is the total processor time the calling process has used in
|
|
executing the instructions of its program.
|
|
|
|
@item clock_t tms_stime
|
|
This is the processor time the system has used on behalf of the calling
|
|
process.
|
|
|
|
@item clock_t tms_cutime
|
|
This is the sum of the @code{tms_utime} values and the @code{tms_cutime}
|
|
values of all terminated child processes of the calling process, whose
|
|
status has been reported to the parent process by @code{wait} or
|
|
@code{waitpid}; see @ref{Process Completion}. In other words, it
|
|
represents the total processor time used in executing the instructions
|
|
of all the terminated child processes of the calling process, excluding
|
|
child processes which have not yet been reported by @code{wait} or
|
|
@code{waitpid}.
|
|
@cindex child process
|
|
|
|
@item clock_t tms_cstime
|
|
This is similar to @code{tms_cutime}, but represents the total processor
|
|
time system has used on behalf of all the terminated child processes
|
|
of the calling process.
|
|
@end table
|
|
|
|
All of the times are given in numbers of clock ticks. Unlike CPU time,
|
|
these are the actual amounts of time; not relative to any event.
|
|
@xref{Creating a Process}.
|
|
@end deftp
|
|
|
|
@comment time.h
|
|
@comment POSIX.1
|
|
@deftypevr Macro int CLK_TCK
|
|
This is an obsolete name for the number of clock ticks per second. Use
|
|
@code{sysconf (_SC_CLK_TCK)} instead.
|
|
@end deftypevr
|
|
|
|
@comment sys/times.h
|
|
@comment POSIX.1
|
|
@deftypefun clock_t times (struct tms *@var{buffer})
|
|
The @code{times} function stores the processor time information for
|
|
the calling process in @var{buffer}.
|
|
|
|
The return value is the number of clock ticks since an arbitrary point
|
|
in the past, e.g. since system start-up. @code{times} returns
|
|
@code{(clock_t)(-1)} to indicate failure.
|
|
@end deftypefun
|
|
|
|
@strong{Portability Note:} The @code{clock} function described in
|
|
@ref{CPU Time} is specified by the @w{ISO C} standard. The
|
|
@code{times} function is a feature of POSIX.1. On @gnusystems{}, the
|
|
CPU time is defined to be equivalent to the sum of the @code{tms_utime}
|
|
and @code{tms_stime} fields returned by @code{times}.
|
|
|
|
@node Calendar Time
|
|
@section Calendar Time
|
|
|
|
This section describes facilities for keeping track of calendar time.
|
|
@xref{Time Basics}.
|
|
|
|
@Theglibc{} represents calendar time three ways:
|
|
|
|
@itemize @bullet
|
|
@item
|
|
@dfn{Simple time} (the @code{time_t} data type) is a compact
|
|
representation, typically giving the number of seconds of elapsed time
|
|
since some implementation-specific base time.
|
|
@cindex simple time
|
|
|
|
@item
|
|
There is also a "high-resolution time" representation. Like simple
|
|
time, this represents a calendar time as an elapsed time since a base
|
|
time, but instead of measuring in whole seconds, it uses a @code{struct
|
|
timeval} data type, which includes fractions of a second. Use this time
|
|
representation instead of simple time when you need greater precision.
|
|
@cindex high-resolution time
|
|
|
|
@item
|
|
@dfn{Local time} or @dfn{broken-down time} (the @code{struct tm} data
|
|
type) represents a calendar time as a set of components specifying the
|
|
year, month, and so on in the Gregorian calendar, for a specific time
|
|
zone. This calendar time representation is usually used only to
|
|
communicate with people.
|
|
@cindex local time
|
|
@cindex broken-down time
|
|
@cindex Gregorian calendar
|
|
@cindex calendar, Gregorian
|
|
@end itemize
|
|
|
|
@menu
|
|
* Simple Calendar Time:: Facilities for manipulating calendar time.
|
|
* High-Resolution Calendar:: A time representation with greater precision.
|
|
* Broken-down Time:: Facilities for manipulating local time.
|
|
* High Accuracy Clock:: Maintaining a high accuracy system clock.
|
|
* Formatting Calendar Time:: Converting times to strings.
|
|
* Parsing Date and Time:: Convert textual time and date information back
|
|
into broken-down time values.
|
|
* TZ Variable:: How users specify the time zone.
|
|
* Time Zone Functions:: Functions to examine or specify the time zone.
|
|
* Time Functions Example:: An example program showing use of some of
|
|
the time functions.
|
|
@end menu
|
|
|
|
@node Simple Calendar Time
|
|
@subsection Simple Calendar Time
|
|
|
|
This section describes the @code{time_t} data type for representing calendar
|
|
time as simple time, and the functions which operate on simple time objects.
|
|
These facilities are declared in the header file @file{time.h}.
|
|
@pindex time.h
|
|
|
|
@cindex epoch
|
|
@comment time.h
|
|
@comment ISO
|
|
@deftp {Data Type} time_t
|
|
This is the data type used to represent simple time. Sometimes, it also
|
|
represents an elapsed time. When interpreted as a calendar time value,
|
|
it represents the number of seconds elapsed since 00:00:00 on January 1,
|
|
1970, Coordinated Universal Time. (This calendar time is sometimes
|
|
referred to as the @dfn{epoch}.) POSIX requires that this count not
|
|
include leap seconds, but on some systems this count includes leap seconds
|
|
if you set @code{TZ} to certain values (@pxref{TZ Variable}).
|
|
|
|
Note that a simple time has no concept of local time zone. Calendar
|
|
Time @var{T} is the same instant in time regardless of where on the
|
|
globe the computer is.
|
|
|
|
In @theglibc{}, @code{time_t} is equivalent to @code{long int}.
|
|
In other systems, @code{time_t} might be either an integer or
|
|
floating-point type.
|
|
@end deftp
|
|
|
|
The function @code{difftime} tells you the elapsed time between two
|
|
simple calendar times, which is not always as easy to compute as just
|
|
subtracting. @xref{Elapsed Time}.
|
|
|
|
@comment time.h
|
|
@comment ISO
|
|
@deftypefun time_t time (time_t *@var{result})
|
|
The @code{time} function returns the current calendar time as a value of
|
|
type @code{time_t}. If the argument @var{result} is not a null pointer,
|
|
the calendar time value is also stored in @code{*@var{result}}. If the
|
|
current calendar time is not available, the value
|
|
@w{@code{(time_t)(-1)}} is returned.
|
|
@end deftypefun
|
|
|
|
@c The GNU C library implements stime() with a call to settimeofday() on
|
|
@c Linux.
|
|
@comment time.h
|
|
@comment SVID, XPG
|
|
@deftypefun int stime (time_t *@var{newtime})
|
|
@code{stime} sets the system clock, i.e., it tells the system that the
|
|
current calendar time is @var{newtime}, where @code{newtime} is
|
|
interpreted as described in the above definition of @code{time_t}.
|
|
|
|
@code{settimeofday} is a newer function which sets the system clock to
|
|
better than one second precision. @code{settimeofday} is generally a
|
|
better choice than @code{stime}. @xref{High-Resolution Calendar}.
|
|
|
|
Only the superuser can set the system clock.
|
|
|
|
If the function succeeds, the return value is zero. Otherwise, it is
|
|
@code{-1} and @code{errno} is set accordingly:
|
|
|
|
@table @code
|
|
@item EPERM
|
|
The process is not superuser.
|
|
@end table
|
|
@end deftypefun
|
|
|
|
|
|
|
|
@node High-Resolution Calendar
|
|
@subsection High-Resolution Calendar
|
|
|
|
The @code{time_t} data type used to represent simple times has a
|
|
resolution of only one second. Some applications need more precision.
|
|
|
|
So, @theglibc{} also contains functions which are capable of
|
|
representing calendar times to a higher resolution than one second. The
|
|
functions and the associated data types described in this section are
|
|
declared in @file{sys/time.h}.
|
|
@pindex sys/time.h
|
|
|
|
@comment sys/time.h
|
|
@comment BSD
|
|
@deftp {Data Type} {struct timezone}
|
|
The @code{struct timezone} structure is used to hold minimal information
|
|
about the local time zone. It has the following members:
|
|
|
|
@table @code
|
|
@item int tz_minuteswest
|
|
This is the number of minutes west of UTC.
|
|
|
|
@item int tz_dsttime
|
|
If nonzero, Daylight Saving Time applies during some part of the year.
|
|
@end table
|
|
|
|
The @code{struct timezone} type is obsolete and should never be used.
|
|
Instead, use the facilities described in @ref{Time Zone Functions}.
|
|
@end deftp
|
|
|
|
@comment sys/time.h
|
|
@comment BSD
|
|
@deftypefun int gettimeofday (struct timeval *@var{tp}, struct timezone *@var{tzp})
|
|
The @code{gettimeofday} function returns the current calendar time as
|
|
the elapsed time since the epoch in the @code{struct timeval} structure
|
|
indicated by @var{tp}. (@pxref{Elapsed Time} for a description of
|
|
@code{struct timeval}). Information about the time zone is returned in
|
|
the structure pointed at @var{tzp}. If the @var{tzp} argument is a null
|
|
pointer, time zone information is ignored.
|
|
|
|
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 ENOSYS
|
|
The operating system does not support getting time zone information, and
|
|
@var{tzp} is not a null pointer. @gnusystems{} do not
|
|
support using @w{@code{struct timezone}} to represent time zone
|
|
information; that is an obsolete feature of 4.3 BSD.
|
|
Instead, use the facilities described in @ref{Time Zone Functions}.
|
|
@end table
|
|
@end deftypefun
|
|
|
|
@comment sys/time.h
|
|
@comment BSD
|
|
@deftypefun int settimeofday (const struct timeval *@var{tp}, const struct timezone *@var{tzp})
|
|
The @code{settimeofday} function sets the current calendar time in the
|
|
system clock according to the arguments. As for @code{gettimeofday},
|
|
the calendar time is represented as the elapsed time since the epoch.
|
|
As for @code{gettimeofday}, time zone information is ignored if
|
|
@var{tzp} is a null pointer.
|
|
|
|
You must be a privileged user in order to use @code{settimeofday}.
|
|
|
|
Some kernels automatically set the system clock from some source such as
|
|
a hardware clock when they start up. Others, including Linux, place the
|
|
system clock in an ``invalid'' state (in which attempts to read the clock
|
|
fail). A call of @code{stime} removes the system clock from an invalid
|
|
state, and system startup scripts typically run a program that calls
|
|
@code{stime}.
|
|
|
|
@code{settimeofday} causes a sudden jump forwards or backwards, which
|
|
can cause a variety of problems in a system. Use @code{adjtime} (below)
|
|
to make a smooth transition from one time to another by temporarily
|
|
speeding up or slowing down the clock.
|
|
|
|
With a Linux kernel, @code{adjtimex} does the same thing and can also
|
|
make permanent changes to the speed of the system clock so it doesn't
|
|
need to be corrected as often.
|
|
|
|
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 EPERM
|
|
This process cannot set the clock because it is not privileged.
|
|
|
|
@item ENOSYS
|
|
The operating system does not support setting time zone information, and
|
|
@var{tzp} is not a null pointer.
|
|
@end table
|
|
@end deftypefun
|
|
|
|
@c On Linux, GNU libc implements adjtime() as a call to adjtimex().
|
|
@comment sys/time.h
|
|
@comment BSD
|
|
@deftypefun int adjtime (const struct timeval *@var{delta}, struct timeval *@var{olddelta})
|
|
This function speeds up or slows down the system clock in order to make
|
|
a gradual adjustment. This ensures that the calendar time reported by
|
|
the system clock is always monotonically increasing, which might not
|
|
happen if you simply set the clock.
|
|
|
|
The @var{delta} argument specifies a relative adjustment to be made to
|
|
the clock time. If negative, the system clock is slowed down for a
|
|
while until it has lost this much elapsed time. If positive, the system
|
|
clock is speeded up for a while.
|
|
|
|
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 by the usual @glibcadj{} functions as an
|
|
elapsed time since a fixed base calendar time. This is convenient for
|
|
computation, but has no relation to the way people normally think of
|
|
calendar time. By contrast, @dfn{broken-down time} is a binary
|
|
representation of calendar time 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 information.
|
|
|
|
A broken-down time value is always relative to a choice of time
|
|
zone, and it also indicates which time zone that is.
|
|
|
|
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 full seconds since the top of the minute (normally
|
|
in the range @code{0} through @code{59}, but 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 full minutes since the top of the hour (in the
|
|
range @code{0} through @code{59}).
|
|
|
|
@item int tm_hour
|
|
This is the number of full hours past midnight (in the range @code{0} through
|
|
@code{23}).
|
|
|
|
@item int tm_mday
|
|
This is the ordinal day of the month (in the range @code{1} through @code{31}).
|
|
Watch out for this one! As the only ordinal number in the structure, it is
|
|
inconsistent with the rest of the structure.
|
|
|
|
@item int tm_mon
|
|
This is the number of full calendar months since the beginning of the
|
|
year (in the range @code{0} through @code{11}). Watch out for this one!
|
|
People usually use ordinal numbers for month-of-year (where January = 1).
|
|
|
|
@item int tm_year
|
|
This is the number of full calendar years since 1900.
|
|
|
|
@item int tm_wday
|
|
This is the number of full days since Sunday (in the range @code{0} through
|
|
@code{6}).
|
|
|
|
@item int tm_yday
|
|
This is the number of full days since the beginning of the year (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 for information about the system clock. It
|
|
contains the following members:
|
|
@table @code
|
|
@item struct timeval time
|
|
This is the current calendar time, expressed as the elapsed time since
|
|
the epoch. The @code{struct timeval} data type is described in
|
|
@ref{Elapsed Time}.
|
|
|
|
@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
|
|
system clock from the true calendar 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 system clock from the true
|
|
calendar 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
|
|
calendar time and the system 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 calendar time.
|
|
|
|
@item long int tick
|
|
The elapsed 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 system clock. The value is expressed in scaled PPM and it denotes
|
|
the difference in frequency between the system 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 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 @theglibc{} did not
|
|
have this function but did have the synonymous @code{adjtimex}.
|
|
|
|
@end deftypefun
|
|
|
|
|
|
@node Formatting Calendar Time
|
|
@subsection Formatting Calendar Time
|
|
|
|
The functions described in this section format calendar 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 you
|
|
specify the calendar time argument 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}, but 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 were first standardized by
|
|
POSIX.2-1992 and by @w{ISO C99}, 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.
|
|
|
|
Using @code{%B} together with @code{%d} produces grammatically
|
|
incorrect results for some locales.
|
|
|
|
@item %c
|
|
The preferred calendar 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 was first standardized by POSIX.2-1992 and by @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 was first standardized by POSIX.2-1992 and by @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 was first standardized by POSIX.2-1992 and by @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 was first standardized by @w{ISO C99} and by POSIX.1-2001.
|
|
|
|
@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 first standardized by @w{ISO C99} and by POSIX.1-2001.
|
|
|
|
@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 first standardized by @w{ISO C99} and by POSIX.1-2001
|
|
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 was first standardized by POSIX.2-1992 and by @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 was first standardized by POSIX.2-1992 and by @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}. In most locales
|
|
@samp{AM}/@samp{PM} format is not supported, in such cases @code{"%p"}
|
|
yields an empty string.
|
|
|
|
@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}. In
|
|
most locales @samp{AM}/@samp{PM} format is not supported, in such cases
|
|
@code{"%P"} yields an empty string.
|
|
|
|
This format is a GNU extension.
|
|
|
|
@item %r
|
|
The complete calendar time using the AM/PM format of the current locale.
|
|
|
|
This format was first standardized by POSIX.2-1992 and by @w{ISO C99}.
|
|
In the POSIX locale, this format is equivalent to @code{%I:%M:%S %p}.
|
|
|
|
@item %R
|
|
The hour and minute in decimal numbers using the format @code{%H:%M}.
|
|
|
|
This format was first standardized by @w{ISO C99} and by POSIX.1-2001
|
|
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 was first standardized by POSIX.2-1992 and by @w{ISO C99}.
|
|
|
|
@item %T
|
|
The time of day using decimal numbers using the format @code{%H:%M:%S}.
|
|
|
|
This format was first standardized by POSIX.2-1992 and by @w{ISO C99}.
|
|
|
|
@item %u
|
|
The day of the week as a decimal number (range @code{1} through
|
|
@code{7}), Monday being @code{1}.
|
|
|
|
This format was first standardized by POSIX.2-1992 and by @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 was first standardized by POSIX.2-1992 and by @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.
|
|
|
|
@item %X
|
|
The preferred time of day representation for the current locale.
|
|
|
|
@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 first standardized by @w{ISO C99} and by POSIX.1-2001
|
|
but was previously available as a GNU extension.
|
|
|
|
In the POSIX locale, 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"@dots{}"} 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
|
|
|
|
The 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 user has to make sure, though, that the input can be parsed in a
|
|
unambiguous way. The string @code{"1999112"} can be parsed using the
|
|
format @code{"%Y%m%d"} as 1999-1-12, 1999-11-2, or even 19991-1-2. It
|
|
is necessary to add appropriate separators to reliably get results.
|
|
|
|
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{60}).
|
|
|
|
Leading zeroes are permitted but not required.
|
|
|
|
@strong{NB:} The Unix specification says the upper bound on this value
|
|
is @code{61}, a result of a decision to allow double leap seconds. You
|
|
will not see the value @code{61} because no minute has more than one
|
|
leap second, 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
|
|
The offset from GMT in @w{ISO 8601}/RFC822 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 whitespace characters in the format string.
|
|
|
|
@strong{Portability Note:} The XPG standard advises applications to use
|
|
at least one whitespace character (as specified by @code{isspace}) or
|
|
other non-alphanumeric characters between any two conversion
|
|
specifications. @Theglibc{} does not have this limitation but
|
|
other libraries might have trouble parsing formats like
|
|
@code{"%d%m%Y%H%M%S"}.
|
|
|
|
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 @glibcadj{} 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 @theglibc{}, 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
|
|
@theglibc{}, @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 @theglibc{}, the
|
|
default time zone is like the specification @samp{TZ=:/etc/localtime}
|
|
(or @samp{TZ=:/usr/local/etc/localtime}, depending on how @theglibc{}
|
|
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. @Theglibc{} 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 in the future. 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 elapsed 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 processor 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 processor time used by the process,
|
|
and processor 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}.
|
|
|
|
To be able to use the alarm function to interrupt a system call which
|
|
might block otherwise indefinitely it is important to @emph{not} set the
|
|
@code{SA_RESTART} flag when registering the signal handler using
|
|
@code{sigaction}. When not using @code{sigaction} things get even
|
|
uglier: the @code{signal} function has to fixed semantics with respect
|
|
to restarts. The BSD semantics for this function is to set the flag.
|
|
Therefore, if @code{sigaction} for whatever reason cannot be used, it is
|
|
necessary to use @code{sysv_signal} and not @code{signal}.
|
|
|
|
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 period between successive timer interrupts. If zero, the
|
|
alarm will only be sent once.
|
|
|
|
@item struct timeval it_value
|
|
This is the period between now and the first timer interrupt. If zero,
|
|
the alarm is disabled.
|
|
@end table
|
|
|
|
The @code{struct timeval} data type is described in @ref{Elapsed Time}.
|
|
@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 period is 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
|
|
@vtable @code
|
|
@item 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
|
|
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
|
|
This constant can be used as the @var{which} argument to the
|
|
@code{setitimer} and @code{getitimer} functions to specify the profiling
|
|
timer.
|
|
@end vtable
|
|
|
|
@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 a short interval. If your program doesn't use signals (except to
|
|
terminate), then you can expect @code{sleep} to wait reliably throughout
|
|
the specified interval. Otherwise, @code{sleep} can return sooner if a
|
|
signal arrives; if you want to wait for a given interval 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 interval is over,
|
|
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 interval.
|
|
|
|
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 calendar time at which the program should stop
|
|
waiting, and keep trying to wait until that calendar 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 @gnusystems{}, 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 sleep interval can be specified in
|
|
nanoseconds. The actual elapsed time of the sleep interval might be
|
|
longer since the system rounds the elapsed time you request up to the
|
|
next integer multiple of the actual resolution the system can deliver.
|
|
|
|
*@code{requested_time} is the elapsed time of the interval you want to
|
|
sleep.
|
|
|
|
The function returns as *@code{remaining} the elapsed time left in the
|
|
interval for which you requested to sleep. If the interval completed
|
|
without getting interrupted by a signal, this is zero.
|
|
|
|
@code{struct timespec} is described in @xref{Elapsed Time}.
|
|
|
|
If the function returns because the interval is over 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
|
|
elapsed 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
|