This chapter describes functions for manipulating dates and times,
including functions for determining what time it is and conversion
between different time representations.
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 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".
We don't speak of a "date", because that is inherent in a calendar
time.
An 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.
An 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.
An 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 period is the elapsed time of an interval between two events,
especially when they are part of a sequence of regularly repeating
events.
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.
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.
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 `time.h'.
The difftime function returns the number of seconds of elapsed
time between calendar time time1 and calendar time time0, as
a value of type double. The difference ignores leap seconds
unless leap second support is enabled.
In the GNU system, you can simply subtract time_t values. But on
other systems, the time_t data type might use some other encoding
where subtraction doesn't work directly.
The GNU C library provides two data types specifically for representing
an elapsed time. They are used by various GNU C library 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.
Data Type:struct timeval
The struct timeval structure represents an elapsed time. It is
declared in `sys/time.h' and has the following members:
long int tv_sec
This represents the number of whole seconds of elapsed time.
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.
Data Type:struct timespec
The struct timespec structure represents an elapsed time. It is
declared in `time.h' and has the following members:
long int tv_sec
This represents the number of whole seconds of elapsed time.
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.
It is often necessary to subtract two values of type struct
timeval or struct timespec. Here is the best way to do
this. It works even on some peculiar operating systems where the
tv_sec member has an unsigned type.
/* Subtract the `struct timeval' values X and Y,
storing the result in RESULT.
Return 1 if the difference is negative, otherwise 0. */
int
timeval_subtract (result, x, y)
struct timeval *result, *x, *y;
{
/* Perform the carry for the later subtraction by updating y. */
if (x->tv_usec < y->tv_usec) {
int nsec = (y->tv_usec - x->tv_usec) / 1000000 + 1;
y->tv_usec -= 1000000 * nsec;
y->tv_sec += nsec;
}
if (x->tv_usec - y->tv_usec > 1000000) {
int nsec = (x->tv_usec - y->tv_usec) / 1000000;
y->tv_usec += 1000000 * nsec;
y->tv_sec -= nsec;
}
/* Compute the time remaining to wait.
tv_usec is certainly positive. */
result->tv_sec = x->tv_sec - y->tv_sec;
result->tv_usec = x->tv_usec - y->tv_usec;
/* Return 1 if result is negative. */
return x->tv_sec < y->tv_sec;
}
Common functions that use struct timeval are gettimeofday
and settimeofday.
There are no GNU C library 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.
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 (see section 21.1 Time Basics) is represented by the data type
clock_t, which is a number of clock ticks. It gives the
total amount of time a process has actively used a CPU since some
arbitrary event. On the GNU system, 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 examinining the process' CPU
time before and after the computation.
In the GNU system, clock_t is equivalent to long int and
CLOCKS_PER_SEC is an integer value. But in other systems, both
clock_t and the macro CLOCKS_PER_SEC can be either integer
or floating-point types. Casting CPU time values to 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
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 section 22. Resource Usage And Limitation.
To get a process' CPU time, you can use the clock function. This
facility is declared in the header file `time.h'.
In typical usage, you call the clock function at the beginning
and end of the interval you want to time, subtract the values, and then
divide by CLOCKS_PER_SEC (the number of clock ticks per second)
to get processor time, like this:
#include <time.h>
clock_t start, end;
double cpu_time_used;
start = clock();
... /* Do the work. */
end = clock();
cpu_time_used = ((double) (end - start)) / CLOCKS_PER_SEC;
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. See section 21.3.2 Processor Time Inquiry.
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.
Macro: int CLOCKS_PER_SEC
The value of this macro is the number of clock ticks per second measured
by the clock function. POSIX requires that this value be one
million independent of the actual resolution.
Macro: int CLK_TCK
This is an obsolete name for CLOCKS_PER_SEC.
Data Type:clock_t
This is the type of the value returned by the clock function.
Values of type clock_t are numbers of clock ticks.
Function: clock_t clock(void)
This function returns the calling process' current CPU time. If the CPU
time is not available or cannot be represented, clock returns the
value (clock_t)(-1).
The times function returns information about a process'
consumption of processor time in a struct tms object, in
addition to the process' CPU time. See section 21.1 Time Basics. You should
include the header file `sys/times.h' to use this facility.
Data Type:struct tms
The tms structure is used to return information about process
times. It contains at least the following members:
clock_t tms_utime
This is the total processor time the calling process has used in
executing the instructions of its program.
clock_t tms_stime
This is the processor time the system has used on behalf of the calling
process.
clock_t tms_cutime
This is the sum of the tms_utime values and the tms_cutime
values of all terminated child processes of the calling process, whose
status has been reported to the parent process by wait or
waitpid; see 26.6 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 wait or
waitpid.
clock_t tms_cstime
This is similar to tms_cutime, but represents the total processor
time system has used on behalf of all the terminated child processes
of the calling process.
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.
See section 26.4 Creating a Process.
Function: clock_t times(struct tms *buffer)
The times function stores the processor time information for
the calling process in buffer.
The return value is the calling process' CPU time (the same value you
get from clock(). times returns (clock_t)(-1) to
indicate failure.
Portability Note: The clock function described in
21.3.1 CPU Time Inquiry is specified by the ISO C standard. The
times function is a feature of POSIX.1. In the GNU system, the
CPU time is defined to be equivalent to the sum of the tms_utime
and tms_stime fields returned by times.
This section describes facilities for keeping track of calendar time.
See section 21.1 Time Basics.
The GNU C library represents calendar time three ways:
Simple time (the time_t data type) is a compact
representation, typically giving the number of seconds of elapsed time
since some implementation-specific base time.
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 struct
timeval data type, which includes fractions of a second. Use this time
representation instead of simple time when you need greater precision.
Local time or broken-down time (the 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.
This section describes the 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 `time.h'.
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 epoch.) POSIX requires that this count not
include leap seconds, but on some systems this count includes leap seconds
if you set TZ to certain values (see section 21.4.7 Specifying the Time Zone with TZ).
Note that a simple time has no concept of local time zone. Calendar
Time T is the same instant in time regardless of where on the
globe the computer is.
In the GNU C library, time_t is equivalent to long int.
In other systems, time_t might be either an integer or
floating-point type.
The function difftime tells you the elapsed time between two
simple calendar times, which is not always as easy to compute as just
subtracting. See section 21.2 Elapsed Time.
Function: time_t time(time_t *result)
The time function returns the current calendar time as a value of
type time_t. If the argument result is not a null pointer,
the calendar time value is also stored in *result. If the
current calendar time is not available, the value
(time_t)(-1) is returned.
Function: int stime(time_t *newtime)
stime sets the system clock, i.e. it tells the system that the
current calendar time is newtime, where newtime is
interpreted as described in the above definition of time_t.
settimeofday is a newer function which sets the system clock to
better than one second precision. settimeofday is generally a
better choice than stime. See section 21.4.2 High-Resolution Calendar.
Only the superuser can set the system clock.
If the function succeeds, the return value is zero. Otherwise, it is
-1 and errno is set accordingly:
The time_t data type used to represent simple times has a
resolution of only one second. Some applications need more precision.
So, the GNU C library 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 `sys/time.h'.
Data Type:struct timezone
The struct timezone structure is used to hold minimal information
about the local time zone. It has the following members:
int tz_minuteswest
This is the number of minutes west of UTC.
int tz_dsttime
If nonzero, Daylight Saving Time applies during some part of the year.
Function: int gettimeofday(struct timeval *tp, struct timezone *tzp)
The gettimeofday function returns the current calendar time as
the elapsed time since the epoch in the struct timeval structure
indicated by tp. (see section 21.2 Elapsed Time for a description of
struct timespec). Information about the time zone is returned in
the structure pointed at tzp. If the tzp argument is a null
pointer, time zone information is ignored.
The return value is 0 on success and -1 on failure. The
following errno error condition is defined for this function:
ENOSYS
The operating system does not support getting time zone information, and
tzp is not a null pointer. The GNU operating system does not
support using struct timezone to represent time zone
information; that is an obsolete feature of 4.3 BSD.
Instead, use the facilities described in 21.4.8 Functions and Variables for Time Zones.
Function: int settimeofday(const struct timeval *tp, const struct timezone *tzp)
The settimeofday function sets the current calendar time in the
system clock according to the arguments. As for gettimeofday,
the calendar time is represented as the elapsed time since the epoch.
As for gettimeofday, time zone information is ignored if
tzp is a null pointer.
You must be a privileged user in order to use 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 stime removes the system clock from an invalid
state, and system startup scripts typically run a program that calls
stime.
settimeofday causes a sudden jump forwards or backwards, which
can cause a variety of problems in a system. Use 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, 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 0 on success and -1 on failure. The
following errno error conditions are defined for this function:
EPERM
This process cannot set the clock because it is not privileged.
ENOSYS
The operating system does not support setting time zone information, and
tzp is not a null pointer.
Function: int adjtime(const struct timeval *delta, struct timeval *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 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 olddelta argument is not a null pointer, the 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 adjtimex function to
permanently change the clock speed.
The return value is 0 on success and -1 on failure. The
following errno error condition is defined for this function:
EPERM
You do not have privilege to set the time.
Portability Note: The gettimeofday, settimeofday,
and adjtime functions are derived from BSD.
Symbols for the following function are declared in `sys/timex.h'.
Calendar time is represented by the usual GNU C library 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, 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 `time.h'.
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.
int tm_sec
This is the number of full seconds since the top of the minute (normally
in the range 0 through 59, but the actual upper limit is
60, to allow for leap seconds if leap second support is
available).
int tm_min
This is the number of full minutes since the top of the hour (in the
range 0 through 59).
int tm_hour
This is the number of full hours past midnight (in the range 0 through
23).
int tm_mday
This is the ordinal day of the month (in the range 1 through 31).
Watch out for this one! As the only ordinal number in the structure, it is
inconsistent with the rest of the structure.
int tm_mon
This is the number of full calendar months since the beginning of the
year (in the range 0 through 11). Watch out for this one!
People usually use ordinal numbers for month-of-year (where January = 1).
int tm_year
This is the number of full calendar years since 1900.
int tm_wday
This is the number of full days since Sunday (in the range 0 through
6).
int tm_yday
This is the number of full days since the beginning of the year (in the
range 0 through 365).
int tm_isdst
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.
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 -5*60*60.
The tm_gmtoff field is derived from BSD and is a GNU library
extension; it is not visible in a strict ISO C environment.
const char *tm_zone
This field is the name for the time zone that was used to compute this
broken-down time value. Like tm_gmtoff, this field is a BSD and
GNU extension, and is not visible in a strict ISO C environment.
The localtime function converts the simple time pointed to by
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 ctime, gmtime,
or localtime. (But no other library function overwrites the contents
of this object.)
The return value is the null pointer if time cannot be represented
as a broken-down time; typically this is because the year cannot fit into
an int.
Using the 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.
The localtime_r function works just like the 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 struct tm to which the parameter
resultp points.
If the conversion is successful the function returns a pointer to the
object the result was written into, i.e., it returns resultp.
Function: struct tm * gmtime(const time_t *time)
This function is similar to 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.
As for the localtime function we have the problem that the result
is placed in a static variable. POSIX.1c also provides a replacement for
gmtime.
This function is similar to localtime_r, except that it converts
just like 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 resultp.
Function: time_t mktime(struct tm *brokentime)
The 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 mktime function ignores the specified contents of the
tm_wday and 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
mktime does is adjust the components of the brokentime
structure (including the tm_wday and tm_yday).
If the specified broken-down time cannot be represented as a simple time,
mktime returns a value of (time_t)(-1) and does not modify
the contents of brokentime.
timelocal is functionally identical to mktime, but more
mnemonically named. Note that it is the inverse of the localtime
function.
Portability note:mktime is essentially universally
available. timelocal is rather rare.
Function: time_t timegm(struct tm *brokentime)
timegm is functionally identical to mktime except it
always takes the input values to be Coordinated Universal Time (UTC)
regardless of any local time zone setting.
Note that timegm is the inverse of gmtime.
Portability note:mktime is essentially universally
available. timegm is rather rare. For the most portable
conversion from a UTC broken-down time to a simple time, set
the TZ environment variable to UTC, call mktime, then set
TZ back.
The ntp_gettime and 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 `sys/timex.h'.
Data Type:struct ntptimeval
This structure is used for information about the system clock. It
contains the following members:
struct timeval time
This is the current calendar time, expressed as the elapsed time since
the epoch. The struct timeval data type is described in
21.2 Elapsed Time.
long int maxerror
This is the maximum error, measured in microseconds. Unless updated
via ntp_adjtime periodically, this value will reach some
platform-specific maximum value.
long int esterror
This is the estimated error, measured in microseconds. This value can
be set by ntp_adjtime to indicate the estimated offset of the
system clock from the true calendar time.
Function: int ntp_gettime(struct ntptimeval *tptr)
The ntp_gettime function sets the structure pointed to by
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 ntp_adjtime
call is necessary.
The return value is 0 on success and other values on failure. The
following errno error conditions are defined for this function:
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.
Data Type:struct timex
This structure is used to control and monitor the system clock. It
contains the following members:
unsigned int modes
This variable controls whether and which values are set. Several
symbolic constants have to be combined with binary or to specify
the effective mode. These constants start with MOD_.
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
MOD_OFFSET is set in modes, the offset (and possibly other
dependent values) can be set. The offset's absolute value must not
exceed MAXPHASE.
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 1 <<
SHIFT_USEC. The value can be set with bit MOD_FREQUENCY, but
the absolute value must not exceed MAXFREQ.
long int maxerror
This is the maximum error, measured in microseconds. A new value can be
set using bit MOD_MAXERROR. Unless updated via
ntp_adjtime periodically, this value will increase steadily
and reach some platform-specific maximum value.
long int esterror
This is the estimated error, measured in microseconds. This value can
be set using bit MOD_ESTERROR.
int status
This variable reflects the various states of the clock machinery. There
are symbolic constants for the significant bits, starting with
STA_. Some of these flags can be updated using the
MOD_STATUS bit.
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 MOD_TIMECONST.
long int precision
This value represents the accuracy or the maximum error when reading the
system clock. The value is expressed in microseconds.
long int tolerance
This value represents the maximum frequency error of the system clock in
scaled PPM. This value is used to increase the maxerror every
second.
struct timeval time
The current calendar time.
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.
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.
long int jitter
This value expresses a median filtered average of the PPS signal's
dispersion in microseconds.
int shift
This value is a binary exponent for the duration of the PPS calibration
interval, ranging from PPS_SHIFT to PPS_SHIFTMAX.
long int stabil
This value represents the median filtered dispersion of the PPS
frequency in scaled PPM.
long int jitcnt
This counter represents the number of pulses where the jitter exceeded
the allowed maximum MAXTIME.
long int calcnt
This counter reflects the number of successful calibration intervals.
long int errcnt
This counter represents the number of calibration errors (caused by
large offsets or jitter).
long int stbcnt
This counter denotes the number of of calibrations where the stability
exceeded the threshold.
Function: int ntp_adjtime(struct timex *tptr)
The ntp_adjtime function sets the structure specified by
tptr to current values.
In addition, ntp_adjtime updates some settings to match what you
pass to it in *tptr. Use the modes element of *tptr
to select what settings to update. You can set offset,
freq, maxerror, esterror, status,
constant, and tick.
modes = zero means set nothing.
Only the superuser can update settings.
The return value is 0 on success and other values on failure. The
following errno error conditions are defined for this function:
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.
EPERM
The process specified a settings update, but is not superuser.
For more details see RFC1305 (Network Time Protocol, Version 3) and
related documents.
Portability note: Early versions of the GNU C library did not
have this function but did have the synonymous adjtimex.
The asctime function converts the broken-down time value that
brokentime points to into a string in a standard format:
"Tue May 21 13:46:22 1991\n"
The abbreviations for the days of week are: `Sun', `Mon',
`Tue', `Wed', `Thu', `Fri', and `Sat'.
The abbreviations for the months are: `Jan', `Feb',
`Mar', `Apr', `May', `Jun', `Jul', `Aug',
`Sep', `Oct', `Nov', and `Dec'.
The return value points to a statically allocated string, which might be
overwritten by subsequent calls to asctime or ctime.
(But no other library function overwrites the contents of this
string.)
This function is similar to asctime but instead of placing the
result in a static buffer it writes the string in the buffer pointed to
by the parameter 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 buffer. Otherwise
return NULL.
Function: char * ctime(const time_t *time)
The ctime function is similar to asctime, except that you
specify the calendar time argument as a time_t simple time value
rather than in broken-down local time format. It is equivalent to
This function is similar to ctime, but places the result in the
string pointed to by buffer. It is equivalent to (written using
gcc extensions, see section `Statement Exprs' in Porting and Using gcc):
This function is similar to the sprintf function (see section 12.14 Formatted Input), but the conversion specifications that can appear in the format
template template are specialized for printing components of the date
and time brokentime according to the locale currently specified for
time conversion (see section 7. Locales and Internationalization).
Ordinary characters appearing in the template are copied to the
output string s; this can include multibyte character sequences.
Conversion specifiers are introduced by a `%' 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:
_
The number is padded with spaces.
-
The number is not padded at all.
0
The number is padded with zeros even if the format specifies padding
with spaces.
^
The output uses uppercase characters, but only if this is possible
(see section 4.2 Case Conversion).
The default action is to pad the number with zeros to keep it a constant
width. Numbers that do not have a range indicated below are never
padded, since there is no natural width for them.
Following the flag an optional specification of the width is possible.
This is specified in decimal notation. If the natural size of the
output is of the field has less than the specified number of characters,
the result is written right adjusted and space padded to the given
size.
An optional modifier can follow the optional flag and width
specification. The modifiers, which are POSIX.2 extensions, are:
E
Use the locale's alternate representation for date and time. This
modifier applies to the %c, %C, %x, %X,
%y and %Y format specifiers. In a Japanese locale, for
example, %Ex might yield a date format based on the Japanese
Emperors' reigns.
O
Use the locale's alternate numeric symbols for numbers. This modifier
applies only to numeric format specifiers.
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 `%' sequence is replaced in the output
string as follows:
%a
The abbreviated weekday name according to the current locale.
%A
The full weekday name according to the current locale.
%b
The abbreviated month name according to the current locale.
%B
The full month name according to the current locale.
%c
The preferred calendar time representation for the current locale.
%C
The century of the year. This is equivalent to the greatest integer not
greater than the year divided by 100.
This format is a POSIX.2 extension and also appears in ISO C99.
%d
The day of the month as a decimal number (range 01 through 31).
%D
The date using the format %m/%d/%y.
This format is a POSIX.2 extension and also appears in ISO C99.
%e
The day of the month like with %d, but padded with blank (range
1 through 31).
This format is a POSIX.2 extension and also appears in ISO C99.
%F
The date using the format %Y-%m-%d. This is the form specified
in the ISO 8601 standard and is the preferred form for all uses.
This format is a ISO C99 extension.
%g
The year corresponding to the ISO week number, but without the century
(range 00 through 99). This has the same format and value
as %y, except that if the ISO week number (see %V) belongs
to the previous or next year, that year is used instead.
This format was introduced in ISO C99.
%G
The year corresponding to the ISO week number. This has the same format
and value as %Y, except that if the ISO week number (see
%V) belongs to the previous or next year, that year is used
instead.
This format was introduced in ISO C99 but was previously available
as a GNU extension.
%h
The abbreviated month name according to the current locale. The action
is the same as for %b.
This format is a POSIX.2 extension and also appears in ISO C99.
%H
The hour as a decimal number, using a 24-hour clock (range 00 through
23).
%I
The hour as a decimal number, using a 12-hour clock (range 01 through
12).
%j
The day of the year as a decimal number (range 001 through 366).
%k
The hour as a decimal number, using a 24-hour clock like %H, but
padded with blank (range 0 through 23).
This format is a GNU extension.
%l
The hour as a decimal number, using a 12-hour clock like %I, but
padded with blank (range 1 through 12).
This format is a GNU extension.
%m
The month as a decimal number (range 01 through 12).
%M
The minute as a decimal number (range 00 through 59).
%n
A single `\n' (newline) character.
This format is a POSIX.2 extension and also appears in ISO C99.
%p
Either `AM' or `PM', according to the given time value; or the
corresponding strings for the current locale. Noon is treated as
`PM' and midnight as `AM'.
%P
Either `am' or `pm', according to the given time value; or the
corresponding strings for the current locale, printed in lowercase
characters. Noon is treated as `pm' and midnight as `am'.
This format was introduced in ISO C99 but was previously available
as a GNU extension.
%r
The complete calendar time using the AM/PM format of the current locale.
This format is a POSIX.2 extension and also appears in ISO C99.
%R
The hour and minute in decimal numbers using the format %H:%M.
This format was introduced in ISO C99 but was previously available
as a GNU extension.
%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.
%S
The seconds as a decimal number (range 00 through 60).
%t
A single `\t' (tabulator) character.
This format is a POSIX.2 extension and also appears in ISO C99.
%T
The time of day using decimal numbers using the format %H:%M:%S.
This format is a POSIX.2 extension.
%u
The day of the week as a decimal number (range 1 through
7), Monday being 1.
This format is a POSIX.2 extension and also appears in ISO C99.
%U
The week number of the current year as a decimal number (range 00
through 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 00.
%V
The ISO 8601:1988 week number as a decimal number (range 01
through 53). ISO weeks start with Monday and end with Sunday.
Week 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 01 of a year can contain days from the previous year.
The week before week 01 of a year is the last week (52 or
53) of the previous year even if it contains days from the new
year.
This format is a POSIX.2 extension and also appears in ISO C99.
%w
The day of the week as a decimal number (range 0 through
6), Sunday being 0.
%W
The week number of the current year as a decimal number (range 00
through 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 00.
%x
The preferred date representation for the current locale.
%X
The preferred time of day representation for the current locale.
%y
The year without a century as a decimal number (range 00 through
99). This is equivalent to the year modulo 100.
%Y
The year as a decimal number, using the Gregorian calendar. Years
before the year 1 are numbered 0, -1, and so on.
%z
RFC 822/ISO 8601:1988 style numeric time zone (e.g.,
-0600 or +0100), or nothing if no time zone is
determinable.
This format was introduced in ISO C99 but was previously available
as a GNU extension.
A full RFC 822 timestamp is generated by the format
`"%a, %d %b %Y %H:%M:%S %z"' (or the equivalent
`"%a, %d %b %Y %T %z"').
%Z
The time zone abbreviation (empty if the time zone can't be determined).
%%
A literal `%' character.
The size parameter can be used to specify the maximum number of
characters to be stored in the array s, including the terminating
null character. If the formatted time requires more than size
characters, strftime returns zero and the contents of the array
s are undefined. Otherwise the return value indicates the
number of characters placed in the array s, not including the
terminating null character.
Warning: This convention for the return value which is prescribed
in 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
"%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:
buf[0] = '\1';
len = strftime (buf, bufsize, format, tp);
if (len == 0 && buf[0] != '\0')
{
/* Something went wrong in the strftime call. */
...
}
If s is a null pointer, strftime does not actually write
anything, but instead returns the number of characters it would have written.
According to POSIX.1 every call to strftime implies a call to
tzset. So the contents of the environment variable TZ
is examined before any output is produced.
The wcsftime function is equivalent to the strftime
function with the difference that it operates on wide character
strings. The buffer where the result is stored, pointed to by s,
must be an array of wide characters. The parameter size which
specifies the size of the output buffer gives the number of wide
character, not the number of bytes.
Also the format string 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 L"..." notation. The parameter
brokentime has the same meaning as in the strftime call.
The wcsftime function supports the same flags, modifiers, and
format specifiers as the strftime function.
The return value of wcsftime is the number of wide characters
stored in s. When more characters would have to be written than
can be placed in the buffer s the return value is zero, with the
same problems indicated in the strftime documentation.
21.4.6 Convert textual time and date information back
The ISO C standard does not specify any functions which can convert
the output of the 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: strptime and
getdate. Both have strange interfaces but at least they are
widely available.
21.4.6.1 Interpret string according to given format
he first function is rather low-level. It is nevertheless frequently
used in software since it is better known. Its interface and
implementation are heavily influenced by the getdate function,
which is defined and implemented in terms of calls to strptime.
The strptime function parses the input string s according
to the format string fmt and stores its results in the
structure tp.
The input string could be generated by a strftime call or
obtained any other way. It does not need to be in a human-recognizable
format; e.g. a date passed as "02:1999:9" is acceptable, even
though it is ambiguous without context. As long as the format string
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 "1999112" can be parsed using the
format "%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 strftime function. The only difference is that the flags
_, -, 0, and ^ are not allowed.
Several of the distinct formats of strftime do the same work in
strptime since differences like case of the input do not matter.
For reasons of symmetry all formats are supported, though.
The modifiers E and O are also allowed everywhere the
strftime function allows them.
The formats are:
%a
%A
The weekday name according to the current locale, in abbreviated form or
the full name.
%b
%B
%h
The month name according to the current locale, in abbreviated form or
the full name.
%c
The date and time representation for the current locale.
%Ec
Like %c but the locale's alternative date and time format is used.
%C
The century of the year.
It makes sense to use this format only if the format string also
contains the %y format.
%EC
The locale's representation of the period.
Unlike %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.
%d
%e
The day of the month as a decimal number (range 1 through 31).
Leading zeroes are permitted but not required.
%Od
%Oe
Same as %d but using the locale's alternative numeric symbols.
Leading zeroes are permitted but not required.
%D
Equivalent to %m/%d/%y.
%F
Equivalent to %Y-%m-%d, which is the ISO 8601 date
format.
This is a GNU extension following an ISO C99 extension to
strftime.
%g
The year corresponding to the ISO week number, but without the century
(range 00 through 99).
Note: Currently, this is not fully implemented. The format is
recognized, input is consumed but no field in tm is set.
This format is a GNU extension following a GNU extension of strftime.
%G
The year corresponding to the ISO week number.
Note: Currently, this is not fully implemented. The format is
recognized, input is consumed but no field in tm is set.
This format is a GNU extension following a GNU extension of strftime.
%H
%k
The hour as a decimal number, using a 24-hour clock (range 00 through
23).
%k is a GNU extension following a GNU extension of strftime.
%OH
Same as %H but using the locale's alternative numeric symbols.
%I
%l
The hour as a decimal number, using a 12-hour clock (range 01 through
12).
%l is a GNU extension following a GNU extension of strftime.
%OI
Same as %I but using the locale's alternative numeric symbols.
%j
The day of the year as a decimal number (range 1 through 366).
Leading zeroes are permitted but not required.
%m
The month as a decimal number (range 1 through 12).
Leading zeroes are permitted but not required.
%Om
Same as %m but using the locale's alternative numeric symbols.
%M
The minute as a decimal number (range 0 through 59).
Leading zeroes are permitted but not required.
%OM
Same as %M but using the locale's alternative numeric symbols.
%n
%t
Matches any white space.
%p
%P
The locale-dependent equivalent to `AM' or `PM'.
This format is not useful unless %I or %l is also used.
Another complication is that the locale might not define these values at
all and therefore the conversion fails.
%P is a GNU extension following a GNU extension to strftime.
%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.
%R
The hour and minute in decimal numbers using the format %H:%M.
%R is a GNU extension following a GNU extension to strftime.
%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.
%s is a GNU extension following a GNU extension to strftime.
%S
The seconds as a decimal number (range 0 through 60).
Leading zeroes are permitted but not required.
Note: The Unix specification says the upper bound on this value
is 61, a result of a decision to allow double leap seconds. You
will not see the value 61 because no minute has more than one
leap second, but the myth persists.
%OS
Same as %S but using the locale's alternative numeric symbols.
%T
Equivalent to the use of %H:%M:%S in this place.
%u
The day of the week as a decimal number (range 1 through
7), Monday being 1.
Leading zeroes are permitted but not required.
Note: Currently, this is not fully implemented. The format is
recognized, input is consumed but no field in tm is set.
%U
The week number of the current year as a decimal number (range 0
through 53).
Leading zeroes are permitted but not required.
%OU
Same as %U but using the locale's alternative numeric symbols.
%V
The ISO 8601:1988 week number as a decimal number (range 1
through 53).
Leading zeroes are permitted but not required.
Note: Currently, this is not fully implemented. The format is
recognized, input is consumed but no field in tm is set.
%w
The day of the week as a decimal number (range 0 through
6), Sunday being 0.
Leading zeroes are permitted but not required.
Note: Currently, this is not fully implemented. The format is
recognized, input is consumed but no field in tm is set.
%Ow
Same as %w but using the locale's alternative numeric symbols.
%W
The week number of the current year as a decimal number (range 0
through 53).
Leading zeroes are permitted but not required.
Note: Currently, this is not fully implemented. The format is
recognized, input is consumed but no field in tm is set.
%OW
Same as %W but using the locale's alternative numeric symbols.
%x
The date using the locale's date format.
%Ex
Like %x but the locale's alternative data representation is used.
%X
The time using the locale's time format.
%EX
Like %X but the locale's alternative time representation is used.
%y
The year without a century as a decimal number (range 0 through
99).
Leading zeroes are permitted but not required.
Note that it is questionable to use this format without
the %C format. The strptime function does regard input
values in the range 68 to 99 as the years 1969 to
1999 and the values 0 to 68 as the years
2000 to 2068. But maybe this heuristic fails for some
input data.
Therefore it is best to avoid %y completely and use %Y
instead.
%Ey
The offset from %EC in the locale's alternative representation.
%Oy
The offset of the year (from %C) using the locale's alternative
numeric symbols.
%Y
The year as a decimal number, using the Gregorian calendar.
%EY
The full alternative year representation.
%z
The offset from GMT in ISO 8601/RFC822 format.
%Z
The timezone name.
Note: Currently, this is not fully implemented. The format is
recognized, input is consumed but no field in tm is set.
%%
A literal `%' character.
All other characters in the format string must have a matching character
in the input string. Exceptions are white spaces in the input string
which can match zero or more white space characters in the format string.
The 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 NULL byte at the end of the string. If an error
occurs, i.e. strptime fails to match all of the format string,
the function returns NULL.
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 tm which are
not directly initialized by the different formats. The
implementations on different Unix systems vary here.
The GNU libc implementation does not touch those fields which are not
directly initialized. Exceptions are the tm_wday and
tm_yday elements, which are recomputed if any of the year, month,
or date elements changed. This has two implications:
Before calling the strptime function for a new input string, you
should prepare the tm structure you pass. Normally this will mean
initializing all values are to zero. Alternatively, you can set all
fields to values like 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 tm was initialized by the function call.
You can construct a struct tm value with several consecutive
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.
The following example shows a function which parses a string which is
contains the date information in either US style or ISO 8601 form:
const char *
parse_date (const char *input, struct tm *tm)
{
const char *cp;
/* First clear the result structure. */
memset (tm, '\0', sizeof (*tm));
/* Try the ISO format first. */
cp = strptime (input, "%F", tm);
if (cp == NULL)
{
/* Does not match. Try the US form. */
cp = strptime (input, "%D", tm);
}
return cp;
}
21.4.6.2 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).
Variable:getdate_err
This variable of type int contains the error code of the last
unsuccessful call to getdate. Defined values are:
1
The environment variable DATEMSK is not defined or null.
2
The template file denoted by the DATEMSK environment variable
cannot be opened.
3
Information about the template file cannot retrieved.
4
The template file is not a regular file.
5
An I/O error occurred while reading the template file.
6
Not enough memory available to execute the function.
7
The template file contains no matching template.
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 time_t variable.
Function: struct tm * getdate(const char *string)
The interface to getdate is the simplest possible for a function
to parse a string and return the value. 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 DATEMSK. This file should contain
lines of valid format strings which could be passed to strptime.
The 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 getdate function call.
The formats recognized by getdate are the same as for
strptime. See above for an explanation. There are only a few
extensions to the strptime behavior:
If the %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.
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 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.
If only the weekday is specified the selected day depends on the current
date. If the current weekday is greater or equal to the tm_wday
value the current week's day is chosen, otherwise the day next week is chosen.
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.
The current hour, minute, and second are used if the appropriate value is
not set through the format.
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.
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):
%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
As you can see, the template list can contain very specific strings like
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.
Input
Match
Result
Mon
%a
Mon Sep 22 12:19:47 EDT 1986
Sun
%a
Sun Sep 28 12:19:47 EDT 1986
Fri
%a
Fri Sep 26 12:19:47 EDT 1986
September
%B
Mon Sep 1 12:19:47 EDT 1986
January
%B
Thu Jan 1 12:19:47 EST 1987
December
%B
Mon Dec 1 12:19:47 EST 1986
Sep Mon
%b %a
Mon Sep 1 12:19:47 EDT 1986
Jan Fri
%b %a
Fri Jan 2 12:19:47 EST 1987
Dec Mon
%b %a
Mon Dec 1 12:19:47 EST 1986
Jan Wed 1989
%b %a %Y
Wed Jan 4 12:19:47 EST 1989
Fri 9
%a %H
Fri Sep 26 09:00:00 EDT 1986
Feb 10:30
%b %H:%S
Sun Feb 1 10:00:30 EST 1987
10:30
%H:%M
Tue Sep 23 10:30:00 EDT 1986
13:30
%H:%M
Mon Sep 22 13:30:00 EDT 1986
The return value of the function is a pointer to a static variable of
type struct tm, or a null pointer if an error occurred. The
result is only valid until the next getdate call, making this
function unusable in multi-threaded applications.
The errno variable is not changed. Error conditions are
stored in the global variable getdate_err. See the
description above for a list of the possible error values.
Warning: The getdate function should never be
used in SUID-programs. The reason is obvious: using the
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.
Function: int getdate_r(const char *string, struct tm *tp)
The getdate_r function is the reentrant counterpart of
getdate. It does not use the global variable getdate_err
to signal an error, but instead returns an error code. The same error
codes as described in the getdate_err documentation above are
used, with 0 meaning success.
Moreover, getdate_r stores the broken-down time in the variable
of type 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 getdate in SUID-programs applies to
getdate_r as well.
In POSIX systems, a user can specify the time zone by means of the
TZ environment variable. For information about how to set
environment variables, see 25.4 Environment Variables. The functions
for accessing the time zone are declared in `time.h'.
You should not normally need to set TZ. If the system is
configured properly, the default time zone will be correct. You might
set 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 TZ variable can be in one of
three formats. With the GNU C library, the most common format is the
last one, which can specify a selection from a large database of time
zone information for many regions of the world. The first two formats
are used to describe the time zone information directly, which is both
more cumbersome and less precise. But the POSIX.1 standard only
specifies the details of the first two formats, so it is good to be
familiar with them in case you come across a POSIX.1 system that doesn't
support a time zone information database.
The first format is used when there is no Daylight Saving Time (or
summer time) in the local time zone:
stdoffset
The 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 offset, so these
restrictions are necessary to parse the specification correctly.
The offset specifies the time value you must add to the local time
to get a Coordinated Universal Time value. It has syntax like
[+|-]hh[:mm[: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 0 and
23, and the minute and seconds between 0 and 59.
For example, here is how we would specify Eastern Standard Time, but
without any Daylight Saving Time alternative:
EST+5
The second format is used when there is Daylight Saving Time:
stdoffsetdst [offset],start[/time],end[/time]
The initial std and offset specify the standard time zone, as
described above. The dst string and offset specify the name
and offset for the corresponding Daylight Saving Time zone; if the
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 start field is when Daylight Saving Time goes into
effect and the end field is when the change is made back to standard
time. The following formats are recognized for these fields:
Jn
This specifies the Julian day, with n between 1 and 365.
February 29 is never counted, even in leap years.
n
This specifies the Julian day, with n between 0 and 365.
February 29 is counted in leap years.
Mm.w.d
This specifies day d of week w of month m. The day
d must be between 0 (Sunday) and 6. The week
w must be between 1 and 5; week 1 is the
first week in which day d occurs, and week 5 specifies the
lastd day in the month. The month m should be
between 1 and 12.
The time fields specify when, in the local time currently in
effect, the change to the other time occurs. If omitted, the default is
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.
EST+5EDT,M4.1.0/2,M10.5.0/2
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:
:characters
Each operating system interprets this format differently; in the GNU C
library, characters is the name of a file which describes the time
zone.
If the TZ environment variable does not have a value, the
operation chooses a time zone by default. In the GNU C library, the
default time zone is like the specification `TZ=:/etc/localtime'
(or `TZ=:/usr/local/etc/localtime', depending on how GNU C library
was configured; see section C. Installing the GNU C Library). Other C libraries use their own
rule for choosing the default time zone, so there is little we can say
about them.
If characters begins with a slash, it is an absolute file name;
otherwise the library looks for the file
`/share/lib/zoneinfo/characters'. The `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,
`America/New_York', `Europe/London', `Asia/Hong_Kong'.
These data files are installed by the system administrator, who also
sets `/etc/localtime' to point to the data file for the local time
zone. The GNU C library comes with a large database of time zone
information for most regions of the world, which is maintained by a
community of volunteers and put in the public domain.
The array tzname contains two strings, which are the standard
names of the pair of time zones (standard and Daylight
Saving) that the user has selected. tzname[0] is the name of
the standard time zone (for example, "EST"), and tzname[1]
is the name for the time zone when Daylight Saving Time is in use (for
example, "EDT"). These correspond to the std and dst
strings (respectively) from the TZ environment variable. If
Daylight Saving Time is never used, tzname[1] is the empty string.
The tzname array is initialized from the TZ environment
variable whenever tzset, ctime, strftime,
mktime, or localtime is called. If multiple abbreviations
have been used (e.g. "EWT" and "EDT" for U.S. Eastern War
Time and Eastern Daylight Time), the array contains the most recent
abbreviation.
The tzname array is required for POSIX.1 compatibility, but in
GNU programs it is better to use the tm_zone member of the
broken-down time structure, since tm_zone reports the correct
abbreviation even when it is not the latest one.
Though the strings are declared as char * the user must refrain
from modifying these strings. Modifying the strings will almost certainly
lead to trouble.
Function: void tzset(void)
The tzset function initializes the tzname variable from
the value of the 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.
The following variables are defined for compatibility with System V
Unix. Like tzname, these variables are set by calling
tzset or the other time conversion functions.
Variable: 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 5*60*60. Unlike the 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 tm_gmtoff, since it contains the correct offset even when
it is not the latest one.
Variable: 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.
Here is an example program showing the use of some of the calendar time
functions.
#include <time.h>
#include <stdio.h>
#define SIZE 256
int
main (void)
{
char buffer[SIZE];
time_t curtime;
struct tm *loctime;
/* Get the current time. */
curtime = time (NULL);
/* Convert it to local time representation. */
loctime = localtime (&curtime);
/* Print out the date and time in the standard format. */
fputs (asctime (loctime), stdout);
/* Print it out in a nice format. */
strftime (buffer, SIZE, "Today is %A, %B %d.\n", loctime);
fputs (buffer, stdout);
strftime (buffer, SIZE, "The time is %I:%M %p.\n", loctime);
fputs (buffer, stdout);
return 0;
}
It produces output like this:
Wed Jul 31 13:02:36 1991
Today is Wednesday, July 31.
The time is 01:02 PM.
The alarm and 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.
Each process has three independent interval timers available:
A real-time timer that counts elapsed time. This timer sends a
SIGALRM signal to the process when it expires.
A virtual timer that counts processor time used by the process. This timer
sends a SIGVTALRM signal to the process when it expires.
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 SIGPROF signal to the process when it expires.
This timer is useful for profiling in interpreters. The interval timer
mechanism does not have the fine granularity necessary for profiling
native code.
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
signal or sigaction before issuing a call to
setitimer or 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. See section 24. Signal Handling.
The setitimer function is the primary means for setting an alarm.
This facility is declared in the header file `sys/time.h'. The
alarm function, declared in `unistd.h', provides a somewhat
simpler interface for setting the real-time timer.
Data Type:struct itimerval
This structure is used to specify when a timer should expire. It contains
the following members:
struct timeval it_interval
This is the period between successive timer interrupts. If zero, the
alarm will only be sent once.
struct timeval it_value
This is the period between now and the first timer interrupt. If zero,
the alarm is disabled.
Function: int setitimer(int which, struct itimerval *new, struct itimerval *old)
The setitimer function sets the timer specified by which
according to new. The which argument can have a value of
ITIMER_REAL, ITIMER_VIRTUAL, or ITIMER_PROF.
If old is not a null pointer, setitimer returns information
about any previous unexpired timer of the same kind in the structure it
points to.
The return value is 0 on success and -1 on failure. The
following errno error conditions are defined for this function:
EINVAL
The timer period is too large.
Function: int getitimer(int which, struct itimerval *old)
The getitimer function stores information about the timer specified
by which in the structure pointed at by old.
The return value and error conditions are the same as for setitimer.
ITIMER_REAL
This constant can be used as the which argument to the
setitimer and getitimer functions to specify the real-time
timer.
ITIMER_VIRTUAL
This constant can be used as the which argument to the
setitimer and getitimer functions to specify the virtual
timer.
ITIMER_PROF
This constant can be used as the which argument to the
setitimer and getitimer functions to specify the profiling
timer.
Function: unsigned int alarm(unsigned int seconds)
The alarm function sets the real-time timer to expire in
seconds seconds. If you want to cancel any existing alarm, you
can do this by calling alarm with a 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, alarm
returns zero.
The alarm function could be defined in terms of setitimer
like this:
If you simply want your process to wait for a given number of seconds,
you should use the sleep function. See section 21.6 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.
Portability Note: The setitimer and getitimer
functions are derived from BSD Unix, while the alarm function is
specified by the POSIX.1 standard. setitimer is more powerful than
alarm, but alarm is more widely used.
The function 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 sleep to wait reliably throughout
the specified interval. Otherwise, sleep can return sooner if a
signal arrives; if you want to wait for a given interval regardless of
signals, use select (see section 13.8 Waiting for Input or Output) and don't specify
any descriptors to wait for.
Function: unsigned int sleep(unsigned int seconds)
The sleep function waits for seconds or until a signal
is delivered, whichever happens first.
If 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 sleep function is declared in `unistd.h'.
Resist the temptation to implement a sleep for a fixed amount of time by
using the return value of sleep, when nonzero, to call
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
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, sleep can do strange things if your program uses
SIGALRM explicitly. Even if SIGALRM signals are being
ignored or blocked when sleep is called, sleep might
return prematurely on delivery of a SIGALRM signal. If you have
established a handler for SIGALRM signals and a SIGALRM
signal is delivered while the process is sleeping, the action taken
might be just to cause sleep to return instead of invoking your
handler. And, if sleep is interrupted by delivery of a signal
whose handler requests an alarm or alters the handling of SIGALRM,
this handler and sleep will interfere.
On the GNU system, it is safe to use sleep and SIGALRM in
the same program, because sleep does not work by means of
SIGALRM.
Function: int nanosleep(const struct timespec *requested_time, struct timespec *remaining)
If resolution to seconds is not enough the 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.
*requested_time is the elapsed time of the interval you want to
sleep.
The function returns as *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.
If the function returns because the interval is over the return value is
zero. If the function returns -1 the global variable errno
is set to the following values:
EINTR
The call was interrupted because a signal was delivered to the thread.
If the remaining parameter is not the null pointer the structure
pointed to by remaining is updated to contain the remaining
elapsed time.
EINVAL
The nanosecond value in the requested_time parameter contains an
illegal value. Either the value is negative or greater than or equal to
1000 million.
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 nanosleep is
called. If the thread gets canceled these resources stay allocated
until the program ends. To avoid this calls to nanosleep should
be protected using cancellation handlers.