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'.
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.
struct timeval structure represents an elapsed time. It is
declared in `sys/time.h' and has the following members:
long int tv_sec
long int tv_usec
struct timespec structure represents an elapsed time. It is
declared in `time.h' and has the following members:
long int tv_sec
long int tv_nsec
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 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 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 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.
clock function. POSIX requires that this value be one
million independent of the actual resolution.
CLOCKS_PER_SEC.
clock function.
Values of type clock_t are numbers of clock ticks.
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 Time Basics. You should
include the header file `sys/times.h' to use this facility.
tms structure is used to return information about process
times. It contains at least the following members:
clock_t tms_utime
clock_t tms_stime
clock_t tms_cutime
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 section 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
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 Creating a Process.
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
section 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 Time Basics.
The GNU C library represents calendar time three ways:
time_t data type) is a compact
representation, typically giving the number of seconds of elapsed time
since some implementation-specific base time.
struct
timeval data type, which includes fractions of a second. Use this time
representation instead of simple time when you need greater precision.
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'.
TZ to certain values (see section 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 Elapsed Time.
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.
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 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:
EPERM
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'.
struct timezone structure is used to hold minimal information
about the local time zone. It has the following members:
int tz_minuteswest
int tz_dsttime
The struct timezone type is obsolete and should never be used.
Instead, use the facilities described in section Functions and Variables for Time Zones.
gettimeofday function returns the current calendar time as
the elapsed time since the epoch in the struct timeval structure
indicated by tp. (see section 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
struct timezone to represent time zone
information; that is an obsolete feature of 4.3 BSD.
Instead, use the facilities described in section Functions and Variables for Time Zones.
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
ENOSYS
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
Portability Note: The gettimeofday, settimeofday,
and adjtime functions are derived from BSD.
Symbols for the following function are declared in `sys/timex.h'.
adjtimex is functionally identical to ntp_adjtime.
See section High Accuracy Clock.
This function is present only with a Linux kernel.
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'.
int tm_sec
0 through 59, but the actual upper limit is
60, to allow for leap seconds if leap second support is
available).
int tm_min
0 through 59).
int tm_hour
0 through
23).
int tm_mday
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
0 through 11). Watch out for this one!
People usually use ordinal numbers for month-of-year (where January = 1).
int tm_year
int tm_wday
0 through
6).
int tm_yday
0 through 365).
int tm_isdst
long int tm_gmtoff
-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
tm_gmtoff, this field is a BSD and
GNU extension, and is not visible in a strict ISO C environment.
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.
Calling localtime has one other effect: it sets the variable
tzname with information about the current time zone. See section Functions and Variables for Time Zones.
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.
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.
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.
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.
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.
Calling mktime also sets the variable tzname with
information about the current time zone. See section Functions and Variables for Time Zones.
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.
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'.
struct timeval time
struct timeval data type is described in
section Elapsed Time.
long int maxerror
ntp_adjtime periodically, this value will reach some
platform-specific maximum value.
long int esterror
ntp_adjtime to indicate the estimated offset of the
system clock from the true calendar time.
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
unsigned int modes
MOD_.
long int offset
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
1 <<
SHIFT_USEC. The value can be set with bit MOD_FREQUENCY, but
the absolute value must not exceed MAXFREQ.
long int maxerror
MOD_MAXERROR. Unless updated via
ntp_adjtime periodically, this value will increase steadily
and reach some platform-specific maximum value.
long int esterror
MOD_ESTERROR.
int status
STA_. Some of these flags can be updated using the
MOD_STATUS bit.
long int constant
MOD_TIMECONST.
long int precision
long int tolerance
maxerror every
second.
struct timeval time
long int tick
long int ppsfreq
long int jitter
int shift
PPS_SHIFT to PPS_SHIFTMAX.
long int stabil
long int jitcnt
MAXTIME.
long int calcnt
long int errcnt
long int stbcnt
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
EPERM
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 functions described in this section format calendar time values as strings. These functions are declared in the header file `time.h'.
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.)
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.
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
asctime (localtime (time))
ctime sets the variable tzname, because localtime
does so. See section Functions and Variables for Time Zones.
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):
({ struct tm tm; asctime_r (localtime_r (time, &tm), buf); })
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.
sprintf function (see section 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 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:
_
-
0
^
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
%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
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
%A
%b
%B
%c
%C
%d
01 through 31).
%D
%m/%d/%y.
This format is a POSIX.2 extension and also appears in ISO C99.
%e
%d, but padded with blank (range
1 through 31).
This format is a POSIX.2 extension and also appears in ISO C99.
%F
%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
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
%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
%b.
This format is a POSIX.2 extension and also appears in ISO C99.
%H
00 through
23).
%I
01 through
12).
%j
001 through 366).
%k
%H, but
padded with blank (range 0 through 23).
This format is a GNU extension.
%l
%I, but
padded with blank (range 1 through 12).
This format is a GNU extension.
%m
01 through 12).
%M
00 through 59).
%n
%p
%P
%r
%R
%H:%M.
This format was introduced in ISO C99 but was previously available
as a GNU extension.
%s
%S
00 through 60).
%t
%T
%H:%M:%S.
This format is a POSIX.2 extension.
%u
1 through
7), Monday being 1.
This format is a POSIX.2 extension and also appears in ISO C99.
%U
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
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
0 through
6), Sunday being 0.
%W
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
%X
%y
00 through
99). This is equivalent to the year modulo 100.
%Y
1 are numbered 0, -1, and so on.
%z
-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 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.
For an example of strftime, see section Time Functions Example.
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.
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.
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.
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 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
%b
%B
%h
%c
%Ec
%c but the locale's alternative date and time format is used.
%C
%y format.
%EC
%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
1 through 31).
Leading zeroes are permitted but not required.
%Od
%Oe
%d but using the locale's alternative numeric symbols.
Leading zeroes are permitted but not required.
%D
%m/%d/%y.
%F
%Y-%m-%d, which is the ISO 8601 date
format.
This is a GNU extension following an ISO C99 extension to
strftime.
%g
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
strftime.
%H
%k
00 through
23).
%k is a GNU extension following a GNU extension of strftime.
%OH
%H but using the locale's alternative numeric symbols.
%I
%l
01 through
12).
%l is a GNU extension following a GNU extension of strftime.
%OI
%I but using the locale's alternative numeric symbols.
%j
1 through 366).
Leading zeroes are permitted but not required.
%m
1 through 12).
Leading zeroes are permitted but not required.
%Om
%m but using the locale's alternative numeric symbols.
%M
0 through 59).
Leading zeroes are permitted but not required.
%OM
%M but using the locale's alternative numeric symbols.
%n
%t
%p
%P
%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
%R
%H:%M.
%R is a GNU extension following a GNU extension to strftime.
%s
%s is a GNU extension following a GNU extension to strftime.
%S
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
%S but using the locale's alternative numeric symbols.
%T
%H:%M:%S in this place.
%u
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
0
through 53).
Leading zeroes are permitted but not required.
%OU
%U but using the locale's alternative numeric symbols.
%V
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
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
%w but using the locale's alternative numeric symbols.
%W
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
%W but using the locale's alternative numeric symbols.
%x
%Ex
%x but the locale's alternative data representation is used.
%X
%EX
%X but the locale's alternative time representation is used.
%y
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 @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 %y completely and use %Y
instead.
%Ey
%EC in the locale's alternative representation.
%Oy
%C) using the locale's alternative
numeric symbols.
%Y
%EY
%z
%a, %d %b %Y %H:%M:%S %z in this place.
This is the full ISO 8601 date and time format.
%Z
%%
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:
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.
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;
}
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).
int contains the error code of the last
unsuccessful call to getdate. Defined values are:
DATEMSK is not defined or null.
DATEMSK environment variable
cannot be opened.
time_t variable.
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:
%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.
tm_wday
value the current week's day is chosen, otherwise the day next week is chosen.
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.
@multitable {xxxxxxxxxxxx} {xxxxxxxxxx} {xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx}
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.
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.
TZ
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 section 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:
std offset
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:
std offset dst [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
1 and 365.
February 29 is never counted, even in leap years.
n
0 and 365.
February 29 is counted in leap years.
Mm.w.d
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
last d 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 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.
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.
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.
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.
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:
SIGALRM signal to the process when it expires.
SIGVTALRM signal to the process when it expires.
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 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.
struct timeval it_interval
struct timeval it_value
The struct timeval data type is described in section Elapsed Time.
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
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
setitimer and getitimer functions to specify the real-time
timer.
ITIMER_VIRTUAL
setitimer and getitimer functions to specify the virtual
timer.
ITIMER_PROF
setitimer and getitimer functions to specify the profiling
timer.
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:
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;
}
There is an example showing the use of the alarm function in
section Signal Handlers that Return.
If you simply want your process to wait for a given number of seconds,
you should use the sleep function. See section 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 Waiting for Input or Output) and don't specify
any descriptors to wait for.
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.
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.
struct timespec is described in See section 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 errno is set to the following values:
EINTR
EINVAL
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.
The nanosleep function is declared in `time.h'.
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