This is libc.info, produced by makeinfo version 4.0c from libc.texinfo.

INFO-DIR-SECTION GNU libraries
START-INFO-DIR-ENTRY
* Libc: (libc).                 C library.
END-INFO-DIR-ENTRY

   This file documents the GNU C library.

   This is Edition 0.10, last updated 2001-07-06, of `The GNU C Library
Reference Manual', for Version 2.2.x.

   Copyright (C) 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2001 Free
Software Foundation, Inc.

   Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.1 or
any later version published by the Free Software Foundation; with the
Invariant Sections being "GNU Lesser General Public License", the
Front-Cover texts being (a) (see below), and with the Back-Cover Texts
being (b) (see below).  A copy of the license is included in the section
entitled "GNU Free Documentation License".

   (a) The FSF's Front-Cover Text is:

   A GNU Manual

   (b) The FSF's Back-Cover Text is:

   You have freedom to copy and modify this GNU Manual, like GNU
software.  Copies published by the Free Software Foundation raise
funds for GNU development.


File: libc.info,  Node: Parsing of Floats,  Prev: Parsing of Integers,  Up: Parsing of Numbers

Parsing of Floats
-----------------

   The `str' functions are declared in `stdlib.h' and those beginning
with `wcs' are declared in `wchar.h'.  One might wonder about the use
of `restrict' in the prototypes of the functions in this section.  It
is seemingly useless but the ISO C standard uses it (for the functions
defined there) so we have to do it as well.

 - Function: double strtod (const char *restrict STRING, char
          **restrict TAILPTR)
     The `strtod' ("string-to-double") function converts the initial
     part of STRING to a floating-point number, which is returned as a
     value of type `double'.

     This function attempts to decompose STRING as follows:

        * A (possibly empty) sequence of whitespace characters.  Which
          characters are whitespace is determined by the `isspace'
          function (*note Classification of Characters::).  These are
          discarded.

        * An optional plus or minus sign (`+' or `-').

        * A floating point number in decimal or hexadecimal format.  The
          decimal format is:
             - A nonempty sequence of digits optionally containing a
               decimal-point character--normally `.', but it depends on
               the locale (*note General Numeric::).

             - An optional exponent part, consisting of a character `e'
               or `E', an optional sign, and a sequence of digits.


          The hexadecimal format is as follows:
             - A 0x or 0X followed by a nonempty sequence of
               hexadecimal digits optionally containing a decimal-point
               character--normally `.', but it depends on the locale
               (*note General Numeric::).

             - An optional binary-exponent part, consisting of a
               character `p' or `P', an optional sign, and a sequence
               of digits.


        * Any remaining characters in the string.  If TAILPTR is not a
          null pointer, a pointer to this tail of the string is stored
          in `*TAILPTR'.

     If the string is empty, contains only whitespace, or does not
     contain an initial substring that has the expected syntax for a
     floating-point number, no conversion is performed.  In this case,
     `strtod' returns a value of zero and the value returned in
     `*TAILPTR' is the value of STRING.

     In a locale other than the standard `"C"' or `"POSIX"' locales,
     this function may recognize additional locale-dependent syntax.

     If the string has valid syntax for a floating-point number but the
     value is outside the range of a `double', `strtod' will signal
     overflow or underflow as described in *Note Math Error Reporting::.

     `strtod' recognizes four special input strings.  The strings
     `"inf"' and `"infinity"' are converted to oo, or to the largest
     representable value if the floating-point format doesn't support
     infinities.  You can prepend a `"+"' or `"-"' to specify the sign.
     Case is ignored when scanning these strings.

     The strings `"nan"' and `"nan(CHARS...)"' are converted to NaN.
     Again, case is ignored.  If CHARS... are provided, they are used
     in some unspecified fashion to select a particular representation
     of NaN (there can be several).

     Since zero is a valid result as well as the value returned on
     error, you should check for errors in the same way as for
     `strtol', by examining ERRNO and TAILPTR.

 - Function: float strtof (const char *STRING, char **TAILPTR)
 - Function: long double strtold (const char *STRING, char **TAILPTR)
     These functions are analogous to `strtod', but return `float' and
     `long double' values respectively.  They report errors in the same
     way as `strtod'.  `strtof' can be substantially faster than
     `strtod', but has less precision; conversely, `strtold' can be
     much slower but has more precision (on systems where `long double'
     is a separate type).

     These functions have been GNU extensions and are new to ISO C99.

 - Function: double wcstod (const wchar_t *restrict STRING, wchar_t
          **restrict TAILPTR)
 - Function: float wcstof (const wchar_t *STRING, wchar_t **TAILPTR)
 - Function: long double wcstold (const wchar_t *STRING, wchar_t
          **TAILPTR)
     The `wcstod', `wcstof', and `wcstol' functions are equivalent in
     nearly all aspect to the `strtod', `strtof', and `strtold'
     functions but it handles wide character string.

     The `wcstod' function was introduced in Amendment 1 of ISO C90.
     The `wcstof' and `wcstold' functions were introduced in ISO C99.

 - Function: double atof (const char *STRING)
     This function is similar to the `strtod' function, except that it
     need not detect overflow and underflow errors.  The `atof' function
     is provided mostly for compatibility with existing code; using
     `strtod' is more robust.

   The GNU C library also provides `_l' versions of these functions,
which take an additional argument, the locale to use in conversion.
*Note Parsing of Integers::.


File: libc.info,  Node: System V Number Conversion,  Prev: Parsing of Numbers,  Up: Arithmetic

Old-fashioned System V number-to-string functions
=================================================

   The old System V C library provided three functions to convert
numbers to strings, with unusual and hard-to-use semantics.  The GNU C
library also provides these functions and some natural extensions.

   These functions are only available in glibc and on systems descended
from AT&T Unix.  Therefore, unless these functions do precisely what you
need, it is better to use `sprintf', which is standard.

   All these functions are defined in `stdlib.h'.

 - Function: char * ecvt (double VALUE, int NDIGIT, int *DECPT, int
          *NEG)
     The function `ecvt' converts the floating-point number VALUE to a
     string with at most NDIGIT decimal digits.  The returned string
     contains no decimal point or sign. The first digit of the string
     is non-zero (unless VALUE is actually zero) and the last digit is
     rounded to nearest.  `*DECPT' is set to the index in the string of
     the first digit after the decimal point.  `*NEG' is set to a
     nonzero value if VALUE is negative, zero otherwise.

     If NDIGIT decimal digits would exceed the precision of a `double'
     it is reduced to a system-specific value.

     The returned string is statically allocated and overwritten by
     each call to `ecvt'.

     If VALUE is zero, it is implementation defined whether `*DECPT' is
     `0' or `1'.

     For example: `ecvt (12.3, 5, &d, &n)' returns `"12300"' and sets D
     to `2' and N to `0'.

 - Function: char * fcvt (double VALUE, int NDIGIT, int *DECPT, int
          *NEG)
     The function `fcvt' is like `ecvt', but NDIGIT specifies the
     number of digits after the decimal point.  If NDIGIT is less than
     zero, VALUE is rounded to the NDIGIT+1'th place to the left of the
     decimal point.  For example, if NDIGIT is `-1', VALUE will be
     rounded to the nearest 10.  If NDIGIT is negative and larger than
     the number of digits to the left of the decimal point in VALUE,
     VALUE will be rounded to one significant digit.

     If NDIGIT decimal digits would exceed the precision of a `double'
     it is reduced to a system-specific value.

     The returned string is statically allocated and overwritten by
     each call to `fcvt'.

 - Function: char * gcvt (double VALUE, int NDIGIT, char *BUF)
     `gcvt' is functionally equivalent to `sprintf(buf, "%*g", ndigit,
     value'.  It is provided only for compatibility's sake.  It returns
     BUF.

     If NDIGIT decimal digits would exceed the precision of a `double'
     it is reduced to a system-specific value.

   As extensions, the GNU C library provides versions of these three
functions that take `long double' arguments.

 - Function: char * qecvt (long double VALUE, int NDIGIT, int *DECPT,
          int *NEG)
     This function is equivalent to `ecvt' except that it takes a `long
     double' for the first parameter and that NDIGIT is restricted by
     the precision of a `long double'.

 - Function: char * qfcvt (long double VALUE, int NDIGIT, int *DECPT,
          int *NEG)
     This function is equivalent to `fcvt' except that it takes a `long
     double' for the first parameter and that NDIGIT is restricted by
     the precision of a `long double'.

 - Function: char * qgcvt (long double VALUE, int NDIGIT, char *BUF)
     This function is equivalent to `gcvt' except that it takes a `long
     double' for the first parameter and that NDIGIT is restricted by
     the precision of a `long double'.

   The `ecvt' and `fcvt' functions, and their `long double'
equivalents, all return a string located in a static buffer which is
overwritten by the next call to the function.  The GNU C library
provides another set of extended functions which write the converted
string into a user-supplied buffer.  These have the conventional `_r'
suffix.

   `gcvt_r' is not necessary, because `gcvt' already uses a
user-supplied buffer.

 - Function: char * ecvt_r (double VALUE, int NDIGIT, int *DECPT, int
          *NEG, char *BUF, size_t LEN)
     The `ecvt_r' function is the same as `ecvt', except that it places
     its result into the user-specified buffer pointed to by BUF, with
     length LEN.

     This function is a GNU extension.

 - Function: char * fcvt_r (double VALUE, int NDIGIT, int *DECPT, int
          *NEG, char *BUF, size_t LEN)
     The `fcvt_r' function is the same as `fcvt', except that it places
     its result into the user-specified buffer pointed to by BUF, with
     length LEN.

     This function is a GNU extension.

 - Function: char * qecvt_r (long double VALUE, int NDIGIT, int *DECPT,
          int *NEG, char *BUF, size_t LEN)
     The `qecvt_r' function is the same as `qecvt', except that it
     places its result into the user-specified buffer pointed to by
     BUF, with length LEN.

     This function is a GNU extension.

 - Function: char * qfcvt_r (long double VALUE, int NDIGIT, int *DECPT,
          int *NEG, char *BUF, size_t LEN)
     The `qfcvt_r' function is the same as `qfcvt', except that it
     places its result into the user-specified buffer pointed to by
     BUF, with length LEN.

     This function is a GNU extension.


File: libc.info,  Node: Date and Time,  Next: Resource Usage And Limitation,  Prev: Arithmetic,  Up: Top

Date and Time
*************

   This chapter describes functions for manipulating dates and times,
including functions for determining what time it is and conversion
between different time representations.

* Menu:

* Time Basics::                 Concepts and definitions.
* Elapsed Time::                Data types to represent elapsed times
* Processor And CPU Time::      Time a program has spent executing.
* Calendar Time::               Manipulation of ``real'' dates and times.
* Setting an Alarm::            Sending a signal after a specified time.
* Sleeping::                    Waiting for a period of time.


File: libc.info,  Node: Time Basics,  Next: Elapsed Time,  Up: Date and Time

Time Basics
===========

   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.


File: libc.info,  Node: Elapsed Time,  Next: Processor And CPU Time,  Prev: Time Basics,  Up: Date and Time

Elapsed Time
============

   One way to represent an elapsed time is with a simple arithmetic data
type, as with the following function to compute the elapsed time between
two calendar times.  This function is declared in `time.h'.

 - Function: double difftime (time_t TIME1, time_t TIME0)
     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.


File: libc.info,  Node: Processor And CPU Time,  Next: Calendar Time,  Prev: Elapsed Time,  Up: Date and Time

Processor And CPU Time
======================

   If you're trying to optimize your program or measure its efficiency,
it's very useful to know how much processor time it uses.  For that,
calendar time and elapsed times are useless because a process may spend
time waiting for I/O or for other processes to use the CPU.  However,
you can get the information with the functions in this section.

   CPU time (*note 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, *Note Resource Usage And Limitation::.

* Menu:

* CPU Time::                    The `clock' function.
* Processor Time::              The `times' function.


File: libc.info,  Node: CPU Time,  Next: Processor Time,  Up: Processor And CPU Time

CPU Time Inquiry
----------------

   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.  *Note Processor Time::.

   Different computers and operating systems vary wildly in how they
keep track of CPU time.  It's common for the internal processor clock
to have a resolution somewhere between a hundredth and millionth of a
second.

 - 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)'.


File: libc.info,  Node: Processor Time,  Prev: CPU Time,  Up: Processor And CPU Time

Processor Time Inquiry
----------------------

   The `times' function returns information about a process'
consumption of processor time in a `struct tms' object, in addition to
the process' CPU time.  *Note 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 *Note 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.  *Note 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 *Note CPU
Time:: 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'.


File: libc.info,  Node: Calendar Time,  Next: Setting an Alarm,  Prev: Processor And CPU Time,  Up: Date and Time

Calendar Time
=============

   This section describes facilities for keeping track of calendar time.
*Note 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.

* Menu:

* Simple Calendar Time::        Facilities for manipulating calendar time.
* High-Resolution Calendar::    A time representation with greater precision.
* Broken-down Time::            Facilities for manipulating local time.
* High Accuracy Clock::         Maintaining a high accuracy system clock.
* Formatting Calendar Time::    Converting times to strings.
* Parsing Date and Time::       Convert textual time and date information back
                                 into broken-down time values.
* TZ Variable::                 How users specify the time zone.
* Time Zone Functions::         Functions to examine or specify the time zone.
* Time Functions Example::      An example program showing use of some of
				 the time functions.


File: libc.info,  Node: Simple Calendar Time,  Next: High-Resolution Calendar,  Up: Calendar Time

Simple Calendar Time
--------------------

   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 (*note TZ Variable::).

     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.  *Note 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'.  *Note 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 process is not superuser.


File: libc.info,  Node: High-Resolution Calendar,  Next: Broken-down Time,  Prev: Simple Calendar Time,  Up: Calendar Time

High-Resolution Calendar
------------------------

   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.

     The `struct timezone' type is obsolete and should never be used.
     Instead, use the facilities described in *Note Time Zone
     Functions::.

 - 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.  (*note 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 *Note
          Time Zone Functions::.

 - 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'.

 - Function: int adjtimex (struct timex *TIMEX)
     `adjtimex' is functionally identical to `ntp_adjtime'.  *Note High
     Accuracy Clock::.

     This function is present only with a Linux kernel.



File: libc.info,  Node: Broken-down Time,  Next: High Accuracy Clock,  Prev: High-Resolution Calendar,  Up: Calendar Time

Broken-down Time
----------------

   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.

 - Function: struct tm * localtime (const time_t *TIME)
     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'.

     Calling `localtime' has one other effect: it sets the variable
     `tzname' with information about the current time zone.  *Note Time
     Zone Functions::.

   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.

 - Function: struct tm * localtime_r (const time_t *TIME, struct tm
          *RESULTP)
     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'.

 - Function: struct tm * gmtime_r (const time_t *TIME, struct tm
          *RESULTP)
     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.

     Calling `mktime' also sets the variable `tzname' with information
     about the current time zone.  *Note Time Zone Functions::.

 - Function: time_t timelocal (struct tm *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.



File: libc.info,  Node: High Accuracy Clock,  Next: Formatting Calendar Time,  Prev: Broken-down Time,  Up: Calendar Time

High Accuracy Clock
-------------------

   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 *Note 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'.


