View Source: gettimeofday(2) - Waikato Linux Users Group

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!!NAME
gettimeofday - get time
!!SYNOPSIS
 __#include <sys/time.h>__
 __#include <time.h>__
 __int gettimeofday(struct timeval *__''tv''__, struct timezone *__''tz''__);__
!!DESCRIPTION
__gettimeofday__ can get the time as well as a timezone. ''tv'' is a __timeval__ struct, as specified in /usr/include/sys/time.h:

 struct timeval {
  long tv_sec;        /* seconds */
  long tv_usec;  /* microseconds */
 };

The ''tv_sec'' member of the struct is the number of seconds since the Epoch (see time(2)), and ''tv_usec'' is the amount of  microseconds past the current ''tv_sec'' value.

The ''tz'' argument is a __timezone__ :

 struct timezone {
  int  tz_minuteswest; /* minutes W of Greenwich */
  int  tz_dsttime;     /* type of dst correction */
 };
The use of the timezone struct is obsolete; the ''tz_dsttime'' field has never been used under Linux - it has not been and will not be supported by libc or glibc. Each and every occurrence of this field in the kernel source (other than the declaration) is a bug. Thus, the following is purely of historic interest.

The field ''tz_dsttime'' contains a symbolic constant (values are given below) that indicates in which part of the year Daylight Saving Time is in force. (Note: its value is constant throughout the year - it does not indicate that DST is in force, it just selects an algorithm.) The daylight saving time algorithms defined are as follows :

 __DST_NONE__ /* not on dst */
 __DST_USA__ /* USA style dst */
 __DST_AUST__ /* Australian style dst */
 __DST_WET__ /* Western European dst */
 __DST_MET__ /* Middle European dst */
 __DST_EET__ /* Eastern European dst */
 __DST_CAN__ /* Canada */
 __DST_GB__ /* Great Britain and Eire */
 __DST_RUM__ /* Rumania */
 __DST_TUR__ /* Turkey */
 __DST_AUSTALT__ /* Australian style with shift in 1986 */

Of course it turned out that the period in which Daylight Saving Time is in force cannot be given by a simple algorithm, one per country; indeed, this period is determined by unpredictable political decisions. So this method of representing time zones has been abandoned.

The following macros are defined to operate on a struct timeval :

 #define       timerisset(tvp)\
               ((tvp)->tv_sec || (tvp)->tv_usec)
 #define       timercmp(tvp, uvp, cmp)\
               ((tvp)->tv_sec cmp (uvp)->tv_sec ||\
               (tvp)->tv_sec == (uvp)->tv_sec &&\
               (tvp)->tv_usec cmp (uvp)->tv_usec)
 #define       timerclear(tvp)\
               ((tvp)->tv_sec = (tvp)->tv_usec = 0)


If either ''tv'' or ''tz'' is null, the corresponding structure is not set or returned.

!!RETURN VALUE
gettimeofday(2)  return 0 for success, or -1 for failure (in which case ''errno'' is set appropriately).

!!ERRORS
;[EINVAL]: Timezone (or something else) is invalid.
;[EFAULT]: One of ''tv'' or ''tz'' pointed outside your accessible address space.

!!NOTE
The defines for __timercmp__, __timerisset__, __timerclear__, __timeradd__, __timersub__ are (since glibc2.2.2) only available if ___BSD_SOURCE__ is defined (either explicitly, or implicitly, by not defining _POSIX_SOURCE or compiling with the -ansi flag).

!!CONFORMING TO
SVr4, BSD 4.3

!!SEE ALSO
date(1), adjtimex(2), time(2), ctime(3), ftime(3)

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Last edited on Tuesday, December 31, 2002 2:24:05 am by PerryLorier

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