GETTIMEOFDAY(2) Linux Programmer's Manual GETTIMEOFDAY(2)
NAME
gettimeofday, settimeofday - get / set time
SYNOPSIS
#include<sys/time.h>
intgettimeofday(structtimeval*tv,structtimezone*tz);
intsettimeofday(conststructtimeval*tv ,conststructtimezone*tz);
DESCRIPTION
The functions gettimeofday and settimeofday can get and set the time as
well as a timezone. The tv argument is a timeval struct, as specified
in /usr/include/sys/time.h:
struct timeval {
long tv_sec; /* seconds */
long tv_usec; /* microseconds */
};
and gives the number of seconds and microseconds since the Epoch (see
time(2)). 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The use of the timezone struct is obsolete; the tz_dsttime field has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 ker-
nel 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. Under
Linux, in a call to settimeofday the tz_dsttime field should be zero.
Under Linux there is some peculiar 'warp clock' semantics associated to
the settimeofday system call if on the very first call (after booting)
that has a non-NULL tz argument, the tv argument is NULL and the
tz_minuteswest field is nonzero. In such a case it is assumed that the
CMOS clock is on local time, and that it has to be incremented by this
amount to get UTC system time. No doubt it is a bad idea to use this
feature.
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.
Only the super user may use settimeofday.
RETURNVALUE
gettimeofday and settimeofday return 0 for success, or -1 for failure
(in which case errno is set appropriately).
ERRORS
EPERMsettimeofday is called by someone other than the superuser.
EINVAL Timezone (or something else) is invalid.
EFAULT One of tv or tz pointed outside your accessible address space.
NOTE
The prototype for settimeofday and the defines for timercmp, timeris-
set, timerclear, timeradd, timersub are (since glibc2.2.2) only avail-
able if _BSD_SOURCE is defined (either explicitly, or implicitly, by
not defining _POSIX_SOURCE or compiling with the -ansi flag).
CONFORMINGTO
SVr4, BSD 4.3
SEEALSO
date(1), adjtimex(2), time(2), ctime(3), ftime(3)
Linux 2.0.32 1997-12-10 GETTIMEOFDAY(2)
Take our 
