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.\"     @(#)time.3	8.1 (Berkeley) 6/4/93
.\"
.Dd April 14, 2022
.Dt TIME 3
.Os
.Sh NAME
.Nm time
.Nd get time of day
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In time.h
.Ft time_t
.Fn time "time_t *tloc"
.Sh DESCRIPTION
The
.Fn time
function
returns the value of time in seconds since 0 hours, 0 minutes,
0 seconds, January 1, 1970, Coordinated Universal Time (UTC).
If an error occurs,
.Fn time
returns the value
.Po Vt time_t Pc Ns \-1 .
.Pp
The return value is also stored in
.No \&* Ns Va tloc ,
provided that
.Va tloc
is non-null.
.Sh ERRORS
The
.Fn time
function may fail for any of the reasons described in
.Xr clock_gettime 2 .
.Sh SEE ALSO
.Xr clock_gettime 2 ,
.Xr gettimeofday 2 ,
.Xr ctime 3
.Sh STANDARDS
The
.Nm
function conforms to
.St -p1003.1-2008 .
.Sh HISTORY
The
.Fn time
system call first appeared in
.At v1 .
Through the
.At v3 ,
it returned 60 Hz ticks since an epoch that changed occasionally, because it
was a 32-bit value that overflowed in a little over 2 years.
.Pp
In
.At v4
the granularity of the return value was reduced to whole seconds,
delaying the aforementioned overflow until 2038.
.Pp
.At v7
introduced the
.Fn ftime
system call, which returned time at a millisecond level,
though retained the
.Fn gtime
system call (exposed as
.Fn time
in userland).
.Fn time
could have been implemented as a wrapper around
.Fn ftime ,
but that wasn't done.
.Pp
.Bx 4.1c
implemented a higher-precision time function
.Fn gettimeofday
to replace
.Fn ftime
and reimplemented
.Fn time
in terms of that.
.Pp
Since
.Fx 9
the implementation of
.Fn time
uses
.Fn clock_gettime "CLOCK_SECOND"
instead of
.Fn gettimeofday
for performance reasons.
.Sh BUGS
Neither
.St -isoC-99
nor
.St -p1003.1-2001
requires
.Fn time
to set
.Va errno
on failure; thus, it is impossible for an application to distinguish
the valid time value \-1 (representing the last UTC second of 1969)
from the error return value.
.Pp
Systems conforming to earlier versions of the C and POSIX
standards (including older versions of
.Fx )
did not set
.No \&* Ns Va tloc
in the error case.