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time(3): Refine history in the manual page

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The time() system call first appeared in Version 1 AT&T UNIX.  Through
the Version 3 AT&T UNIX, 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.

In Version 4 AT&T UNIX the granularity of the return value was reduced to
whole seconds, delaying the aforementioned overflow until 2038.

Version 7 AT&T UNIX introduced the ftime() system call, which returned
time at a millisecond level, though retained the gtime() system call
(exposed as time() in userland).  time() could have been implemented as a
wrapper around ftime(), but that wasn't done.

4.1cBSD implemented a higher-precision time function gettimeofday() to
replace ftime() and reimplemented time() in terms of that.

Since FreeBSD 9 the implementation of time() uses
clock_gettime(CLOCK_SECOND) instead of gettimeofday() for performance
reasons.

With most valuable input from Warner (imp@).

Reviewed by:	0mp, jilles, imp
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D34751
This commit is contained in:
Gordon Bergling 2022-04-14 10:04:14 +02:00
parent 239170f292
commit 3e0f3678ec
1 changed files with 49 additions and 8 deletions
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57
lib/libc/gen/time.3
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@ -32,7 +32,7 @@
.\" @(#)time.3 8.1 (Berkeley) 6/4/93
.\" $FreeBSD$
.\"
.Dd March 4, 2022
.Dd April 14, 2022
.Dt TIME 3
.Os
.Sh NAME
@ -49,7 +49,7 @@ The
.Fn time
function
returns the value of time in seconds since 0 hours, 0 minutes,
0 seconds, January 1, 1970, Coordinated Universal Time.
0 seconds, January 1, 1970, Coordinated Universal Time (UTC).
If an error occurs,
.Fn time
returns the value
@ -73,12 +73,54 @@ function may fail for any of the reasons described in
The
.Nm
function conforms to
.St -p1003.1-2001 .
.St -p1003.1-2008 .
.Sh HISTORY
A
The
.Fn time
function appeared in
.At v6 .
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
@ -92,8 +134,7 @@ 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
.Tn POSIX
Systems conforming to earlier versions of the C and POSIX
standards (including older versions of
.Fx )
did not set