timer_create.2: Document CLOCK_BOOTTIME

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

diff --git a/man2/timer_create.2 b/man2/timer_create.2
index aec4ec299868be7d388eb7b17a956c780175dd9d..7dbb02e7228f4960efcfcbd842aa1370d4893dde 100644 (file)
--- a/man2/timer_create.2
+++ b/man2/timer_create.2
@@ -23,9 +23,6 @@
 .\" the source, must acknowledge the copyright and authors of this work.
 .\" %%%LICENSE_END
 .\"
-.\" FIXME Linux 2.6.39 adds CLOCK_BOOTTIME, which needs be documented
-.\" Does this also affect timerfd_create()?
-.\"
 .\" FIXME Linux 3.0 adds CLOCK_BOOTTIME_ALARM and CLOCK_REALTIME_ALARM,
 .\" which need be documented
 .\" Does this also affect timerfd_create()?
@@ -84,6 +81,21 @@ A clock that measures (user and system) CPU time consumed by
 the calling thread.
 .\" The CLOCK_MONOTONIC_RAW that was added in 2.6.28 can't be used
 .\" to create a timer -- mtk, Feb 2009
+.TP
+.BR CLOCK_BOOTTIME " (Since Linux 2.6.39)"
+.\" commit 70a08cca1227dc31c784ec930099a4417a06e7d0
+Like
+.BR CLOCK_MONOTONIC ,
+this is a monotonically increasing clock.
+However, whereas the
+.BR CLOCK_MONOTONIC
+clock does not measure the time while a system is suspended, the
+.BR CLOCK_BOOTTIME
+clock does include the time during which the system is suspended.
+This is useful for applications that need to be suspend-aware.
+.BR CLOCK_REALTIME
+is not suitable for such applications, since that clock is affected
+by discontinuous changes to the system clock.
 .PP
 As well as the above values,
 .I clockid