Use a monotonic clock instead of a realtime clock

Using a realtime clock is a bad idea: it is affected by any kind of time
change, which can happen when the administrator modifies the system time,
or more simply when a laptop suspends to RAM and then wakes up from sleep.

With the current approach of using a realtime clock:

- if the system time jumps forward (e.g. when resuming after a
  suspend-to-RAM), bmon would take 100% CPU and display random graph data
  extremely fast, until it "catches up" with the new time.

- if the system time jumps backwards, bmon would freeze until *time*
  "catches up" to the point it was before.  bmon then (incorrectly)
  displays a spike in the graph, because lots of packets have been
  sent/received since the last update.

Instead of using gettimeofday(), switch to clock_gettime() with
CLOCK_MONOTONIC on systems that support it.  OS X does not provide
clock_gettime(), so this commit also adds a Mach-specific implementation.

This change has been tested on Linux 4.1 with glibc and musl, and on
FreeBSD 10.0-RELEASE-p12.

1 files changed, 4 insertions, 1 deletions

diff --git a/configure.ac b/configure.ac
index 70adab3..f1439e1 100644
--- a/configure.ac
+++ b/configure.ac
@@ -58,7 +58,7 @@ AC_CHECK_HEADERS(sys/param.h sys/socket.h)
 
 AC_CHECK_TYPES(suseconds_t)
 
-AC_CHECK_FUNCS(atexit gettimeofday memset pow socket strcasecmp)
+AC_CHECK_FUNCS(atexit clock_gettime memset pow socket strcasecmp)
 AC_CHECK_FUNCS(strchr strdup strerror strncasecmp strstr strtol)
 AC_CHECK_FUNCS(uname getdate)
 
@@ -86,6 +86,9 @@ esac
 
 AC_CHECK_LIB(m, pow, [], AC_MSG_ERROR([requires libm]))
 
+# Don't fail if not found (for instance, OS X does not have clock_gettime)
+AC_CHECK_LIB(rt, clock_gettime, [], [])
+
 BMON_LIB=""
 
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