charts: show ??s when proc_stat->start is unset

When libvmon fails to successfully sample proc_stat, it will
leave this value as 0, which isn't really otherwise a normal
process start value.

Handle this by producing "??s" for the Wall time normally derived
from (sys_stat->boottime - proc_stat->start), to prevent
producing an incorrect Wall time equal to sys_stat->boottime.

There should probably be a more robust means of communicating
these libvmon sampling failures to vwm/vmon, but I've thus far
been resisting adding something like an errno to every sample
store, or worse every sample store's datum.  It's kind of
non-trivial to do without bloating the sample stores, especially
since the stores consolidate multiple proc files under a single
store/want.  Having a single errno in the store would prevent
letting the valid portions of the store be usable while ignoring
the errored portions.  Perhaps just a per-store errno with a
bitfield to indicate which subset are errored would suffice...

1 files changed, 2 insertions, 0 deletions

diff --git a/src/charts.c b/src/charts.c
index 0edb7e6..11680d0 100644
--- a/src/charts.c
+++ b/src/charts.c
@@ -794,6 +794,8 @@ static void draw_columns(vwm_charts_t *charts, vwm_chart_t *chart, vwm_column_t
 		case VWM_COLUMN_PROC_WALL: /* User Sys Wall times */
 			if (!row)
 				str_len = snpf(str, sizeof(str), "Wall");
+			else if (!proc_stat->start)
+				str_len = snpf(str, sizeof(str), "??s");
 			else
 				str_len = snpf(str, sizeof(str), "%.2fs",
 						(float)(sys_stat->boottime - proc_stat->start) / (float)charts->vmon.ticks_per_sec);