OpenBSD client is broken and I'm not sure how long this has been the case. Attached patch against 4.3.18 branch fixes it. However, I'm noticing another problem I cannot explain: in the cpu output the "top" data has this error in it:
warning: process count should be a non-negative number -- using default
Reading through the Client Data top seems to be running with the wrong arguments: top -d -20
After further testing I can only get this message to appear in my own invocations of "top" if I give a negative number with the -d argument. Unfortunately while grepping through the code I cannot find where this is being called from -- it's certainly not in the xymonclient-openbsd.sh file.
A note about the patch:
The change in lib/sendmsg.c is due to OpenBSD not having msglen_t in <sys/msg.h>. It is unsigned long on FreeBSD and I assume it's the same on other platforms as well or there would be issues. The sendmsg.c probably won't apply cleanly to trunk because it appears much has changed in sendmsg.c and this code is about 200 lines earlier.