On Fri, Mar 26, 2010 at 12:51 PM, J.C. Roberts <list-...@designtools.org> wrote: ... > Think about it this way; you use the "boot> " prompt to boot up your > experimental kernel named "/bsd.exp". It dumps core before you can get > a dmesg, so you want to see what the heck was going on. You reboot with > your normal kernel "/bsd" and use dmesg on the core file, and tell it > where to find the corresponding names (i.e. in your experimental kernel) > > # dmesg -M bsd.exp.core -N /bsd.exp > > Obviously, the default input for values, /dev/kmem, will only give > values of the currently running kernel, hence to get those values you > either need to running a particular kernel, or crash it to dump core.
Yes, exactly. > As for dmesg picking the right kernel file for getting the names of the > currently running kernel, there is either some magic in demsg which > picks the right kernel from disk, or a properly running kernel does not > need to read names from the kernel file since they're already in memory. It's simpler than that: when run without the -N or -M options, dmesg gets the info via sysctl() instead of trying to read from /dev/kmem. This makes it ABI-proof against various kernel or libkvm changes that break code that reads /dev/kmem. That's true not just of dmesg but of all the other programs that need kernel info: ps, netstat, fstat, ipcs, systat, etc. $ sysctl -A kern 2>&1 >/dev/null | grep 'use ' | head sysctl: use pstat to view kern.vnode information sysctl: use ps to view kern.proc information sysctl: use pstat to view kern.file information sysctl: use pstat -t to view kern.tty.ttyinfo information sysctl: use dmesg to view kern.msgbuf sysctl: use netstat to view kern.mbstat sysctl: use ps to view kern.proc2 information sysctl: use fstat to view kern.file2 information sysctl: use vmstat or systat to view vm.vmmeter information sysctl: use vmstat or systat to view vm.uvmexp information $ Make sense? Philip Guenther
