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dmesg is said to examine or control the kernel ring buffer. But it has option --kernel and --userspace.
Does dmesg get kernel messages from the kernel ring buffer?
Does the kernel ring buffer store only messages generated by the kernel, not those by processes?
Where does dmesg get user space messages?
Do user space messages include those messages generated by all the processes, for example, by cron?
Does
dmesgget kernel messages from the kernel ring buffer?
Yes, as described in man dmesg:
dmesgis used to examine or control the kernel ring buffer.
Does the kernel ring buffer store only messages generated by the kernel, not those by processes?
No, processes can add messages to the kernel ring buffer, by writing to /dev/kmsg.
Where does
dmesgget user space messages?
From the kernel ring buffer, using either /dev/kmsg or the syslog system call (not to be confused with the syslog system logger and associated services). Messages are distinguished by their “facility”; kernel messages use the kern facility, user messages the user, mail, daemon etc. facilities (see dmesg --help for a list of supported facilities).
Do user space messages include those messages generated by all the processes, for example, by cron?
No, only those explicitly added to the kernel ring buffer.
syslog dæmons can typically read from the kernel ring buffer on Linux, but that’s the only connection. I’m not aware of anything that writes to the buffer apart from the kernel (which doesn’t need a device to do so), and systemd (which will write to the kernel buffer if it can’t write to its journal or to syslog).
