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I'd like to write a statement to dmesg. [How] can I do this?

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Why do you want to do this? – Nils Apr 3 '12 at 20:36
Does it matter? :) – stephelton Apr 3 '12 at 20:38
Two reasons I've wanted to do this before: (1) to see what "now is" in dmesg-timestamp format, and (2) to know when I last looked at dmesg. – stephelton Apr 3 '12 at 20:39
I'm pretty sure now is seconds from boot, and I think it takes a kernel call to write to it so a userland program can't. – Kevin Apr 3 '12 at 21:22
@Kevin, so you know without looking how many seconds have elapsed since you booted? – stephelton Apr 4 '12 at 1:28

3 Answers

up vote 7 down vote accepted

Write to /dev/kmsg (not /proc/kmsg as suggested by @Nils). See linux/drivers/char/mem.c kmsg_write for the kernel-side implementation and systemd/src/log.c write_to_kmsg for an example of usage.

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up vote 2 down vote

Assuming nobody else comes up with an official way to do this ...

You can write a kernel module that calls the printk function. There's an example here that might just do the job for you.

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a. No need, others have written it already (e.g. kecho). b. Really no need for an extra module, see my answer. – ephemient Apr 3 '12 at 21:44
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If I understood man dmesg correctly, you should be able to write to /proc/kmsg.

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# echo "test" >> /proc/kmsg [yields =>] -su: echo: write error: Input/output error – stephelton Apr 3 '12 at 20:50

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