What is the first column in dmesg?

Question

I'm trying the dmesg command which outputs the message buffer of the kernal. The format returned on my setup is in the format:

...
[    0.000000]   3 base 100000000 mask F00000000 write-back
...
[   95.709163] wlo1: associated
[   95.709218] IPv6: ADDRCONF(NETDEV_CHANGE): wlo1: link becomes ready
...

The number in the first column increases with each entry but when I read man dmesg or the Wikipedia entry I don't see any information on what this value is.

What is the significance of the value in the first column?

Answer 1

That’s the timestamp of the event, i.e. the time at which it occurred, measured in seconds (with 0 being equal to the time at which the kernel booted).

You can print actual times using -T instead, skip the timestamps with -t, or specify a format with --time-format. See man dmesg for details.

Note: dmesg, as its manpage says, simply displays the contents of the “kernel ring buffer”, i.e. however many kernel messages fit in the buffer. That might extend all the way back to the kernel boot, but not necessarily. The end of dmesg is the last message printed by the kernel.