what‘s the difference between shell-cmd ’dmesg‘ and file ’/var/log/dmesg‘

I think the ouput of the command dmesg should be the subset of the file /var/log/dmesg. But when I am debugging,the segmentation fault information can only be seen in the output of the command dmesg.

What is the difference between them ?

Answer

The kernel message buffer is a ring buffer with limited space – when new messages arrive old may get dropped.

dmesg outputs the current buffer.

Normally /var/log/dmesg is filled directly after boot, so that the boot messages do not get lost.