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stackbiz
Asked: 2022-11-26 19:38:32 +0800 CST

What does 176 mean in Linux magic SysRq key

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Here is the value set in Ubuntu 22.04:

root@test:~# cat /proc/sys/kernel/sysrq
176

I can't find the value from this link:

https://www.kernel.org/doc/html/latest/admin-guide/sysrq.html

What does it mean?

networking

1 Answers

  1. Best Answer

    For interpretation of the SysReq value you need to convert it into binary,
    then check which 'bits' are 1 telling whether the corresponding function is enabled.

    Typing e.g.

    $ python -c 'print( bin(176) )'
0b10110000

    ... at the bash prompt will tell the bits prepended with a 0b (for "binary").
    Note: start from the right with the value 1 for the first bit, then 2 for the next, doubling the value for each step left.

    If you wish to have the actual bitvalues printed, then it requires some more code;

    $ cat cvt.py 
#!/bin/env python

import sys

b=bin(int(sys.argv[1])).replace("0b","")
v=2**(len(b)-1)
for bit in b: 
  print( f"{v:>4}", "bit" if bit=="1" else "bit, not included" )
  v=v//2

    which executed shows this:

    $ python cvt.py 176
 128 bit
  64 bit, not included
  32 bit
  16 bit
   8 bit, not included
   4 bit, not included
   2 bit, not included
   1 bit, not included

    So, 176 means that

     16 =  0x10 - enable sync command
 32 =  0x20 - enable remount read-only
128 =  0x80 - allow reboot/poweroff

    ... functions are enabled.

    Note: 16+32+128 = 176

    $ echo ' 16+32+128' | bc
176
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