How to list processes attached to a shared memory segment in linux

Question

How do I determine what process is attached to a shared memory segment?

awagner@tree:/home/awagner$ ipcs -m

------ Shared Memory Segments --------
key        shmid      owner      perms      bytes      nattch     status      
0x00000000 0          root       777        102400     1                       
0x00000000 32769      root       774        96         1          dest         
0x00000000 98306      awagner    600        393216     2          dest         
0x00000000 131075     awagner    600        393216     2          dest

i.e. how do I figure out which two processes are attached to shmid 98306?

User · Answer

Use ipcs -a  it gives detailed information of all resources  semaphore  shared-memory etc   Here is the image of the output

User · Answer

I don t think you can do this with the standard tools  You can use ipcs -mp to get the process ID of the last process to attach detach but I m not aware of how to get all attached processes with ipcs   With a two-process-attached segment  assuming they both stayed attached  you can possibly figure out from the creator PID cpid and last-attached PID lpid which are the two processes but that won t scale to more than two processes so its usefulness is limited   The cat  proc sysvipc shm method seems similarly limited but I believe there s a way to do it with other parts of the  proc filesystem  as shown below   When I do a grep on the procfs maps for all processes  I get entries containing lines for the cpid and lpid processes   For example  I get the following shared memory segment from ipcs -m   ------ Shared Memory Segments -------- key        shmid      owner      perms      bytes      nattch     status       0x00000000 123456     pax        600        1024       2          dest   and  from ipcs -mp  the cpid is 3956 and the lpid is 9999 for that given shared memory segment  123456    Then  with the command grep 123456  proc   maps  I see    proc 3956 maps  blah blah blah 123456  SYSV000000  deleted   proc 9999 maps  blah blah blah 123456  SYSV000000  deleted    So there is a way to get the processes that attached to it  I m pretty certain that the dest status and  deleted  indicator are because the creator has marked the segment for destruction once the final detach occurs  not that it s already been destroyed   So  by scanning of the  proc   maps  files   you should be able to discover which PIDs are currently attached to a given segment

User · Answer

given your example above - to find processes attached to shmid 98306  lsof   egrep  98306 COMMAND

User · Answer

I wrote a tool called who attach shm pl  it parses  proc  pid  maps to get the information   you can download it from github  sample output   shm attach process list  group by shm key                                                                     0x2d5feab4      home curu mem dumper  home curu playd 0x4e47fc6c      home curu playd 0x77da6cfe      home curu mem dumper  home curu playd  home curu scand                                                                     process shm usage                                                                     home curu mem dumper  2      0x2d5feab4 0x77da6cfe  home curu playd  3      0x2d5feab4 0x4e47fc6c 0x77da6cfe  home curu scand  1      0x77da6cfe

User · Answer

Just in case someone is interest only in what kind of process created the shared moeries, call

ls -l /dev/shm

It lists the names that are associated with the shared memories - at least on Ubuntu. Usually the names are quite telling.

[linux] How to list processes attached to a shared memory segment in linux?

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