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

本文整理自网络，参考网页(references):

1. http://stackoverflow.com/questions/5658568/how-to-list-processes-attached-to-a-shared-memory-segment-in-linux
2. http://publib.boulder.ibm.com/infocenter/iseries/v5r4/index.jsp?topic=%2Frzahz%2Fipcs.htm

[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 segment 98305?

[Answer]:

I don't think you can do this with the standard tools. You can useipcs -mpto get the process ID of thelastprocess to attach/detach but I'm not aware of how to getallattached processes withipcs.

With a two-process-attached segment, assuming they bothstayedattached, you can possibly figure out from the creator PIDcpidand last-attached PIDlpidwhich are the two processes but that won't scale to more than two processes so its usefulness is limited.

Thecat /proc/sysvipc/shmmethod seems similarly limited but I believe there's a way to do it with other parts of the/procfilesystem, as shown below:

When I do agrepon theprocfsmaps for all processes, I get entries containing lines for thecpidandlpidprocesses.

For example, I get the following shared memory segment fromipcs -m:

------ Shared Memory Segments --------
key        shmid      owner      perms      bytes      nattch     status      
0x00000000 123456     pax        600        1024       2          dest

and, fromipcs -mp, thecpidis 3956 and thelpidis 9999 for that given shared memory segment (123456).

Then, with the commandgrep 123456/proc/*/maps, I see:

/proc/3956/maps: blah blah blah 123456 /SYSV000000 (deleted)
/proc/9999/maps: blah blah blah 123456 /SYSV000000 (deleted)

So thereisa way to get the processes that attached to it. I'm pretty certain that thedeststatus 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.

---

CPID(-p, -a) The process ID of the job that created the shared memory segment.

LPID(-p, -a) The process ID of the last job to attach or detach from the shared memory segment or change the semaphore value.