I wonder how to get processes currently running semaphores by /proc ? I guess it's possible by SysVIPC subdirectory.But I don't know how to use this commands.
Ubuntu 12.10
1 Answer
My only experience in dealing with semaphores and shared memory is through the use of the command ipcs. Take a look at the ipcs man page for more details.
This command shows you what processes have semaphores:
$ ipcs -s
------ Semaphore Arrays --------
key semid owner perms nsems
0x4d114854 65536 saml 600 8
With the semid known we can query for addition info about the PIDs that have semaphores (note there are 8 - the nsems column):
$ ipcs -s -i 65536
Semaphore Array semid=65536
uid=500 gid=501 cuid=500 cgid=501
mode=0600, access_perms=0600
nsems = 8
otime = Sun May 12 14:44:53 2013
ctime = Wed May 8 22:12:15 2013
semnum value ncount zcount pid
0 1 0 0 0
1 1 0 0 0
2 1 0 0 2265
3 1 0 0 2265
4 1 0 0 0
5 1 0 0 0
6 1 0 0 4390
7 1 0 0 4390
The pid column are these processes. You can either look them up using ps or look through the /proc file-system, /proc/<pid>.
For example:
$ more /proc/2265/cmdline
mono
POSIX & SystemV
Building off of a comment left by @lgeorget I dug into my PID 2265's /proc/2265/map contents and did find the following /dev/shm references:
$ grep shm /proc/2265/maps
7fa38e7f6000-7fa38ebdf000 rw-s 00000000 00:11 18517 /dev/shm/mono-shared-500-shared_fileshare-grinchy-Linux-x86_64-40-12-0
7fa38f0ca000-7fa38f0cb000 rw-s 00000000 00:11 18137 /dev/shm/mono.2265
7fa3967be000-7fa3967d3000 rw-s 00000000 00:11 18516 /dev/shm/mono-shared-500-shared_data-grinchy-Linux-x86_64-328-12-0
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2As far as I know, only System V semaphores (those you get with
semget(2)) show up inipcs -sso if you're using POSIX semaphores (those you get withsem_open(2)), you have to use another method.– lgeorgetMay 12, 2013 at 18:50 -
Know of any terminal commands that will return the sem_open(2) variety? Just curious. Everything I know about semaphores is in the answer above 8-).– slm ♦May 12, 2013 at 18:52
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No, that's the problem. ;) As the
ipcsmanpage say: "The Linux ipcs utility is not fully compatible to the POSIX ipcs utility." so I'm not sure they even thought of something for POSIX IPC. Maybe a future release ofipcs:).– lgeorgetMay 12, 2013 at 18:55 -
the contents of the
mapare both variety's of semaphores thought, correct?– slm ♦May 12, 2013 at 18:59 -
I guess so. One way or another, semaphores have to be attached somewhere in the process memory. But I was not sure SysV semaphores were also created in the pseudo-filesystem
/dev/shm. Now, we have the answer. :)– lgeorgetMay 12, 2013 at 19:03
/proc/PID/maps, you can see the memory mapping of a process and POSIX semaphores show up as attached files in/dev/shm. I'm not sure about SysV semaphores though./proc/PID/mapsfile for a given PID./dev/shmand they are pretty indistinguishable from semaphores (except if they have clever names).