Linux list shared memory

Technique to find shared memory names currently used

When one opens a shared memory object (using shm_open ), you provide a name for that object. Each object must have a distinct name. Is there a way to identify the names of all of the currently allocated shared objects? And if so, how? Update: It would appear that, in my case, these are (as @HristoLliev calls them) System V shared memory segments. ipcs -m reports a list of segments similar to what I expect, but does not show the names.

1 Answer 1

shm_open(3) on Linux relies on tmpfs , usually mounted under /dev/shm . What shm_open() does is to convert the object name into a file path by prepending it with the mount point of the tmpfs filesystem. As long as the shared memory object is not unlinked, it will be visible in the filesystem and all you need to do is issue a simple ls command:

$ ls /dev/shm pulse-shm-1 pulse-shm-2 pulse-shm-3 . 

Some Linux distributions have tmpfs mounted under a different mountpoint. In order to find out where, issue:

$ mount -t tmpfs tmpfs on /dev/shm type tmpfs (rw) 

If you want to know which process(es) has/have mapped shared memory objects, the lsof command is your friend:

$ lsof /dev/shm COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME kded4 30657 hristo mem REG 0,16 67108904 294877 /dev/shm/pulse-shm-1 kded4 30657 hristo mem REG 0,16 67108904 294868 /dev/shm/pulse-shm-2 knotify4 30687 hristo mem REG 0,16 67108904 294876 /dev/shm/pulse-shm-3 pulseaudi 30717 hristo mem REG 0,16 67108904 294868 /dev/shm/pulse-shm-4 shm.x 31878 hristo DEL REG 0,16 305893 /dev/shm/asd 

Unlinked shared objects are no longer visible in the filesystem, though they might still persist if mapped in by some process as is the case with the last process in the list above — DEL in the FD field means that the corresponding file was deleted.

Note that System V shared memory segments (as obtained by ipcs -m ) live in a different name space and do not have corresponding object names, just numeric keys.

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How to check shared memory in linux

List shared memory segments: List message queues: Remove shared memory segment created with : Remove shared memory segment identified by : Remove message queue created with : Remove message queue identified by : Solution: The column in does not report the same thing that s entry does. is reporting the amount of memory that is shared with other processes because it’s in a dynamic library that is loaded once into memory, and all processes using that library include the same pages in their image, since those pages are read-only. seems to be showing «shared memory» segments, which are data pages that may be read-write or read-only, and are shared between processes with the and related functions. In order to find out where, issue: If you want to know which process(es) has/have mapped shared memory objects, the command is your friend: Unlinked shared objects are no longer visible in the filesystem, though they might still persist if mapped in by some process as is the case with the last process in the list above — in the field means that the corresponding file was deleted.

How do we check the «locked» shared memory of a process in linux?

You should use the ipcs(1) command, as:

The ipcs command show the ‘locked’ status on the ‘status’ field, as shown by the ipcs.c code:

 printf (" %-10ju %-6s %-6s\n", shmdsp->shm_nattch, shmdsp->shm_perm.mode & SHM_DEST ? _("dest") : " ", shmdsp->shm_perm.mode & SHM_LOCKED ? _("locked") : " "); 

Buffer and Cache Memory in Linux, We can use the free command to check the sizes of the buffer memory and cache memory: $ free -h total used free shared buff/cache available Mem: 7.6Gi 6.4Gi 170Mi 402Mi 1.1Gi 573Mi Swap: 2.0Gi 589Mi 1.4Gi. We’re passing the -h flag to display the results in a human-readable format.

Command to check status of message queue and shared memory in linux?

ipcs(1) provides information on the IPC facilities and ipcrm(1) can be used to remove the IPC objects from the system.

List shared memory segments:

Remove shared memory segment created with shmkey :

Remove shared memory segment identified by shmid :

Remove message queue created with msgkey :

Remove message queue identified by msgid :

Linux — shm_open — how to know if I have opened an, According to this shm_open manual use the O_EXCL oflag to detect if the shared memory object already exists. That’s true, but there appears to be no way to avoid a race condition. If shm_open fails because the object already exists, it could easily be created by another process before you get a chance to try again.

How to get accurate shared memory size of a process?

The SHR column in top does not report the same thing that pmap s shared entry does. top is reporting the amount of memory that is shared with other processes because it’s in a dynamic library that is loaded once into memory, and all processes using that library include the same pages in their image, since those pages are read-only. pmap seems to be showing «shared memory» segments, which are data pages that may be read-write or read-only, and are shared between processes with the shmget() and related functions.

How to use shared memory with Linux in C, I have a bit of an issue with one of my projects. I have been trying to find a well documented example of using shared memory with fork() but to no success.. Basically the scenario is that when the user starts the program, I need to store two values in shared memory: current_path which is a char* and a …

Technique to find shared memory names currently used

shm_open(3) on Linux relies on tmpfs , usually mounted under /dev/shm . What shm_open() does is to convert the object name into a file path by prepending it with the mount point of the tmpfs filesystem. As long as the shared memory object is not unlinked, it will be visible in the filesystem and all you need to do is issue a simple ls command:

$ ls /dev/shm pulse-shm-1 pulse-shm-2 pulse-shm-3 . 

Some Linux distributions have tmpfs mounted under a different mountpoint. In order to find out where, issue:

$ mount -t tmpfs tmpfs on /dev/shm type tmpfs (rw) 

If you want to know which process(es) has/have mapped shared memory objects, the lsof command is your friend:

$ lsof /dev/shm COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME kded4 30657 hristo mem REG 0,16 67108904 294877 /dev/shm/pulse-shm-1 kded4 30657 hristo mem REG 0,16 67108904 294868 /dev/shm/pulse-shm-2 knotify4 30687 hristo mem REG 0,16 67108904 294876 /dev/shm/pulse-shm-3 pulseaudi 30717 hristo mem REG 0,16 67108904 294868 /dev/shm/pulse-shm-4 shm.x 31878 hristo DEL REG 0,16 305893 /dev/shm/asd 

Unlinked shared objects are no longer visible in the filesystem, though they might still persist if mapped in by some process as is the case with the last process in the list above — DEL in the FD field means that the corresponding file was deleted.

Note that System V shared memory segments (as obtained by ipcs -m ) live in a different name space and do not have corresponding object names, just numeric keys.

How to Check Linux Memory Usage, The programs you run on your computer store small bits of information in the RAM so that it can access it when needed as fast as possible. Unfortunately, RAM is very expensive and limited. Without enough free RAM, your computer won’t function properly. Checking RAM usage is one of the most important tasks in Linux. In …

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List Processes Attached to a Shared Memory Segment in Linux

*1. Overview
* In Linux, a shared memory segment is part of the memory that two or more processes can share. It’s a mechanism meant to facilitate the communication and sharing of data between processes. However, monitoring these processes attached to the shared memory segment can be demanding for beginners. In this tutorial, we’ll discuss an option that we can use to list these processes. First, we’ll look at the ipcs command and then follow this up with the pstree command. *2. List Processes Attached to a Shared Memory Segment
* First, let’s start by taking a look at all the shared memory segments in our system:

$ ipcs -m ------ Shared Memory Segments -------- key shmid owner perms bytes nattch status 0x00000000 6 francis 600 524288 2 dest 

From the example above, we’re using the ipcs command to display the shared memory segments. In detail, the ipcs command is a utility for displaying information on inter-process communication (IPC) mechanisms. Also, we add the -m option to filter the output to only show information about the shared memory segments available. So, we can see that there’s only one segment in our system. Further, from the visible column names, we have to note the shared memory segment identifier (shmid) value which acts as the process identifier for the shared memory segment. This is because, for us to list the processes attached to a shared memory segment, we have to first determine the process ID of this shared memory segment that we’re interested in. Secondly, we can view information specific to a shared memory segment of our choosing:

$ ipcs -m -i 6 Shared memory Segment shmid=6 uid=1000 gid=1000 cuid=1000 cgid=1000 mode=01600 access_perms=0600 bytes=524288 lpid=4766 cpid=1574 nattch=2 att_time=Sun Jan 29 15:40:28 2023 det_time=Sun Jan 29 15:40:28 2023 change_time=Sun Jan 29 15:25:30 2023 

Above, we’re displaying detailed information about the shared memory segment whose identifier is 6. Next, we’re going to use the pstree command with shmid to list the attached processes:

The pstree command lists the processes attached to this shared memory segment in a tree-like structure. This command shows the relationship between processes by presenting the hierarchy of parent processes with their child processes. Additionally, we use the -p option to instruct pstree to include process identifiers (PIDs) in the output. *3. Conclusion
* In this tutorial, we’ve learned that shared memory is an important feature for processes to share data. Also, by using the ipcs command we get to view the shared memory segments in our system. Additionally, by utilizing the pstree command we get to list processes generated from the shared memory segment using its identifier (shmid)

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

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.

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