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.
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:
$ pstree -p 6 rcu_par_gp(6)
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.
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).