IPC Inter-Process-Communication
Shared memory is the memory that may be accessed by multiple processes; i.e. a memory
region that can be shared between different processes and a better way of passing data
between two processes. Shared memory is the fastest form of Inter-Process
Communication which is currently available. Assume that the program will create a memory
portion, which another process can access (if permitted). A shared segment can be
attached multiple times by the same process. Whenever the memory is mapped into the
address space of the process, i.e. sharing the common memory region, the kernel will not
involve while passing data between the processes.
Different Types of IPCS
There are various IPC’s which allows a process to communicate with another processes,
either in the same computer or different computer in the same network.
Pipes – Provides a way for processes to communicate with each another by exchanging messages. Named pipes provide a way for processes running on different computer systems to communicate over the network.
Shared Memory – Processes can exchange values in the shared memory. One process will create a portion of memory which other process can access.
Message Queue – It is a structured and ordered list of memory segments where processes store or retrieve data.
Semaphores – Provides a synchronizing mechanism for processes that are accessing the same resource. No data is passed with a semaphore; it simply coordinates access to shared
resources.
List all the IPC facility
ipcs command with -a option lists all the IPC facilities which has read access for the current
process. It provides details about message queue, semaphore and shared memory.
ipcs -a
>ipcs -a
------ Shared Memory Segments --------
key shmid owner perms bytes nattch status
0x00000000 1671168 ravi 600 524288 2 dest
0x00000000 393217 ravi 600 524288 2 dest
0x00000000 1277954 ravi 600 524288 2 dest
0x00000000 1310723 ravi 600 134217728 2 dest
0x00000000 622596 ravi 600 524288 2 dest
0x00000000 884741 ravi 600 524288 2 dest
0x00000000 983046 ravi 600 393216 2 dest
0x00000000 1081351 ravi 600 524288 2 dest
0x00000000 2818056 ravi 600 524288 2 dest
------ Semaphore Arrays --------
key semid owner perms nsems
------ Message Queues --------
key msqid owner perms used-bytes messages
All the IPC facility has unique key and identifier, which is used to identify an IPC facility.
List all the Message Queue
ipcs with option -q, lists only message queues for which the current process has read
access.
ipcs -q
------ Message Queues --------
key msqid owner perms used-bytes messages
0x000005a4 32768 root 644 0 0
List all the Semaphores
ipcs -s option is used to list the accessible semaphores.
ipcs -s
------ Semaphore Arrays --------
key semid owner perms nsems
0x0103eefd 0 root 664 1
0x0103eefe 32769 root 664 1
List all the Shared Memory
ipcs -m option with ipcs command lists the shared memories.
ipcs -m
------ Shared Memory Segments --------
key shmid owner perms bytes nattch status
0x00000000 1671168 ravi 600 524288 2 dest
0x00000000 393217 ravi 600 524288 2 dest
0x00000000 1277954 ravi 600 524288 2 dest
0x00000000 1310723 ravi 600 134217728 2 dest
Lists the Limits for IPC facility
ipcs -l option gives the system limits for each ipc facility.
ipcs -l
------ Shared Memory Limits --------
max number of segments = 4096
max seg size (kbytes) = 18014398509465599
max total shared memory (kbytes) = 18446744073642442748
min seg size (bytes) = 1
------ Semaphore Limits --------
max number of arrays = 32000
max semaphores per array = 32000
max semaphores system wide = 1024000000
max ops per semop call = 500
semaphore max value = 32767
------ Messages Limits --------
max queues system wide = 32000
max size of message (bytes) = 8192
default max size of queue (bytes) = 16384
List Creator and Owner Details for IPC Facility
ipcs -c option lists creator userid and groupid and owner userid and group id. This option
can be combined with -m, -s and -q to view the creator details for specific IPC facility.
ipcs -m -c
------ Shared Memory Segment Creators/Owners --------
shmid perms cuid cgid uid gid
1671168 600 ravi ravi ravi ravi
393217 600 ravi ravi ravi ravi
1277954 600 ravi ravi ravi ravi
1310723 600 ravi ravi ravi ravi
Process ids that accessed IPC facility recently
ipcs -p option displays creator id, and process id which accessed the corresponding ipc
facility very recently.
ipcs -m -p
------ Shared Memory Creator/Last-op PIDs --------
shmid owner cpid lpid
1671168 ravi 3124 10093
393217 ravi 2647 2981
1277954 ravi 2836 24419
1310723 ravi 2836 24419
Last Accessed Time
ipcs -t option displays last operation time in each ipc facility. This option can also be
combined with -m, -s or -q to print for specific type of ipc facility. For message queue, -t
option displays last sent and receive time, for shared memory it displays last attached
(portion of memory) and detached timestamp and for semaphore it displays last operation
and changed time details.
ipcs -t
------ Shared Memory Attach/Detach/Change Times --------
shmid owner attached detached changed
1671168 ravi Nov 14 08:12:16 Nov 14 08:12:16 Nov 6 19:20:58
393217 ravi Nov 6 19:20:15 Nov 6 19:20:15 Nov 6 19:19:50
Status of current usage
ipcs with -u command displays current usage for all the IPC facility. This option can be
combined with a specific option to display the status for a particular IPC facility.
ipcs -u
------ Shared Memory Status --------
segments allocated 24
pages allocated 53664
pages resident 10535
pages swapped 691
Swap performance: 0 attempts 0 successes
------ Semaphore Status --------
used arrays = 49
allocated semaphores = 252
------ Messages: Status --------
allocated queues = 1
used headers = 0
used space = 0 bytes
How to Delete/remove a share memory ?
Delete/remove a share memory
ipcrm
usage: ipcrm [ [-q msqid] [-m shmid] [-s semid]
[-Q msgkey] [-M shmkey] [-S semkey] ... ]
Delete/remove a Shared Memory segment
$ ipcrm -m 10715155
Delete a message queue
$ipcrm -s 163843
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