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