IPC Parameters

If IPC (interprocess communication) structures are incorrectly set, certain system calls will fail and return errors.

Before increasing the size of an IPC parameter, investigate the problem by using ipcs(1) to see if the IPC resources are being removed when no longer needed. For example, shmget returns the error ENOSPC if applications do not remove semaphores, memory segments, or message queues.

Note that IPC objects differ from most IRIX objects in that they are not automatically freed when all active references to them are gone. In particular, they are not deallocated when the program that created them exits.

Table A-1 lists error messages, system calls that cause the error, and parameters to adjust. Subsequent paragraphs explain the details you need to know before you increase the parameters listed in this table.

System Call Errors and IPC Parameters to Adjust

Message	System Call	Parameter
EAGAIN	msgsnd()	see below
EINVAL	msgsnd() shmget()	msgmax shmmax
EMFILE	shmat()	sshmseg
ENOSPC	semget() shmget()	msgmni semmni, semmns, shmmni

EAGAIN

If IPC_NOWAIT is set, msgsnd can return EAGAIN for a number of reasons:

• The total number of bytes on the message queue exceeds msgmnb.

• The total number of bytes used by messages in the system exceeds msgseg * msgsz.

• The total number of system-wide message headers exceeds msgmax.

EINVAL

shmget (which gets a new shared memory segment identifier) will fail with EINVAL if the given size is not within shmmin and shmmax. Since shmmin is set to the lowest possible value (1), and shmmax is very large, it should not be necessary to change these values.

EMFILE

shmat will return EMFILE if it attaches more than sshmseg shared memory segments. sshmseg is the total number of system-shared memory segments per process.

ENOSPC

shmget will return ENOSPC if:

• shmmni (the system-wide number of shared memory segments) is too small. However, applications may be creating shared memory segments and forgetting to remove them. So, before making a parameter change, use ipcs(1) to get a listing of currently active shared memory segments.

  semget will return ENOSPC if:

• semmni is too small, indicating that the total number of semaphore identifiers is exceeded.

• semmns (the system-wide number of semaphores) is exceeded. Use ipcs to see if semaphores are being removed as they should be.

  msgget will return ENOSPC if:

• msgmni is too small. Use ipcs to see if message queues are being removed as they should be.