HP-UX Reference (11i v1 05/09) - 5 Miscellaneous Topics (vol 9)

s
shmem(5) shmem(5)
(Tunable Kernel Parameters)
NAME
shmem - enable or disable System V shared memory
VALUES
Failsafe
1 (on)
Default
1 (on)
Allowed values
0 (off) or 1 (on)
DESCRIPTION
Shared memory is an efficient InterProcess Communications (IPC) mechanism. One process creates a
shared memory segment and attaches it to its address space. Any processes looking to communicate with
this process through the shared memory segment, then attach the shared memory segment to their
corresponding address spaces as well. Once attached, a process can read from or write to the segment
depending on the permissions specified while attaching it.
This tunable controls the availability of System V shared memory in the HP-UX kernel. Setting it to off
changes the tunables
shmmax, shmmni, and shmseg to 0 (off), effectively allowing no shared memory
segments to be created.
Who Is Expected to Change This Tunable?
Anyone.
Restrictions on Changing
Changes to this tunable take effect on the next reboot.
When Should the Tunable Be Turned On?
If System V shared memory is disabled and a system user wishes to access this functionality, setting the
value to 1 (on) will enable it.
What Are the Side Effects of Turning the Tunable On?
The tunables shmmax, shmmni, and shmseg use the values set in the tunable configuration files instead
of the value of 0 (off).
When Should the Tunable Be Turned Off?
If System V shared memory segments are not needed, this functionality can be disabled. This leaves the
option of shared memory via the mmap() system call instead.
What Are the Side Effects of Turning the Tunable Off?
Code that is trying to use System V shared memory functionality will receive the [EINVAL] error message
because any request for a segment will be greater than the system-imposed maximum set to zero.
What Other Tunables Should Be Changed at the Same Time?
If the System V shared memory is being enabled, a check of the values for shmmax, shmmni, and
shmseg for reasonable bounds should be performed. These values represent the maximum size of a seg-
ment, number of identifiers, and maximum number of shared segments per process respectively.
WARNINGS
All HP-UX kernel tunable parameters are release specific. This parameter may be removed or have its
meaning changed in future releases of HP-UX.
AUTHOR
shmem was developed by HP.
SEE ALSO
shmmax(5), shmmni(5), shmseg(5).
Section 5−−338 Hewlett-Packard Company − 1 − HP-UX 11i Version 1: September 2005