NAME
shmem — enable or disable System V shared memory
VALUES
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 segment, 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.
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