shmget.2 (2011 03)

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shmget(2) shmget(2)

NAME

shmget() - get shared memory segment

SYNOPSIS

#include <sys/shm.h>

int shmget(key_t key, size_t size, int shmflg);

DESCRIPTION

shmget() returns the shared memory identiﬁer associated with key .

A shared memory identiﬁer and associated data structure and shared memory segment of size size bytes

(see glossary (9)) are created for key if one of the following is true:

• key is equal to

IPC_PRIVATE. This call creates a new identiﬁer, subject to available resources.

The identiﬁer will never be returned by another call to

shmget() until it has been released by

a call to

shmctl(). The identiﬁer should be used among the calling process and its descen-

dents; however, it is not a requirement. The resource can be accessed by any process having the

proper permissions.

• key does not already have a shared memory identiﬁer associated with it, and (shmﬂg &

IPC_CREAT) is "true". If

IPC_CREAT is set in shmﬂg the shared memory segment created can

only be shared by processes of the same executable type. That is, an application compiled as a

32-bit process will be able to share the same memory segment with other 32-bit processes, and an

application compiled as a 64-bit process will be able to share the same memory segment with

other 64-bit processes. If a 64-bit bit process want to create a shared memory segment which can

also be shared with 32-bit processes, the 64-bit process must specify

IPC_SHARE32 in addition

to IPC_CREAT in shmﬂg. The 32-bit process does not need to specify IPC_SHARE32.On

Itanium(R)-based platforms, if the Adaptive Address Space feature is being used, then additional

rules may apply. See the section below on Adaptive Address Space.

NOTE: A shared memory segment receives zero-initialized pages upon ﬁrst reference.

Upon creation, the data structure associated with the new shared memory identiﬁer is initialized as fol-

lows:

•

shm_perm.cuid, shm_perm.uid

, shm_perm.cgid, and shm_perm.gid are set equal to

the effective user ID and effective group ID, respectively, of the calling process.

•

shm_perm.cuid, the low-order 9 bits of

shm_perm.mode are set equal to the low-order 9 bits

of shmﬂg.

shm_segsz is set equal to the value of size.

•

shm_lpid, shm_nattch, shm_atime, and shm_dtime are set equal to 0.

•

shm_ctime is set equal to the current time.

Specifying

IPC_GLOBAL in a call to shmget(), along with IPC_CREAT, forces the shared memory seg-

ment to be allocated from the global quadrant and not from the local quadrants. If this is not possible,

shmget() will fail. Refer to setmemwindow (1M) for more information.

Specifying

IPC_RELAXED_ISOLATION in a call to shmget(), along with IPC_CREAT, can improve

the performance of co-operating processes in cases where these processes are accessing a large number of

shared memory segments. This ﬂag is treated as a hint.

The ﬂags

IPC_MEM_INTERLEAVED, IPC_MEM_LOCAL

, and IPC_MEM_FIRST_TOUCH involve the

placement of physical memory. This is important only on Cache Coherent Non-Uniform Memory Archi-

tecture (ccNUMA) systems. They will have no effect otherwise. For more information regarding ccNUMA

and locality domains, see mpctl(2). These ﬂags are hints to the system. If memory of the desired type is

not available, the next most suitable memory is returned instead.

IPC_MEM_INTERLEAVED is set in shmﬂg, physical memory will come from interleaved memory.

Interleaved memory is a hardware construct which combines the memory from several locality domains

into one region of memory. This memory is striped together with a very ﬁne granularity. Interleaved

memory is a good place to put shared objects, the kernel, and objects that could be accessed from any part

of the system. This is the default behavior for shared memory segments created with this call.

IPC_MEM_LOCAL is set in shmﬂg, physical memory for this mapping will always be allocated from the

locality domain where the call to shmget() was made. Note that this ﬂag implicitly uses the current

locality domain, so the caller should be aware of locality domain conﬁgurations and bindings. See

mpctl (2).

HP-UX 11i Version 3: March 2011 − 1 − Hewlett-Packard Company 1

Summary of content (2 pages)

PAGE 1
shmget(2) shmget(2) NAME shmget() - get shared memory segment SYNOPSIS #include int shmget(key_t key, size_t size, int shmflg); DESCRIPTION shmget() returns the shared memory identifier associated with key . A shared memory identifier and associated data structure and shared memory segment of size size bytes (see glossary (9)) are created for key if one of the following is true: • key is equal to IPC_PRIVATE. This call creates a new identifier, subject to available resources.
PAGE 2
shmget(2) shmget(2) If IPC_MEM_FIRST_TOUCH is set in shmflg, physical memory will come from the locality domain of the first processor to read or write that memory. If the shared memory is paged out and then back in again, physical memory will come from the locality domain of the processor bringing in that memory.