NAME [Toc] [Back]
shmget - get shared memory segment
SYNOPSIS [Toc] [Back]
int shmget(key_t key, size_t size, int shmflg);
DESCRIPTION [Toc] [Back]
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 descendents;
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 wants 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
Upon creation, the data structure associated with the new shared
memory identifier is initialized as follows:
+ 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.
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+ shm_lpid, shm_nattch, shm_atime, and shm_dtime are set equal
+ 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 segment to be allocated from the global
quadrant and not from the local quadrants. If this is not possible,
shmget() will fail. Please refer to setmemwindow(1M) for more
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 Architecture (ccNUMA) systems. They
will have no effect otherwise. For more information regarding ccNUMA
and locality domains, see the mpctl(2) manual page. These flags are
hints to the system. If memory of the desired type is not available,
the next most suitable memory is returned instead.
If 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.
If 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 the mpctl(2) manual page.
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.
System V Shared Memory Under Adaptive Address Space [Toc] [Back]
Itanium-based platforms have the capability to create two types of
processes: MGAS (Mostly Global Address Space), which is the default,
and MPAS (Mostly Private Address Space). See ld(1) and chatr(1).
If 64-bit MPAS and 64-bit MGAS processes want to share a mapping among
themselves without incurring the cost of aliasing, then (shmflg &
IPC_GLOBAL) should be ``true'', but (shmflg & IPC_SHARE32) should be
``false''. 64-bit MPAS processes should also specify the IPC_GLOBAL
flag to the shmat(2) call. Such a mapping cannot be shared with 32-
bit MGAS processes.
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See the Adaptive Address Space Whitepaper for details.
EXAMPLES [Toc] [Back]
The following call to shmget() returns a unique shmid for the newly
created shared memory segment of 4096 bytes:
myshmid = shmget (IPC_PRIVATE, 4096, 0600);
RETURN VALUE [Toc] [Back]
Upon successful completion, a non-negative integer, namely a shared
memory identifier, is returned. Otherwise, a value of -1 is returned
and errno is set to indicate the error.
ERRORS [Toc] [Back]
shmget() fails if any of the following conditions are encountered:
[EINVAL] size is less than the system-imposed minimum or
greater than the system-imposed maximum.
[EINVAL] A shared memory identifier exists for key but is
in 64-bit address space and the process performing
the request has been compiled as a 32-bit
executable. In order to avoid receiving this
error, both IPC_SHARE32 and IPC_CREAT must be set
in shmflg by the 64-bit processes upon segment
[EACCES] A shared memory identifier exists for key but
operation permission (see glossary(9)) as
specified by the low-order 9 bits of shmflg would
not be granted.
[EINVAL] A shared memory identifier exists for key but the
size of the segment associated with it is less
than size and size is not equal to zero.
[ENOENT] A shared memory identifier does not exist for key
and (shmflg & IPC_CREAT) is ``false''.
[ENOSPC] A shared memory identifier is to be created but
the system-imposed limit on the maximum number of
allowed shared memory identifiers system wide
would be exceeded.
[ENOMEM] A shared memory identifier and associated shared
memory segment are to be created, but the amount
of available physical memory is not sufficient to
fill the request.
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[ENOMEM] A shared memory segment with IPC_CREAT or
IPC_GLOBAL specified was to be created; however,
there is not enough space in the global quadrant.
[EEXIST] A shared memory identifier exists for key but
((shmflg & IPC_CREAT) && (shmflg & IPC_EXCL)) is
SEE ALSO [Toc] [Back]
chatr(1), ld(1), ipcrm(1), ipcs(1), mpctl(2), shmctl(2), shmop(2),
ftok(3C), Adaptive Address Space Whitepaper.
STANDARDS CONFORMANCE [Toc] [Back]
shmget(): SVID2, SVID3, XPG2, XPG3, XPG4
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