shmget - Return (and possibly create) the ID for a shared
int flags );
Application developers might want to specify #include
statements for <sys/types.h> and <sys/ipc.h> before the
one for <sys/shm.h> if programs are being developed for
multiple platforms. The additional #include statements
are not required on Tru64 UNIX systems or by ISO or XSH
specifications, but might be required on other vendors'
systems that conform to these standards.
Interfaces documented on this reference page conform to
industry standards as follows:
shmget(): XSH4.0, XSH4.2, XSH5.0
Refer to the standards(5) reference page for more information
about industry standards and associated tags.
Specifies the key that identifies the shared memory
region. The value for the key parameter can be IPC_PRIVATE
or a random number other than zero (0). If the value of
key is IPC_PRIVATE, it can be used to assure the return of
the identifier of a new, unused shared memory region.
Specifies the minimum number of bytes to allocate for the
region. Specifies the creation options. Possible values
are: If the specified key does not exist, the shmget()
function creates a shared memory identifier for the specified
If the specified key does exist, and IPC_EXCL is
not set, the shared memory identifier for the specified
key is returned.
If the specified key does exist and IPC_EXCL is
set, the shmget() function fails and returns an
error. If the key already exists, the shmget()
function fails and returns an error notification.
The shmget() function returns (and possibly creates) the
ID for the shared memory region identified by the key
parameter. If IPC_PRIVATE is used for the key parameter,
the shmget() function returns the ID for a private (that
is, newly created) shared memory region. The flags parameter
supplies creation options for the shmget() function.
If the key parameter does not already exist, the IPC_CREAT
flag instructs the shmget() function to create a new
shared memory region for the key and return the kernelassigned
ID for the region.
After creating a new shared memory region ID, the shmget()
function initializes the shmid_ds structure associated
with the ID as follows: The shm_perm.cuid and shm_perm.uid
fields are set equal to the effective user ID of the calling
process. The shm_perm.cgid and shm_perm.gid fields
are set equal to the effective group ID of the calling
process. The low-order nine bits of the shm_perm.mode
field are set equal to the low-order nine bits of flags.
The shm_segsz field is set equal to size. The shm_lpid,
shm_nattch, shm_atime, and shm_dtime fields are all set
equal to 0 (zero). The shm_ctime field is set equal to
the current time. [Tru64 UNIX] The shm_cpid field is set
to the process ID of the calling process.
[Tru64 UNIX] To reduce the overhead associated with managing
large shared memory regions among many processes,
the Tru64 UNIX kernel makes use of shared page tables,
also referred to as segmented shared memory (SSM). When
the segment size requested by shmget() is greater than or
equal to the value of the ssm_threshold system attribute,
the memory region is managed using shared page tables.
When attaching to such a segment, the segment must always
be aligned and sized correctly. The alignment and size
factor is given by the system-defined SSM_SIZE value (see
<machine/pmap.h>). When the attachment occurs, the segment's
memory region is aligned on an SSM_SIZE boundary
and its size is rounded up to the next SSM_SIZE boundary--thus,
the segment size specified by shmget() does not
need to be a multiple of SSM_SIZE. Use of segmented
shared memory can be disabled by setting the ssm_threshold
attribute to zero.
For more information on segmented shared memory and interprocess
communication, see the system attribute descriptions
in sys_attrs_ipc(5) and the discussion of tuning
interprocess communication limits in the System Configuration
and Tuning guide.
The librt library contains alternative interfaces for
interprocess communication. The names of these routines
adhere to the format shm_* and their reference pages are
listed in SEE ALSO.
Upon successful completion, a shared memory identifier is
returned. If the shmget() function fails, a value of -1
is returned and errno is set to indicate the error.
The shmget() function sets errno to the specified values
for the following conditions: A shared memory region ID
already exists for the key parameter, but operation permission
as specified by the low-order nine bits of the
flags parameter was not granted. A shared memory region
ID already exists for the key parameter, but IPC_CREAT and
IPC_EXCL were specified for the flags parameter. The
value of the size parameter is less than the systemdefined
minimum or greater than the system-defined maximum.
Or, a shared memory region ID already exists for the
key parameter, but the number of bytes allocated for the
region is less than size and size is not equal to 0
(zero). A shared memory region ID does not exist for the
key parameter, and IPC_CREAT was not used for the flags
parameter. An attempt was made to create a shared memory
region ID and its associated shmid_ds structure, but there
was not enough physical memory available. An attempt to
create a new shared memory region ID exceeded the systemwide
limit on the maximum number of IDs allowed.
Functions: shmat(2), shmctl(2), shmdt(2), sysconfig(8),
table(2), ftok(3), shm_open(3), shm_unlink(3)
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