shmget - Returns (and possibly creates) the ID for a shared memory region
If programs are being developed for multiple platforms, application
developers may want to specify #include statements for <<sys/types.h>> and
<<sys/ipc.h>> before the one for <<sys/shm.h>>. The additional #include state-
ments are not required on Tru64 UNIX systems or by ISO or X/Open standards,
but may be required on other vendors' systems that conform to these stan-
Interfaces documented on this reference page conform to industry standards
Refer to the standards(5) reference page for more information about indus-
try standards and associated tags.
key 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 a new, unused shared memory
size Specifies the minimum number of bytes to allocate for the region.
flags Specifies the creation flags. Possible values are:
IPC_CREAT If the key specified does not exist, the shmget() func-
tion creates an ID using the specified key.
If the specified key does exist, and IPC_EXCL is not
set, the ID 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.
IPC_EXCL 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 kernel-assigned ID
for the region.
After creating a new shared memory region ID, the shmget() function ini-
tializes the shmid_ds structure associated with the ID as follows:
+ The shm_perm.cuid and shm_perm.uid fields are set equal to the effec-
tive user ID of the calling process.
+ The shm_perm.cgid and shm_perm.gid fields are set equal to the effec-
tive 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 cal-
[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 ssm-threshold (an attribute in /etc/sysconfigtab), the memory
region is managed using shared page tables. When attaching to such a seg-
ment, the segment must always be aligned and sized correctly. The align-
ment 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.)
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
[EACCES] 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.
[EEXIST] A shared memory region ID already exists for the key parameter,
but IPC_CREAT and IPC_EXCL were used for the flags parameter.
[EINVAL] The value of the size parameter is less than the system-defined
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).
[ENOENT] A shared memory region ID does not exist for the key parameter,
and IPC_CREAT was not used for the flags parameter.
[ENOMEM] An attempt was made to create a shared memory region ID and its
associated shmid_ds structure, but there was not enough physical
[ENOSPC] An attempt to create a new shared memory region ID exceeded the
system-wide limit on the maximum number of IDs allowed.
Functions: shmat(2), shmctl(2), shmdt(2), sysconfig(8), table(2), ftok(3)