shmget - get shared memory segment
int shmget(key_t key, size_t size, int shmflg);
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 HP Scalable Computing Architecture (SCA) systems with multiple
locality domains, specifying IPC_MEM_LOCAL in shmflag targets the
shared memory to the locality on which the allocating thread is
executing at the time of the call. This overrides the default
allocation policy of "first touch", in which memory is allocated from
the locality domain on which the thread is executing at the time of
the fault. When IPC_MEM_LOCAL is specified:
+ Should there not be enough memory available in the specified
locality domain to satisfy the request, memory is allocated
from the domain on which the thread is currently executing.
+ On systems with a single locality domain, the call has no
Hewlett-Packard Company - 1 - HP-UX Release 11i: November 2000
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.
+ shm_lpid, shm_nattch, shm_atime, and shm_dtime are set equal
+ shm_ctime is set equal to the current time.
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);
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.
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.
Hewlett-Packard Company - 2 - HP-UX Release 11i: November 2000
[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.
[EEXIST] A shared memory identifier exists for key but
((shmflg & IPC_CREAT) && (shmflg & IPC_EXCL)) is
ipcrm(1), ipcs(1), shmctl(2), shmop(2), stdipc(3C).
shmget(): SVID2, SVID3, XPG2, XPG3, XPG4
Hewlett-Packard Company - 3 - HP-UX Release 11i: November 2000