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shmget(2)
NAME
shmget - Return (and possibly create) the ID for a shared memory region
SYNOPSIS
#include <sys/shm.h>
int shmget(
key_t key,
size_t size,
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.
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.
PARAMETERS
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 the identifier of a new, unused shared memory region.
size
Specifies the minimum number of bytes to allocate for the region.
flags
Specifies the creation options. Possible values are:
IPC_CREAT
If the specified key does not exist, the shmget() function
creates a shared memory identifier for the specified key.
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.
IPC_EXCL
If the key already exists, the shmget() function fails and
returns an error notification.
DESCRIPTION
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
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.
NOTES
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.
RETURN VALUES
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.
ERRORS
The shmget() function sets errno to the specified values for the following
conditions:
[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 specified 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 memory
available.
[ENOSPC]
An attempt to create a new shared memory region ID exceeded the
system-wide limit on the maximum number of IDs allowed.
SEE ALSO
Functions: shmat(2), shmctl(2), shmdt(2), sysconfig(8), table(2), ftok(3),
shm_open(3), shm_unlink(3)
Files: shmid_ds(4)
Standards: standards(5)
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