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Sharing memory between processes occurs on a virtual segment basis. There is only one copy of each individual shared memory segment existing in the UNIX operating system at any time.
Before sharing of memory can be realized, a uniquely identified shared memory segment and data structure must be created. The unique identifier created is called the shared memory identifier (shmid); it is used to identify or refer to the associated data structure. This identifier is accessible by any process in the system, subject to normal access restrictions.
The data structure includes the following for each shared memory segment:
In UnixWare, the definition for the associated shared-memory segment data structure shmid_ds is as follows:
/*
* There is a shared mem id data structure for each segment in the system.
*/
struct shmid_ds {
struct ipc_perm shm_perm; /* operation permission struct */
int shm_segsz; /* segment size */
struct region *shm_reg; /* ptr to region structure */
char pad[4]; /* for swap compatibility */
pid_t shm_lpid; /* pid of last shmop */
pid_t shm_cpid; /* pid of creator */
ushort shm_nattch; /* used only for shminfo */
ushort shm_cnattch; /* used only for shminfo */
time_t shm_atime; /* last shmat time */
time_t shm_dtime; /* last shmdt time */
time_t shm_ctime; /* last change time */
};
The C programming language data structure definition for the shared memory segment data structure shmid_ds is located in the sys/shm.h header file.
Note that the
shm_perm
member of
this structure uses
ipc_perm
as a template.
The
ipc_perm
data structure is the same for all IPC
facilities; it is located in the
sys/ipc.h
header file
and shown in
``ipc_perm data structure''.
The shmget system call performs two tasks:
The task performed is determined by the value of the key argument passed to the shmget system call.
For the first task, if the key is not already in use for an existing shared memory identifier at the security level of the calling process and the IPC_CREAT flag is set in shmflg, a new identifier is returned with an associated shared memory segment data structure created for it provided no system-tunable parameters would be exceeded.
There is also a provision
for specifying a
key
of value zero which is
known as the private
key
(IPC_PRIVATE);
when specified, a new
shmid
is always returned
with an associated shared memory segment data structure
created for it unless a system-tunable parameter would be exceeded.
The
ipcs
command will show
the
key
field for the
shmid
as all zeros.
For the second task, if a shmid exists for the key specified, the value of the existing shmid is returned. If it is not desired to have an existing shmid returned, a control command (IPC_EXCL) can be specified (set) in the shmflg argument passed to the system call. ``Using shmget'' discusses how to use this system call.
When performing the first task, the process that calls shmget becomes the owner/creator, and the associated data structure is initialized accordingly. Remember, ownership can be changed, but the creating process always remains the creator (see ``Controlling shared memory''). The creator of the shared memory segment also determines the initial operation permissions for it.
Once a uniquely identified shared memory segment data structure is created, shmop (shared memory segment operations) and shmctl (shared memory control) can be used.
Shared memory segment operations consist of attaching and detaching shared memory segments. shmat and shmdt are provided for each of these operations (see ``Operations for shared memory'' for details of the shmat and shmdt system calls).
The shmctl system call permits you to control the shared memory facility in the following ways:
See the section ``Controlling shared memory'' for details of the shmctl system call.