shmid_ds man page on OSF1

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shmid_ds(4)							   shmid_ds(4)

NAME
       shmid_ds - Defines a shared memory region

SYNOPSIS
       #include <sys/shm.h>

       struct shmid_ds(
	       struct ipc_perm shm_perm,
	       int shm_segsz,
	       u_short shm_lpid,
	       u_short shm_cpid,
	       u_short shm_nattch,
	       time_t shm_atime,
	       time_t shm_dtime,
	       time_t shm_ctime );

DESCRIPTION
       The shmid_ds structure defines a shared memory region associated with a
       shared memory region ID. There is one  shared  memory  region  per  ID.
       Collectively, the shared memory regions are maintained in a shared mem‐
       ory table, with the shared memory region IDs identifying the entries in
       the table.

       The  IPC permissions for the shared memory regions are implemented in a
       separate, but associated, ipc_perm structure.

       A shared memory region is created indirectly via the shmget() call.  If
       shmget()	 is  called  with  a non-existent shared memory region ID, the
       kernel allocates a new shmid_ds structure, initializes it, and  returns
       the ID that is to be associated with the region.

       The  kernel  allocates  actual  memory  of  shm_segsz bytes only when a
       process attaches a region to its address space.	Attached  regions  are
       maintained  in a separate region table.	The entries in the shared mem‐
       ory table point to the associated attached regions in the region table.
       The  same  shared  memory region can be attached multiple times, by the
       same or different processes.  Each attachment of the region  creates  a
       new entry in the region table.

       After  a	 process  attaches  a shared memory region, the region becomes
       part of the process's virtual address space.  Processes	access	shared
       memory  regions	by  using the same machine instructions used to access
       any virtual address.

       The following sections describe the fields in  the  shmid_ds  structure
       and show the associated ipc_perm structure.

FIELDS
       The ipc_perm structure that defines permissions for shared memory oper‐
       ations.	The size of the shared memory region, in bytes.	  The  process
       ID  of  the  process  that  created  the	 shared memory region ID.  The
       process ID of the last process that  performed  a  shmat()  or  shmdt()
       operation  on  the  shared memory region.  The number of processes that
       currently have this region attached.  The  time	of  the	 last  shmat()
       operation.   The	 time  of the last shmdt() operation.  The time of the
       last shmctl() operation that changed a member of the ipc_perm structure
       designated by the shm_perm field.

THE IPC_PERM STRUCTURE
       The  shm_perm  field  identifies the associated ipc_perm structure that
       defines the permissions for operations on  the  shared  memory  region.
       The ipc_perm structure (from the sys/ipc.h header file) is as follows:

       struct ipc_perm {
	       ushort	 uid;	     /* owner's user id		   */
	       ushort	 gid;	     /* owner's group id	   */
	       ushort	 cuid;	     /* creator's user id	   */
	       ushort	 cgid;	     /* creator's group id	   */
	       ushort	 mode;	     /* access modes		   */
	       ushort	 seq;	     /* slot usage sequence number */
	       key_t	 key;	     /* key			   */ };

       The  mode  field	 is a nine-bit field that contains the permissions for
       shared memory operations.  The first three bits identify owner  permis‐
       sions;  the  second three bits identify group permissions; and the last
       three bits identify other permissions.  In each group,  the  first  bit
       indicates  read	permission; the second bit indicates write permission;
       and the third bit is not used.

SEE ALSO
       Functions: shmat(2), shmdt(2), shmctl(2), shmget(2)

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