shmid_ds(4)						     Kernel Interfaces Manual						       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 memory 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 memory table point to the associated attached regions in the  region  ta-
       ble.  The same shared memory region can be attached multiple times, by the same or different processes.	Each attachment of the region cre-
       ates 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.

FIELDS

       The  ipc_perm structure that defines permissions for shared memory operations.  See NOTES.  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  shmid_ds
       structure.

NOTES

       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 shown here.

       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.

RELATED INFORMATION

       Functions: shmat(2), shmdt(2), shmctl(2), shmget(2) delim off

																       shmid_ds(4)