Index for Section 4

Alphabetical listing for S

Bottom of page

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 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
  shm_perm
      The ipc_perm structure that defines permissions for shared memory
      operations.

  shm_segsz
      The size of the shared memory region, in bytes.

  shm_cpid
      The process ID of the process that created the shared memory region ID.

  shm_lpid
      The process ID of the last process that performed a shmat() or shmdt()
      operation on the shared memory region.

  shm_nattch
      The number of processes that currently have this region attached.

  shm_atime
      The time of the last shmat() operation.

  shm_dtime
      The time of the last shmdt() operation.

  shm_ctime
      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 permissions; 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)

Index for Section 4

Alphabetical listing for S

Top of page