shmid_ds - Defines a shared memory region
#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 );
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.
The ipc_perm structure that defines permissions for shared
memory operations. 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 [Toc] [Back] 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.
Functions: shmat(2), shmdt(2), shmctl(2), shmget(2)
shmid_ds(4)
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