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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 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.



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 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) delim off




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Time: 02:40:08 GMT, October 02, 2010