Kernel shmid_ds structure
Keywords: lpg

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#### Kernel `shmid_ds` structure

As with message queues and semaphore sets, the kernel maintains a special internal data structure for each shared memory segment which exists within its addressing space. This structure is of type `shmid_ds`, and is defined in `linux/shm.h` as follows:

            /* One shmid data structure for each shared memory segment in the system. */
            struct shmid_ds {
                    struct ipc_perm shm_perm;        /* operation perms */
                    int     shm_segsz;               /* size of segment (bytes) */
                    time_t  shm_atime;               /* last attach time */
                    time_t  shm_dtime;               /* last detach time */
                    time_t  shm_ctime;               /* last change time */
                    unsigned short  shm_cpid;        /* pid of creator */
                    unsigned short  shm_lpid;        /* pid of last operator */
                    short   shm_nattch;              /* no. of current attaches */

                                                     /* the following are private */

                    unsigned short   shm_npages;     /* size of segment (pages) */
                    unsigned long   *shm_pages;      /* array of ptrs to frames -> SHMMAX */
                    struct vm_area_struct *attaches; /* descriptors for attaches */
            };

Operations on this structure are performed by a special system call, and should not be tinkered with directly. Here are descriptions of the more pertinent fields:

`shm_perm`

  This is an instance of the `ipc_perm` structure, which is defined for us in `linux/ipc.h`. This holds the permission information for the segment, including the access permissions, and information about the creator of the segment (uid, etc).

`shm_segsz`

  Size of the segment (measured in bytes).

`shm_atime`

  Time the last process attached the segment.

`shm_dtime`

  Time the last process detached the segment.

`shm_ctime`

  Time of the last change to this structure (mode change, etc).

`shm_cpid`

  The PID of the creating process.

`shm_lpid`

  The PID of the last process to operate on the segment.

`shm_nattch`

  Number of processes currently attached to the segment.