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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.