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 */
};

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.