(ipc.info.gz) Shared Memory
1.7 Shared Memory
Shared memory is distinct from the sharing of read-only code pages or
the sharing of unaltered data pages that is available due to the
copy-on-write mechanism. The essential difference is that the shared
pages are dirty (in the case of Shared memory) and can be made to
appear at a convenient location in the process' address space.
A shared segment is described by :
struct ipc_perm shm_perm;
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 */
ulong *shm_pages; /* internal page table */
ushort shm_cpid; /* pid, creator */
ushort shm_lpid; /* pid, last operation */
short shm_nattch; /* no. of current attaches */
A shmget allocates a shmid_ds and an internal page table. A shmat
maps the segment into the process' address space with pointers into the
internal page table and the actual pages are faulted in as needed. The
memory associated with the segment must be explicitly destroyed by
calling shmctl with IPC_RMID.
* shmlimits Limits imposed by this implementation.
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