Linux Kernel Architecture

Chapter 5: Locking and Interprocess Communication

q_messages

q_receivers

q_senders

struct msg_receiver

struct task_struct

ipcs_idr struct msg_msg

struct msg_queue

struct

kern_ipc_perm

ID to pointer mapping

struct

ipc_ids

Figure 5-6: Data structures for System V message queues.

5.3.4 Shared Memory

From the user and kernel perspective,shared memory— the third and final SysV concept for interprocess

communication — uses similar structures for its implementation to the first two mechanisms described

above. Its essential aspects do not differ from those of semaphores and message queues.

❑ Applications request an IPC object that can be accessed via a common magic number and a

kernel-internal identifier via the current namespace.

❑ Access to memory can be restricted by means of a system of privileges.

❑ System calls are used to allocate memory that is associated with the IPC object and that can be

accessed by all processes with the appropriate authorization.

Kernel-side implementation also adopts very similar concepts to those described above. I will therefore

make do with a brief description of the data structures shown in Figure 5-7.

shm_file

shmid_kernel struct

address_

space

struct

file

ipcs_idr

ID to pointer mapping

struct

ipc_ids kern_ipc_perm

Figure 5-7: Data structures for System V shared memory.

Once again, a combination ofkern_ipc_permandshmid_kernelheld in theentriesarray of thesmd_ids

global variable is used to facilitate management of the access permissions to the IPC object. A dummy