IPC == OS-supported mechanisms for interaction among processes (coordination & communication)
– message passing
e.g. sockets, pipes, message queues
– memory-based IPC
shared memory, mem mapped files…
– higher-level semantics
files, RPC
synchronization primitives
Message Passing
send/recv of messages
– buffer, FIFO queue…
OS provides interface to processes – a port
-processes send/write messages to a port
-processes recv/read messages from a port
send: system call + data copy
recv: system call + data copy
simplicity: kernel does channel management and synchronization
Shared memory IPC
read and write to shared memory region
– OS establishes shared channel b/w the processes
1. physical pages mapped into virtual address space
2. VA(PI) and VA(P2) map to the same physical address
3. VA(PI) =/ VA(P2)
4. physical memory does not need to be contiguous
Copy(messages) vs. map(shared memory)
Goal: transfer data from one into target address space
copy:
– cpu cycles to copy data to/from port
map:
– cpu cycles to map memory address space
SysV shared memory overview
1. create
– os assigns unique key
2. Attach
– map virtual => physical addresses
3. Detach
– invalidate addr. mappings
4. Destroy
– only remove when explicitly deleted (or reboot)
1. shmget(shmid, size, flag)
– create or open
ftok (pathname, prg-id)
– same args => same key
2. shmat(shmid, addr, flags)
– addr = NULL => arbitrary
– cast addr to arbitrary type
3. shmdt(shmid)
4. shmctl(shmid, cmd, buf)
– destroy wit IPC.RHID
