Working with the Unix OS

Interprocess Communication
When processes A and B run simultaneously, a situation could arise whereby process A has locked semaphore 0
and wants to lock semaphore 1, but process B has locked semaphore 1 and wants to lock semaphore 0.
Both processes sleep, unable to continue. They are deadlocked and exit only on receipt of a signal.
To avoid deadlock use multiple semaphore operations simultaneously:
struct sembuf psernbuf[2];
psernbuf[0].sem_nurn = 0;
psernbuf[l].sem_nurn = 1;
psernbuf[0].sem_op = -1;
psernbuf[0].sem_op = -1;
semop(semid, psernbuf, 2);
! Undo Structures for Semaphores
Per process
Undo Headers
:
-----
Undo Structures
desc
num
value
desc
num
value
desc
num
value
-----
-----
:
desc
num
value
-----
Each undo structure is an array of triples consisting of:
- a semaphore ID,
- a semaphore number in the set identified by ID,
- and an adjustment value.
Sequence of Undo Structures
! After first operation
semaphore id semid
semaphore nurn 0
adjustment 1
! After second operation
seamphore id semid semid
semaphore nurn 0 1
adjustment 1 1
! After third operation
semaphore id semid
semaphore nurn 0
adjustment 1
! After fourth operation
empty
BERKELEY Sockets
The Application Program Interface (API) is the interface to a programmer. For UNIX there is Berleley Sockets and
System V Transport Layer Interface (TLI).
Network I/O includes File I/O system calls: open, creat, close, read, write, & lseek
! Network I/O considerations
- client or server?
- connection-oriented or connectionless
- process names are more important in networking
- more parameters for a network connection
- communication protocol record boundaries
- support multiple communication protocols
163