Working with the Unix OS

Processes II
int msgctl(int msqid, int crod, struct msqid_ds *buff);
cmd of IPC_RMID to remove message queue from system
Multiplexed Messages
client 1
pid=123
client 21
pid=456
client 3
pid=789
type=1 123 1 456 1 789
type=123/456/789 1
server
message
queue
Features:
- read in any order
- assign priorities
- read any message
Semaphores
Semaphores are a synchronization primitive. We will use to synchronize access to shared memory segments.
A semaphore is a integer resource counter. If we have one resource, a shared file, then valid values are 0 & 1.
process A
process B
kernel
semaphore: 0 or 1
Since our use of semaphores is to provide resource synchronization between different processes, the semaphore
value must be stored in the kernel.
To obtain a resource that is controlled by a semaphore, a process needs to test its current value, and if value> 0,
decrement the value by 1.
If value = 0, the process must wait until value> 0 (wait for some other process to release resource).
To release resource, a process increments the value. System V implementation of semaphores is done in the kernel
- guarantee a group of operations is atomic.
#include
#include /* defines ipc-perm structure */
struct semid_ds {
struct ipc-perm sem-perm; /* operation perm struct */
struct sem *sem_base; /* ptr to 1st semaphore in set */
ushort sem_nsems; /* # of semaphores in set */
time_t sem_otime; /* time of last semop */
time_t sem_ctime; /* time of last change */
};
struct sem {
ushort semval; /* semaphore value, non -ve */
short sempid; /* pid of last operation */
ushort semncnt; /* # awaiting semval > cval */
ushort semzcnt; /* # awaiting semval = 0 */
};
Kernel also maintains for each value in the set:
- process ID of process that did last operation
- number of processes waiting for value to increase
- number of processes waiting for value to = zero
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