Working with the Unix OS

Processes II
Kernel data structures for a semaphore set
#include
#include
#include
semid
sem-perm
sem_base
sem_nsems
sem_otime
sem ctime
semval
sempid
semncnt
semzcnt
semval
sempid
semncnt
semzcnt
[0]
[0]
[0]
[0]
[1]
[1]
[1]
[1]
int semget(key_t key, int nsems, int semflag);
semflag
0400 SEM_R alter by owner
0200 SEM_A read by owner
0040 SEM_R >> 3 alter by group
0020 SEM_A >> 3 alter by group
0004 SEM_R >> 6 read by world
0002 SEM_A >> 6 alter by world
IPC_CREAT
IPC_EXCL
int semop(int semid, struct sembuf **opsptr, unsigned int nops);
struct sembuf {
ushort sem_num; /* semaphore # */
short sem_op; /* semaphore operation */
short sem_flg; /* operation flags */
};
Semaphore operations:
- if sem_op > 0, sere_val is added to semaphore value (release of resources)
- if sem_op = 0, caller waits until semaphore value = 0
- if sem_op < 0, caller waits until semaphore value >= absolute of sem_op
int semctl(int semid, int semnum, int cmd, union semnum arg);
union semun {
int val; /* used for SETVAL only */
struct semid_ds *buffi /* used for IPC_STAT & IPC_SET */
ushort *arrarY; /* used for IPC_GETALL & IPC_SETALL */
} arg;
File Locking with Semaphores
To lock the semaphore call semop to do two operations atomically. First, wait for sem#0 to become 0, then
secondly increment sem#0 by 1.
To unlock the resource, call semop to decrement sem#0 by 1. Explicitly set IPC_NOWAIT, so that cannot wait if
"impossible condition" occurs.
With System V, it is hard to initialize a semaphore to a value other than zero.
If the process aborts for any reason while it has the lock, the semaphore value is left at one.
Any other process that tries to obtain the lock waits forever when it does the locking semop that first waits for the
value to become zero.
System V solution is to tell the kernel (when obtaining lock) that if this process terminates before releasing lock,
release it for the process.
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