/*
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* This is a R/W lock implementation using the SYS semaphores.
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* Based on http://www.experts-exchange.com/Programming/Languages/C/Q_23939132.html
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*/
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#ifndef __HASHTABLE_SEM__
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#define __HASHTABLE_SEM__
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#include <sys/types.h>
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#include <sys/ipc.h>
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#include <sys/sem.h>
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#include <stdio.h>
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#define SEM_READER 0
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#define SEM_WRITER 1
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struct sembuf read_start[] =
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{ {SEM_READER, 1, SEM_UNDO}, {SEM_WRITER, 0, SEM_UNDO} };
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struct sembuf read_end[] = { {SEM_READER, -1, SEM_UNDO} };
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struct sembuf write_start1[] = { {SEM_WRITER, 1, SEM_UNDO} };
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struct sembuf write_start2[] =
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{ {SEM_READER, 0, SEM_UNDO}, {SEM_READER, 1, SEM_UNDO} };
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struct sembuf write_fail_end[] = { {SEM_WRITER, -1, SEM_UNDO} };
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struct sembuf write_end[] =
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{ {SEM_READER, -1, SEM_UNDO}, {SEM_WRITER, -1, SEM_UNDO} };
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#define SEMOP(semid,tbl,exc) { if(0>semop (semid, tbl, sizeof(tbl)/sizeof(struct sembuf) )){perror("semop: ");return exc;}}
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//Necessary stuff for locking.
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int
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write_end_proc (int semid)
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{
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SEMOP (semid, write_fail_end, 0);
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return -1;
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}
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#define READ_LOCK(semid) SEMOP(semid,read_start,-1)
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#define READ_UNLOCK(semid) SEMOP(semid,read_end,-1)
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#define WRITE_LOCK_READERS(semid) (SEMOP(semid,write_start1,-1))
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#define WRITE_LOCK_TO_WRITE(semid) (SEMOP(semid,write_start2,write_end_proc(semid)))
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//This macro returns a 0 different value if something goes wrong with the locking.
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#define WRITE_LOCK(semid) (WRITE_LOCK_READERS(semid) && WRITE_LOCK_TO_WRITE(semid))
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#define WRITE_UNLOCK(semid) SEMOP(semid,write_end, -1)
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int
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read_lock (int semid)
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{
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READ_LOCK (semid);
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return 0;
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}
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int
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read_unlock (int semid)
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{
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READ_UNLOCK (semid);
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return 0;
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}
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int
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write_lock (int semid)
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{
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WRITE_LOCK_READERS (semid);
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WRITE_LOCK_TO_WRITE (semid);
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return 0;
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}
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int
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write_unlock (int semid)
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{
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WRITE_UNLOCK (semid);
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return 0;
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}
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#endif // __HASHTABLE_SEM__
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