Mutual Exclusion Locks

Syntax

A mutual exclusion lock is a special case of a semaphore when the initial value is 1. The type of a lock is mutex_t. A lock must be initialized with mutex_init() before its use. Then, a lock can be acquired with mutex_lock() and then released with mutex_unlock(). All mutex lock related functions start with mutex_. The specification of these functions are shown below:

#include  <synch.h>

mutex_t   Lock;

int  mutex_init(
          mutex_t  *Lock,   /* pointer to a lock */
          USYNC_THREAD,     /* use this in CS270 */
          (void *) NULL);   /* always use this   */


int  mutex_lock(
          mutex_t  *Lock);  /* pointer to a lock */


int  mutex_unlock(
          mutex_t  *Lock);  /* pointer to a lock */

• A lock is declared to be of type mutex_t.
• Locks are initialized with function mutex_init(). It requires three arguments. The first is a pointer to the lock to be initialized, the second is USYNC_THREAD for our purpose, and the third is a pointer to void.
• Both mutex_lock() and mutex_unlock() only require one argument, the lock being used.
• To acquire a lock (i.e., to lock a lock), use function mutex_lock(). When the lock is already locked, the calling thread blocks until the lock becomes available. When mutex_lock() returns, the lock is locked and the calling thread is the owner.
• To release a lock (i.e., to unlock a lock), use function mutex_unlock(). The lock must be locked and the calling thread must be the owner that last acquired the lock. When any other threads are waiting for the lock, one of them will be released to be the next owner.
• When a lock is initialized, it is unlocked. So, if you want a lock locked at the very beginning, you should call mutex_lock() right after the lock is initialized.
• Since locks are simplified version of semaphores, they can be used to construct mutual exclusion. For example, a pair of mutex_lock() and mutex_unlock() will form a critical section and guarantee mutual exclusion.

  #include  <synch.h>
  
  mutex_t   Lock;
  
  mutex_lock(&Lock);
       ..... in critical section .....
  mutex_unlock(&Lock);