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src/PREEMPTIVE/sem.c

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+/************************************************************************
+ *                                                                      *
+ * file: sem.c                                                          *
+ *                                                                      *
+ * Module implementing semaphores.                                      *
+ *                                                                      *
+ ************************************************************************/
+
+#include "sem.h"
+#include <stdlib.h> 
+
+pthread_mutex_t sem_mutex; /* protect sem module from race conditions */
+
+/************************************************************************
+ *  INTERFACE: sem_create()                                             *
+ *                                                                      *
+ * Make and return a semaphore (i.e. return a pointer to a Sem struct). *
+ ************************************************************************/
+Sem *sem_create(int val)
+{
+        Sem *s;
+        static int firstcall = 1;
+
+	/* We, perhaps naively, assume that sem_create() will not be 
+	 * called concurrently!
+	 */
+        if (firstcall) {
+                firstcall = 0;
+                pthread_mutex_init(&sem_mutex, NULL);
+        }
+
+        if (val < 0) return (Sem *)0;
+
+        pthread_mutex_lock(&sem_mutex);
+
+        if ((s = (Sem *)malloc(sizeof(Sem))) == (Sem *)0) {
+                return (Sem *)0;
+        }
+        s->val = val; 
+        pthread_cond_init(&s->cond, NULL);
+
+        pthread_mutex_unlock(&sem_mutex);
+
+        return s;
+}
+
+/************************************************************************
+ *  INTERFACE: sem_destroy()                                            *
+ *                                                                      *
+ * Destroy a semaphore.                                                 *
+ * Forbid destruction (return -1) if the semaphore has waiting threads. *
+ * Otherwise return 1 for success.                                      *
+ ************************************************************************/
+int 
+sem_destroy(Sem *s)
+{
+        pthread_mutex_lock(&sem_mutex);
+
+        if (s->val < 0) {
+                /* Mustn't destroy a semaphore with waiting threads
+                 * on it.
+                 */
+
+                pthread_mutex_unlock(&sem_mutex);
+                return -1;
+        }
+          pthread_cond_destroy(&s->cond);
+        free((void *)s);
+
+        pthread_mutex_unlock(&sem_mutex);
+
+        return 1;
+}
+
+/************************************************************************
+ *  INTERFACE: sem_P()                                                  *
+ *                                                                      *
+ * Do a P on a semaphore(!)                                             *
+ ************************************************************************/
+void 
+sem_P(Sem *s)
+{
+        pthread_mutex_lock(&sem_mutex);
+
+        if (--s->val < 0) {
+                /* wait on the semaphore's condition */
+                pthread_cond_wait(&s->cond, &sem_mutex);
+        }
+        
+        pthread_mutex_unlock(&sem_mutex);
+}
+
+/************************************************************************
+ *  INTERFACE: sem_V()                                                  *
+ *                                                                      *
+ * Do a V on a semaphore.                                               *
+ ************************************************************************/
+void 
+sem_V(Sem *s)
+{
+        pthread_mutex_lock(&sem_mutex);
+
+        if (++s->val <= 0) {
+                /* assert the semaphore's condition */
+                      pthread_cond_signal(&s->cond);
+        }
+        
+        pthread_mutex_unlock(&sem_mutex);
+
+}
+
+/* end file: sem.c */

src/PREEMPTIVE/sem.h

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+/* file: sem.h -- definitions for semaphores. */
+
+#ifndef SEM_DEFINED
+#define SEM_DEFINED
+
+/* Include public interface. */
+#include "../thread.h"
+#include <pthread.h>
+
+typedef struct semtype {
+        int val;
+	pthread_cond_t cond;
+} Sem;
+
+Sem *create_sem(int val);
+int destroy_sem(Sem *s);
+void sem_P(Sem *s);
+void sem_V(Sem *s);
+
+#endif
+/* end file: sem.h */