Semaphore implementaion using condition variables and mutex

Implementation of semaphore using condition variable and mutex

        To implement a semaphore, we need 3 important functions. A sem_post or up function, sem_wait or down function and sem_init function. At first we have to define a structure with a interger value, mutex of pthread_mutex_t type and  cond of pthread_cond_t type.

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struct semaphore                   

{                                              

  int val;                     

         pthread_mutex_t mutex;

         pthread_cond_t cond;    

}                                            

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 Now we need three functions:

 --> init(semaphore *s); --contains the initial value of the semaphore.

 --> up(semaphore *s);  --increments the value of val.

 --> down(semaphore *s); -- decrements the value of val and if val is zero it waits for an up()

Code for init()

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void init(semaphore *s, int n)                                                     

{                                                                   

s->val = n;                                

}                                                                  

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Code for up()

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void up(semaphore *s)                                    

{                                                                      

pthread_mutex_lock(&(s->mutex));

s->val++;                                        

     pthread_cond_broadcast(&(s->cond));

pthread_mutex_lock(&(s->mutex)); 

}                                                                      

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Code for down()

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void down(semaphore *s)                                              

{                                                                                      

pthread_mutex_lock(&(s->mutex));                

while(s->val == 0)                                         

                 pthread_cond_wait(&(s->cond),&(s->mutex));

s->val--;                                                           

pthread_mutex_unlock(&(s->mutex));             

}                                                                                     

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