The task of the reader-writers

  #include <pthread.h> 
 #include <stdio.h> 
 #include <math.h> 
 #include <stdlib.h> 
 #include <semaphore.h> 
 #define M 4 // num of WR
 #define N 3 // num of RE
 unsigned int iter ;  // iteration
 sem_t accessM , readresM , orderM ;  // sem.
 unsigned int readers = 0 ;  // Number of readers accessing the resource

 void * reader ( void * prm )
 {
	 int num1 = * ( int * ) prm ;
	 int i = 0 , r ;
	 for ( i ; i < iter ; i ++ )
	 {
		
		 if ( sem_wait ( & orderM ) == 0 ) printf ( "% d Reader% d in queue __________% d \ n " , i , num1 , num1 );  // Remember our order of arrival
		 sem_wait ( & readresM );  // We will manipulate the readers counter
		 if ( readers == 0 ) // If there are currently no readers (we came first) ...
			 sem_wait ( & accessM );  // ... requests exclusive access to the resource for readers
		 readers ++ ;  // Note that there is now one more reader
		 sem_post ( & orderM );  // Release order of arrival semaphore (we have been served)
		 sem_post ( & readresM );  // We are done accessing the number of readers for now

		 printf ( "% d Reader% d ________________ H% d \ n " , i , num1 , num1 );  // Here you can read the resource
		 r = 1 + rand () % 4 ;
		 sleep ( r );
		 sem_wait ( & readresM );  // We will manipulate the readers counter
		 readers - ;  // We are leaving, there is one less reader
		 if ( readers == 0 ) // If there are no more readers currently reading ...
			 sem_post ( & accessM );  // ... release exclusive access to the resource
		 sem_post ( & readresM );  // We are done accessing the number of readers for now
	 }
 }

 void * writer ( void * prm )
 {
	 int num2 = * ( int * ) prm ;
	 int j = 0 , r ;
	 for ( j ; j < iter ; j ++ )
	 {
		 if ( sem_wait ( & orderM ) == 0 ) printf ( "% d Writer% d in queue __________ P% d \ n " , j , num2 , num2 );  // Remember our order of arrival
		 sem_wait ( & accessM );  // Request exclusive access to the resource
		 sem_post ( & orderM );  // Release order of arrival semaphore (we have been served)

		 printf ( "% d Writer% d ________________ P% d \ n " , j , num2 , num2 );  // Here you can write
		 r = 1 + rand () % 4 ;
		 sleep ( r );
		 sem_post ( & accessM );  // Release exclusive access to the resource
	 }
 }

 void main ()
 {	
	 pthread_t threadRE [ N ];
	 pthread_t threadWR [ M ];
	 sem_init ( & accessM , 0 , 1 );
	 sem_init ( & readresM , 0 , 1 );
	 sem_init ( & orderM , 0 , 1 );

	 printf ( "Enter the number of iterations:" );
	 scanf ( "% d" , & iter );
	 printf ( "Iter QUEENING / PERFORMANCE \ n " );
	 int i ;
	 for ( i = 0 ; i < M ; i ++ )
	 {
		 pthread_create ( & ( threadWR [ i ]), NULL , writer , ( void * ) & i );
	 }
	 for ( i = 0 ; i < N ; i ++ )
	 {
		 pthread_create ( & ( threadRE [ i ]), NULL , reader , ( void * ) & i );
	 }
	

	 for ( i = 0 ; i < N ; i ++ )
	 {
		 pthread_join ( threadRE [ i ], NULL );
	 }
	 for ( i = 0 ; i < M ; i ++ )
	 {
		 pthread_join ( threadWR [ i ], NULL );
	 }
	
	 sem_destroy ( & accessM );
	 sem_destroy ( & readresM );
	 sem_destroy ( & orderM );
 }