Concurrent Servers. Overview. In our current assignment we have the following changes:

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1 Concurrent Servers Overview In our current assignment we have the following changes: Concurrent server Session command with an argument of the session name Shutdown command 2

2 Concurrent Server When a client arrives, a new process is spawned for the client Perform the desired command Make sure that the session specific resources are protected from race conditions Keep track of the number of processes accessing session resources Needs to behave correctly for the Quit command When all processes Quit a session, then the session resources can be reclaimed 3 Session Command Has an argument session name If this is a new session Allocate resources for new session Dictionary Score table Etc. Returns a session handle which uniquely identifies the game session The session handle is passed as an argument to all commands to identify the session 4

3 Shutdown Command Shutdown the server This happens only when all players and sessions have shutdown The server process exits Once Shutdown has been called, no further connections are allowed 5 client.c if ((create_socket = socket(af_inet,sock_stream,0)) > 0) printf("the Socket was created\n"); address.sin_family = AF_INET; hostinfo = gethostbyname( argv[1] ); if ( hostinfo == NULL ) return 1; address.sin_addr = *(( struct in_addr *)hostinfo->h_addr); address.sin_port = htons( PORT ); if (connect(create_socket, (struct sockaddr *) &address, sizeof(address)) == 0) printf("the connection was accepted with the server %s...\n", inet_ntoa(address.sin_addr)); 6

4 client.c (cont) do{ recv(create_socket,buffer,bufsize,0); printf("message recieved: %s\n",buffer); if (strcmp(buffer,"/q")){ printf("message to send: "); gets(buffer); send(create_socket,buffer,bufsize,0); } }while (strcmp(buffer,"/q")); close(create_socket); } 7 fork() Duplicates a process Both processes execute the same code starting after the fork() call How do the processes differ? Child process is new and has a different process ID fork() provides different return values to the parent and the child processes Parent process ID of child Child a zero 8

5 server.c sd = socket( PF_INET, SOCK_STREAM, 0 ); memset( &addr, 0, sizeof( addr ) ); addr.sin_family = AF_INET; addr.sin_port = htons( PORT ); addr.sin_addr.s_addr = INADDR_ANY; bind( sd, &addr, sizeof( addr ) ); /* listen on socket, and can queue 5 connect requests */ listen( sd, 5 ); 9 server.c (cont) while(1) { addr_len = sizeof(addr); clientsd = accept(sd, &addr, &addr_len); pid = fork(); if ( pid == 0 ) { /* a child since, pid==0 */ close( sd );/*child don't need to listen on original socket*/ memset( buffer, 0, BUFSIZE ); do { /* child stuff */ } while( ); close( clientsd ); exit(0); } /* if child*/ else /* parent */ close( clientsd ); /* doesn t need since server just listens */ } /* while */ 10

6 Process Semaphores Used to synchronize processes Process semaphores are: Allocated Used Deallocated Although we usually only need a single semaphore, they come in sets 11 Include Files #include <sys/ipc.h> #include <sys/sem.h> #include <sys/types.h> 12

7 Allocate Semaphore int semget( key_t key, int num, int flags); semid = semget( (key_t)4996, 1, IPC_CREATE 0666 ); Arguments Key specifying the semaphore Number of semaphores Permission flags 13 Deallocate Semaphores int semctl( int semid, int num, int flag, union semun arg ); int semctl( semid, 1, IPC_RMID, ignored_argument ); Semaphores are persistent The last process to use the semaphore set must explicitly remove it to ensure that the OS doesn t run out of semaphores Arguments Semaphore identifier Number of semaphores IPC_RMID Any value as the argument is ignored 14

8 Allocate and Deallocate union semun { int val; struct semid_ds *buf; unsigned short int *array; struct seminfo * buf; }; /* semun */ int binary_semaphore_allocate (key_t key) { return semget (key, 1, IPC_CREATE 0666); } /* binary_semaphore_allocate */ int binary_semaphore_deallocate (int semid) { union semun ignored_argument; return semctl (semid, 1, IPC_RMID, ignored_argument); } /* binary_semaphore_deallocate 15 Calls to Allocate and Deallocate Semaphores semid = binary_semaphore_allocate( 4996 ); status = binary_semaphore_deallocate( semid ); 16

9 Initializing Semaphores union semun { int val; struct semid_ds *buf; unsigned short int *array; struct seminfo * buf; }; /* semun */ int binary_semaphore_initialize (int semid) { union semun argument; unsigned short values[1]; values[0] = 1; argument.array = values; return semctl (semid, 0, SETALL, argument); } /* binary_semaphore_initialize */ 17 Initializing Semaphores We use semctl again as follows: status = semctl( semid ); 18

10 Wait (P) and Signal (V) int binary_semaphore_wait (int semid) { struct sembuf operations[1]; operations[0].sem_num = 0; operations[0].sem_op = -1; operations[0].sem_flg = SEM_UNDO; return semop (semid, operations, 1); } /* binary_semaphore_wait */ int binary_semaphore_signal (int semid) { struct sembuf operations[1]; operations[0].sem_num = 0; operations[0].sem_op = 1; operations[0].sem_flg = SEM_UNDO; return semop (semid, operations, 1); } /* binary_semaphore_signal */ 19 Wait (P) and Signal (V) status = binary_semaphore_wait( semid ); status = binary_semaphore_signal( semid ); 20

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