Threads and Locking Ioctl operations

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Threads and LockingIoctl operations
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Threads

• Lightweight processes
• Whats wrong with processes?
• fork() is expensive 10 to 100 times slower
• Inter process communication
• For returning information from child to parent

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Threads

• Shared components
• Global memory
• Instructions
• Most data
• Open descriptors (files, sockets etc)
• Signal handlers
• Not shared
• Thread ID
• Registers, Program counter, stack pointer
• Stack
• Errno
• Signal mask
• Priority

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Creation

• Thread equivalent of fork()
• int pthread_create(
• pthread_t thread,
• pthread_attr_t attr,
• void (start_routine)(void ),
• void arg
• )
• Returns 0 is OK, and non-zero (gt 0) if error.

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Termination

• Return from initial fuction.
• void pthread_exit(void status)
• exit() called by any thread
• main() returns

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Waiting for child thread to exit

• int pthread_join(pthread_t tid, void status)
• Equivalent of waitpid()

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Detaching a thread

• The detached thread can act as daemon thread
• The parent thread doesnt need to wait
• int pthread_detach(pthread_t tid)
• Detaching self
• pthread_detach(pthread_self())

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Echo client-server
Server
listenfd
S1
S2
Read
Write
Read
Write
Client2
Client1
Read Thread
Write Thread
Read Thread
Write Thread
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Thread-based Echo Server
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• main()
• int listenfd, connfd
• int len
• / Start the usual way /
• listenfd Socket()
• Bind(listenfd, )
• Listen(listenfd, )
• for ( )
• len addrlen
• connfd Accept(listenfd, )
• / Create a thread in service_func routine /
• Pthread_create(NULL, NULL, service_func, (void )
  connfd)

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• void service_func(void arg)
• int local_connfd
• / release parent from waiting /
• Pthread_detach(pthread_self())
• / extract connfd from argument /
• local_connfd (int) arg
• / receive and echo clients message /
• str_echo(local_connfd)
• / Terminate the connection /
• Close(local_connfd)
• return(NULL)

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Thread-based Echo Client
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• int sockfd
• FILE fp
• main()
• pthread_t tid
• fp fopen()
• / Start the usual way /
• sockfd Socket()
• Connect()
• / Create a thread to send data /
• Pthread_create(tid, NULL, write_func, NULL)
• / read data from sockfd /
• read_func()

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• void write_func(void arg)
• char sendlineMAXLINE
• while( more data in fp)
• Read from fp into sendline
• write sendline into sockfd
• Shutdown(sockfd, SHUT_WR)
• return(NULL)
• void read_func()
• char recvlineMAXLINE
• while ( more data from sockfd)
• read from sockfd into recvline
• write from recvline to stdout

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Mutex for mutual exclusion

• int counter 0
• void thread_func(void arg)
• int val
• / unprotected code why? /
• val counter
• counter val 1
• return NULL

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Mutex

• int counter 0
• ptread_mutex_t mutex PTHREAD_MUTEX_INITIALIZER
• void thread_func(void arg)
• int val
• / protected by mutex /
• Pthread_mutex_lock( mutex )
• val counter
• counter val 1
• Pthread_mutex_unlock( mutex )
• return NULL

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Condition Variable for signaling

• Think of Producer consumer problem
• Producers and consumers run in separate threads.
• Producer produces data and consumer consumes
  data.
• Consumer has to inform producer when data is
  available

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Without condition variables
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• / Globals /
• int data_avail 0
• int pthread_mutex_t data_mutex PTHREAD_MUTEX_INIT
  IALIZER
• void producer(void )
• Pthread_mutex_lock(data_mutex)
• Produce data
• Insert data into queue
• data_avail
• Pthread_mutex_unlock(data_mutex)
• consume_data()

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• void consumer(void )
• Pthread_mutex_lock(data_mutex)
• while( !data_avail )
• / do nothing keep looping!!/
• Extract data from queue
• if (queue is empty) data_avail 0
• Pthread_mutex_unlock(data_mutex)
• consume_data()

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With condition variables
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• int data_avail 0
• int pthread_mutex_t data_mutex PTHREAD_MUTEX_INIT
  IALIZER
• int pthread_cont_t data_cond PTHREAD_COND_INITIA
  LIZER
• void producer(void )
• Pthread_mutex_lock(data_mutex)
• Produce data
• Insert data into queue
• data_avail
• Pthread_cond_signal(data_cond)
• Pthread_mutex_unlock(data_mutex)
• consume_data()

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• void consumer(void )
• Pthread_mutex_lock(data_mutex)
• while( !data_avail )
• / sleep on condition variable/
• Pthread_cond_wait(data_cond, data_mutex)
• /woken up /
• Extract data from queue
• if (queue is empty) data_avail 0
• Pthread_mutex_unlock(data_mutex)
• consume_data()

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ioctl()
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ioctl()

• Handles miscellaneous properties of a file/device
  referenced by a descriptor.
• In our case, network interfaces.
• int ioctl(int fd, int request, void arg)
• Socket operations
• File operations
• Interface configuration
• ARP cache
• Routing table

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• SIOSPGRP/SIOGPGRP
• set/get process/group ID of a socket
• FIONREAD
• Return number of bytes in socket buffer
• SIOCGIFCONF
• Get list of all interfaces
• SIOCGIFBRDADDR/ SIOCSIFBRDADDR
• Get/set broadcast address
• SIOCGARP/SIOCSARP/SIOCDARP
• Get/modify/delete ARP cache entry.
• SIOCADDRT/SIOCDELRT
• Add/delete routes