Pthreads & Concurrency
Acknowledgements

The material in this tutorial is
based in part on:
 POSIX Threads Programming , by
Blaise Barney



History
Advantages
Content
History

Different versions of threads
 each hardware vendor created own version

IEEE POSIX 1003.1c Standard

C programming types and procedure
calls
Advantages

Potential program performance gains
Platform
fork()
pthread_create()
real
user
sys
real
user
sys
AMD 2.4 GHz Opteron
41.07
60.08
9.01
0.66
0.19
0.43
IBM 1.9 GHz POWER5 p5-575
64.24
30.78 27.68
1.75
0.69
1.10
IBM 1.5 GHz POWER4
104.05 48.64 47.21
2.01
1.00
1.52
INTEL 2.4 GHz Xeon
54.95
1.64
0.67
0.90
1.54
20.78
Advantages

More efficient inter-thread
communication

Threaded applications
 Overlap CPU work and I/O
 Priority/Real-time scheduling
 Asynchronous event handling
The Pthreads API

Thread management
 Create, detach, join
 Set/query thread attributes

Mutex
 Mutual Exclusion

Condition variables
 Address communications between threads

Non-mutex synchronization
 Read/write locks
 Barriers
The Pthreads API

Naming conventions
Routine Prefix
Functional Group
pthread_
Thread themselves and miscellaneous subroutines
pthread_attr_
Thread attributes objects
pthread_mutex_
Mutexes
pthread_mutexattr_
Mutex attributes objects
pthread_cond_
Condition variables
pthread_condattr_
Condition attributes objects
pthread_key_
Thread-specific data keys


Create & Terminate
Program: Hello World
Creation

Routine
 pthread_create(
thread,
attr,
start_routine,
arg)
Termination

Four ways for a Pthread to terminate
 Returns from start_routine
 Call pthread_exit routine
 Cancel by another thread through
pthread_cancel routine
 exit call
Example: Hello World

Program Requirements
 Print “Hello World” through standard output
 Use two threads to finish the task, one
should print “Hello”, the other should print
“World”
 Terminate all related threads related after
finishing the printing task
Example: Hello World
#include <stdio.h>
#include <pthread.h>
……
Example: Hello World
void print_message_function( void *ptr )
{
char *message;
message = (char *) ptr;
printf("%s ", message);
}
Example: Hello World
main()
{
pthread_t thread1, thread2;
char *message1 = "Hello";
char *message2 = "World";
……
}
Example: Hello World
main()
{
pthread_t thread1, thread2;
char *message1 = "Hello";
char *message2 = "World";
……
}
Example: Hello World
main()
{
pthread_t thread1, thread2;
char *message1 = "Hello";
char *message2 = "World";
……
}
Example: Hello World
main()
{
……
pthread_create(&thread1, pthread_attr_default,
(void*)&print_message_function, (void*) message1);
pthread_create(&thread2, pthread_attr_default,
(void*)&print_message_function, (void*) message2);
exit(0);
}
Example: Hello World
main()
{
……
pthread_create(&thread1, pthread_attr_default,
(void*)&print_message_function, (void*) message1);
pthread_create(&thread2, pthread_attr_default,
(void*)&print_message_function, (void*) message2);
exit(0);
}
Example: Hello World
main()
{
……
pthread_create(&thread1, pthread_attr_default,
(void*)&print_message_function, (void*) message1);
pthread_create(&thread2, pthread_attr_default,
(void*)&print_message_function, (void*) message2);
exit(0);
}
Example: Hello World
main()
{
……
pthread_create(&thread1, pthread_attr_default,
(void*)&print_message_function, (void*) message1);
pthread_create(&thread2, pthread_attr_default,
(void*)&print_message_function, (void*) message2);
exit(0);
}
Example: Hello World
main()
{
……
pthread_create(&thread1, pthread_attr_default,
(void*)&print_message_function, (void*) message1);
pthread_create(&thread2, pthread_attr_default,
(void*)&print_message_function, (void*) message2);
exit(0);
}
Example: Hello World
main()
{
……
pthread_create(&thread1, pthread_attr_default,
(void*)&print_message_function, (void*) message1);
pthread_create(&thread2, pthread_attr_default,
(void*)&print_message_function, (void*) message2);
exit(0);
}
Example: Hello World
main()
{
……
pthread_create(&thread1, pthread_attr_default,
(void*)&print_message_function, (void*) message1);
pthread_create(&thread2, pthread_attr_default,
(void*)&print_message_function, (void*) message2);
exit(0);
}
Example: Hello World
Thread 1
default thread
Thread 2
Example: Hello World
Thread 1
default thread
Thread 2
Example: Hello World
Thread 1
default thread
Thread 2
Example: Hello World
Thread 1
default thread
Thread 2
Example: Hello World
Thread 1
default thread
Thread 2
Example: Hello World
Thread 1
Printf
default thread
Thread 2
Printf
Example: Hello World
Thread 1
Printf
default thread
Exit
Thread 2
Printf
Example: Hello World
Thread 1
standard
output
Thread 2
Thread 1
default
thread
Thread 2
Return
Example: Hello World
void print_message_function( void *ptr )
{
char *message;
message = (char *) ptr;
printf("%s ", message);
}
main()
{
……
pthread_exit(); //exit(0);
}
Example: Hello World
void print_message_function( void *ptr )
{
char *message;
message = (char *) ptr;
printf("%s ", message);
pthread_exit();
}
main()
{
……
pthread_exit(); //exit(0);
}
Passing parameters to threads
The pthread_create() routine permits the
programmer to pass one argument to the
thread start routine.
 For cases where multiple arguments must
be passed, this limitation is overcome by
creating a structure which contains all of
the arguments, and then passing a pointer
to that structure.
 All arguments must be passed by
reference and cast to (void *).

Joining and detaching threads

pthread_join (threadid,status)
 blocks the calling thread until the specified threadid thread
terminates.
 target thread's termination return status placed in status if
specified in the target thread's call to pthread_exit().
 A joining thread can match one pthread_join() call.
 It is a logical error to attempt multiple joins on the same
thread.

pthread_detach (threadid, status)
 to explicitly detach a thread


pthread_attr_setdetachstate (attr,detachstate)
pthread_attr_getdetachstate (attr, detachstate)



Create & Destroy
Lock & Unlock
Program: Bounded Buffer
Mutex

Mutual Exclusion

Act as a “lock” to prevent race conditions

Typical sequence of use
 create, initialize
 lock, unlock
 destroy

Losers in the competition for the mutex
variable have to block
Creating & Destroying

Type: pthread_mutex_t

Initializing
 Statically
○ pthread_mutex_t mymutex = PTHREAD_MUTEX_INITIALIZER
 Dynamically
○ pthread_mutex_init (mutex, attr)
 pthread_mutex_attr_t attr specifies protocol for priorities, priority ceiling,
and process sharing

Destroying
 pthread_mutex_destroy – to delete a mutex object
 pthread_mutex_attr_destroy – to delete a mutex attr object
Lock & Unlock

Lock
 pthread_mutex_lock (mutex)
○ blocks calling thread if mutex already locked
 pthread_mutex_trylock(mutex)
○ returns with “busy” code if already locked

Unlock
 pthread_mutex_unlock (mutex)
 Error
○ mutex is released
○ mutex owned by others