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- Thread operations include thread creation, termination,
synchronization (joins,blocking), scheduling, data management and
process interaction.
- A thread does not maintain a list of created threads, nor does it know the
thread that created it.
- All threads within a process share the same address space.
- Threads in the same process share:
- Process instructions
- Most data
- open files (descriptors)
- signals and signal handlers
- current working directory
- User and group id
- Each thread has a unique:
- Thread ID
- set of registers, stack pointer
- stack for local variables, return addresses
- signal mask
- priority
- Return value: errno
- pthread functions return "0" if OK.
| Thread Creation and Termination: |
Example: pthread1.c
-
#include
#include
#include
void *print_message_function( void *ptr );
main()
{
pthread_t thread1, thread2;
char *message1 = "Thread 1";
char *message2 = "Thread 2";
int iret1, iret2;
/* Create independent threads each of which will execute function */
iret1 = ( &thread1, NULL, print_message_function, (void*) message1);
iret2 = pthread_create( &thread2, NULL, print_message_function, (void*) message2);
/* Wait till threads are complete before main continues. Unless we */
/* wait we run the risk of executing an exit which will terminate */
/* the process and all threads before the threads have completed. */
( thread1, NULL);
pthread_join( thread2, NULL);
printf("Thread 1 returns: %d\n",iret1);
printf("Thread 2 returns: %d\n",iret2);
exit(0);
}
void *print_message_function( void *ptr )
{
char *message;
message = (char *) ptr;
printf("%s \n", message);
}
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Compile:
- C compiler: cc -lpthread pthread1.c
or
- C++ compiler: g++ -lpthread pthread1.c
Run: ./a.out
Results:
-
Thread 1
Thread 2
Thread 1 returns: 0
Thread 2 returns: 0
Details:
- In this example the same function is used in each thread. The arguments
are different. The functions need not be the same.
- Threads terminate by explicitly calling pthread_exit,
by letting the function return, or by a call to the function exit
which will terminate the process including any threads.
- Function call:
int pthread_create(pthread_t * thread,
const pthread_attr_t * attr,
void * (*start_routine)(void *),
void *arg);
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Arguments:
- thread - returns the thread id. (unsigned long int defined in bits/pthreadtypes.h)
- attr - Set to NULL if default thread attributes are used.
(else define members of the struct pthread_attr_t defined in
bits/pthreadtypes.h)
Attributes include:
- detached state (joinable? Default: PTHREAD_CREATE_JOINABLE. Other option: PTHREAD_CREATE_DETACHED)
- scheduling policy (real-time? PTHREAD_INHERIT_SCHED,PTHREAD_EXPLICIT_SCHED,SCHED_OTHER)
- scheduling parameter
- inheritsched attribute (Default: PTHREAD_EXPLICIT_SCHED Inherit from parent thread: PTHREAD_INHERIT_SCHED)
- scope (Kernel threads: PTHREAD_SCOPE_SYSTEM User threads: PTHREAD_SCOPE_PROCESS Pick one or the other not both.)
- guard size
- stack address (See unistd.h and bits/posix_opt.h _POSIX_THREAD_ATTR_STACKADDR)
- stack size (default minimum PTHREAD_STACK_SIZE
set in pthread.h),
- void * (*start_routine) - pointer to the function to be threaded. Function has a single argument: pointer to void.
- *arg - pointer to argument of function. To pass multiple arguments, send a pointer to a structure.
- Function call:
void pthread_exit(void *retval);
Arguments:
- retval - Return value of thread.
This routine kills the thread. The pthread_exit function never
returns. If the thread is not detached, the thread id and return value
may be examined from another thread by using pthread_join.
Note: the return pointer *retval, must not be of local scope
otherwise it would cease to exist once the thread terminates.
- [C++ pitfalls]: The above sample program
will compile with the GNU C and C++ compiler g++.
The following function pointer representation below
will work for C but not C++. Note the subtle differences and avoid the
pitfall below:
void print_message_function( void *ptr );
...
...
iret1 = pthread_create( &thread1, NULL, (void*)&print_message_function, (void*) message1);
...
...
The threads library provides three synchronization mechanisms:
- mutexes - Mutual exclusion lock: Block access to variables
by other threads. This enforces exclusive access by a thread to a
variable or set of variables.
- joins - Make a thread wait till others are complete (terminated).
- condition variables - data type pthread_cond_t
Mutexes:
Mutexes are used to prevent data inconsistencies due to race conditions.
A race condition often occurs when two or more threads need to perform
operations on the same memory area, but the results of computations depends on
the order in which these operations are performed. Mutexes are used for
serializing shared resources. Anytime a global resource is accessed by more
than one thread the resource should have a Mutex associated with it.
One can apply a mutex to protect a segment of memory ("critical region") from other threads.
Mutexes can be applied only to threads in a single process and do not work
between processes as do semaphores.
Example threaded function:
-
| Without Mutex |
With Mutex |
int counter=0;
/* Function C */
void functionC()
{
counter++
}
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/* Note scope of variable and mutex are the same */
pthread_mutex_t mutex1 = PTHREAD_MUTEX_INITIALIZER;
int counter=0;
/* Function C */
void functionC()
{
pthread_mutex_lock( &mutex1 );
counter++
pthread_mutex_unlock( &mutex1 );
}
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| Possible execution sequence |
| Thread 1 |
Thread 2 |
Thread 1 |
Thread 2 |
|
counter = 0
|
counter = 0
|
counter = 0
|
counter = 0
|
|
counter = 1
|
counter = 1
|
counter = 1
|
Thread 2 locked out.
Thread 1 has exclusive use of variable counter
|
|
|
|
counter = 2
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If register load and store operations for the incrementing of variable counter
occurs with unfortunate timing, it is theoretically possible to have each
thread increment and overwrite the same variable with the same value.
Another possibility is that thread two would first increment counter
locking out thread one until complete and then thread one would increment it to 2.
-
| Sequence |
Thread 1 |
Thread 2 |
| 1 |
counter = 0 |
counter=0 |
| 2 |
Thread 1 locked out.
Thread 2 has exclusive use of variable counter
|
counter = 1 |
| 3 |
counter = 2 |
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Code listing: mutex1.c
-
#include
#include
#include
void *functionC();
pthread_mutex_t mutex1 = PTHREAD_MUTEX_INITIALIZER;
int counter = 0;
main()
{
int rc1, rc2;
pthread_t thread1, thread2;
/* Create independent threads each of which will execute functionC */
if( (rc1=pthread_create( &thread1, NULL, &functionC, NULL)) )
{
printf("Thread creation failed: %d\n", rc1);
}
if( (rc2=pthread_create( &thread2, NULL, &functionC, NULL)) )
{
printf("Thread creation failed: %d\n", rc2);
}
/* Wait till threads are complete before main continues. Unless we */
/* wait we run the risk of executing an exit which will terminate */
/* the process and all threads before the threads have completed. */
pthread_join( thread1, NULL);
pthread_join( thread2, NULL);
exit(0);
}
void *functionC()
{
( &mutex1 );
counter++;
printf("Counter value: %d\n",counter);
( &mutex1 );
}
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Compile: cc -lpthread mutex1.c
Run: ./a.out
Results:
-
Counter value: 1
Counter value: 2
When a mutex lock is attempted against a mutex which is held by another thread,
the thread is blocked until the mutex is unlocked.
When a thread terminates, the mutex does not unless explicitly unlocked.
Nothing happens by default.
Joins:
A join is performed when one wants to wait for a thread to finish. A thread
calling routine may launch multiple threads then wait for them to finish
to get the results. One wait for the completion of the threads with a join.
Sample code: join1.c
-
#include
#include
#define NTHREADS 10
void *thread_function(void *);
pthread_mutex_t mutex1 = PTHREAD_MUTEX_INITIALIZER;
int counter = 0;
main()
{
pthread_t thread_id[NTHREADS];
int i, j;
for(i=0; i < NTHREADS; i++)
{
( &thread_id[i], NULL, thread_function, NULL );
}
for(j=0; j < NTHREADS; j++)
{
( thread_id[j], NULL);
}
/* Now that all threads are complete I can print the final result. */
/* Without the join I could be printing a value before all the threads */
/* have been completed. */
printf("Final counter value: %d\n", counter);
}
void *thread_function(void *dummyPtr)
{
printf("Thread number %ld\n", ());
pthread_mutex_lock( &mutex1 );
counter++;
pthread_mutex_unlock( &mutex1 );
}
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Compile: cc -lpthread join1.c
Run: ./a.out
Results:
-
Thread number 1026
Thread number 2051
Thread number 3076
Thread number 4101
Thread number 5126
Thread number 6151
Thread number 7176
Thread number 8201
Thread number 9226
Thread number 10251
Final counter value: 10
Condition Variables:
A condition variable is a variable of type pthread_cond_t and is
used with the appropriate functions for waiting and later, process continuation.
The condition variable mechanism allows threads to suspend execution and
relinquish the processor until some condition is true.
A condition variable must always be associated with a mutex
to avoid a race condition created by one thread preparing to wait and another
thread
which may signal the condition before the first thread actually waits on it
resulting in a deadlock.
The thread will be perpetually waiting for a signal that is never sent.
Any mutex can be used, there is no explicit link between the mutex and the
condition variable.
Functions used in conjunction with the condition variable:
- Creating/Destroying:
-
- pthread_cond_t cond = PTHREAD_COND_INITIALIZER;
-
- Waiting on condition:
-
- - place limit on how long it will block.
- Waking thread based on condition:
-
- - wake up all threads blocked by the specified condition variable.
Example code: cond1.c
-
#include
#include
#include
pthread_mutex_t count_mutex = PTHREAD_MUTEX_INITIALIZER;
pthread_mutex_t condition_mutex = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t condition_cond = PTHREAD_COND_INITIALIZER;
void *functionCount1();
void *functionCount2();
int count = 0;
#define COUNT_DONE 10
#define COUNT_HALT1 3
#define COUNT_HALT2 6
main()
{
pthread_t thread1, thread2;
pthread_create( &thread1, NULL, &functionCount1, NULL);
pthread_create( &thread2, NULL, &functionCount2, NULL);
pthread_join( thread1, NULL);
pthread_join( thread2, NULL);
exit(0);
}
void *functionCount1()
{
for(;;)
{
( &condition_mutex );
while( count >= COUNT_HALT1 && count <= COUNT_HALT2 )
{
( &condition_cond, &condition_mutex );
}
( &condition_mutex );
pthread_mutex_lock( &count_mutex );
count++;
printf("Counter value functionCount1: %d\n",count);
pthread_mutex_unlock( &count_mutex );
if(count >= COUNT_DONE) return(NULL);
}
}
void *functionCount2()
{
for(;;)
{
pthread_mutex_lock( &condition_mutex );
if( count < COUNT_HALT1 || count > COUNT_HALT2 )
{
( &condition_cond );
}
pthread_mutex_unlock( &condition_mutex );
pthread_mutex_lock( &count_mutex );
count++;
printf("Counter value functionCount2: %d\n",count);
pthread_mutex_unlock( &count_mutex );
if(count >= COUNT_DONE) return(NULL);
}
}
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Compile: cc -lpthread cond1.c
Run: ./a.out
Results:
-
Counter value functionCount1: 1
Counter value functionCount1: 2
Counter value functionCount1: 3
Counter value functionCount2: 4
Counter value functionCount2: 5
Counter value functionCount2: 6
Counter value functionCount2: 7
Counter value functionCount1: 8
Counter value functionCount1: 9
Counter value functionCount1: 10
Counter value functionCount2: 11
Note that functionCount1() was halted while count was between the values COUNT_HALT1 and COUNT_HALT2.
The only thing that has been ensures is that functionCount2 will
increment the count between the values COUNT_HALT1 and COUNT_HALT2.
Everything else is random.
The logic conditions (the "if" and "while" statements) must be chosen to
insure that the "signal" is executed if the "wait" is ever processed.
Poor software logic can also lead to a deadlock condition.
Note: Race conditions abound with this example because count is used as the
condition and can't be locked in the while statement without causing deadlock.
I'll work on a cleaner example but it is an example of a condition variable.
When this option is enabled, each thread may have its own scheduling properties.
Scheduling attributes may be specified:
- during thread creation
- by dynamically by changing the attributes of a thread already created
- by defining the effect of a mutex on the thread's scheduling when creating a mutex
- by dynamically changing the scheduling of a thread during synchronization operations.
The threads library provides default values that are sufficient for most cases.
- - register handlers to be called at fork(2) time
- [pthread_attr_init] - thread creation attributes
- [pthread_attr_init] - thread creation attributes
- [pthread_attr_init] - thread creation attributes
- [pthread_attr_init] - thread creation attributes
- [pthread_attr_init] - thread creation attributes
- [pthread_attr_init] - thread creation attributes
- - thread creation attributes
- [pthread_attr_init] - thread creation attributes
- [pthread_attr_init] - thread creation attributes
- [pthread_attr_init] - thread creation attributes
- [pthread_attr_init] - thread creation attributes
- [pthread_attr_init] - thread creation attributes
- - thread cancellation
- [pthread_cleanup_push] - install and remove cleanup handlers
- [pthread_cleanup_push] - install and remove cleanup handlers
- - install and remove cleanup handlers
- [pthread_cleanup_push] - install and remove cleanup handlers
- [pthread_condattr_init] - condition creation attributes
- - condition creation attributes
- [pthread_cond_init] - operations on conditions
- [pthread_cond_init] - operations on conditions
- - operations on conditions
- [pthread_cond_init] - operations on conditions
- [pthread_cond_init] - operations on conditions
- [pthread_cond_init] - operations on conditions
- - create a new thread
- - put a running thread in the detached state
- - compare two thread identifiers
- - terminate the calling thread
- [pthread_setschedparam] - control thread scheduling parameters
- [pthread_key_create] - management of thread-specific data
- - wait for termination of another thread
- - management of thread-specific data
- [pthread_key_create] - management of thread-specific data
- - terminate all threads in program except calling thread
- [pthread_sigmask] - handling of signals in threads
- [pthread_mutexattr_init] - mutex creation attributes
- [pthread_mutexattr_init] - mutex creation attributes
- - mutex creation attributes
- [pthread_mutexattr_init] - mutex creation attributes
- [pthread_mutex_init] - operations on mutexes
- - operations on mutexes
- [pthread_mutex_init] - operations on mutexes
- [pthread_mutex_init] - operations on mutexes
- [pthread_mutex_init] - operations on mutexes
- - once-only initialization
- - return identifier of current thread
- [pthread_cancel] - thread cancellation
- [pthread_cancel] - thread cancellation
- - control thread scheduling parameters
- [pthread_key_create] - management of thread-specific data
- - handling of signals in threads
- [pthread_cancel] - thread cancellation
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