More on pthread_create( )

  • pthread_create( ): 

      int pthread_create(pthread_t *threadID, const pthread_attr_t *attr,
                     void *(*start_routine) (void *), void *arg);

  Effect:  create a new thread that starts executing 
           in the function start_routine

  Return value:   0 --> success
                ≠ 0 --> error code

  Parameters:

    threadID        contains the ID of the new thread upon return
    attr            specify the attributes for the new thread (later)
    start_routine   start function for the new thread
    arg             parameter for the start_routine (later)

  • Typical way to used pthread_create( ): 

        if ( pthread_create(&tid, &attr, startFunction, &x) != 0 )
    {
        perror("pthread_create"); // Print cause of error
        exit(1);
    }

Specifying thread attributes when a thread is created

  • The attributes for a newly created thread can be speficied using a variable of the type pthread_attr_t: 

       pthread_attr_t    attr;

  • You must first reset all the thread properties to their default values using the function pthread_attr_init(): 

       pthread_attr_init( &attr );  // Pass the var attr by reference

  • You can create a thread with the default attribute values with: 

         pthread_attr_t    attr;  
     pthread_attr_init( &attr );
     pthread_create(&tid, &attr, worker, NULL);

 or:
     pthread_create(&tid, NULL, worker, NULL);

Attributes of threads

  • Pthreads can be created with different attributes

  • Attibutes of a thread that can be changed:

    Attribute Default value Meaning
    contenstionscope PTHREAD_SCOPE_PROCESS Thread competes for resources within the process
    detachstate PTHREAD_CREATE_JOINABLE Thread is joinable by other threads
    stackaddr NULL Stack used by thread is allocated by the OS
    stacksize NULL Use default size (8 MB) determined by the OS
    priority 0 Priority to run (lower value is a higher priority)
    schedulingpolicy SCHED_OTHER The scheduling policy is determined by the system
    inheritsched PTHREAD_EXPLICIT_SCHED scheduling policy and parameters are not inherited but is explicitly defined by the attribute object
    guardsize PAGESIZE size of guard area for a thread's stack (this area will help determine if the thread has exceeded the total amount of stack space that was allocated for the thread)

  • Comment:

    • You mostly want to use the default value

    • You may want to change the stacksize if the thread executes a recursive function....

Changing some attributes of a thread

  • After initializing the thread attribute object attr with the default values, you can change some attribute value using:

    • pthread_attr_setscope(): change the contentionscope
    • pthread_attr_setdetachstate(): change the detachstate
    • pthread_attr_setstackaddr(): change the stack location
    • pthread_attr_setstacksize(): change the stack size
    • .. etc

  • For more information:

    • Use this command to find the pthread_attr_set...( ) functions:

      • man pthread_attr_init

    • Use this command to find the details on how to use a specific function pthread_attr_setFunName:

      • man pthread_attr_setFunName

How to change the stacksize of a thread

  • Default stack size = 8 M Bytes (see demo program)

  • When to use a larger stack:

    • Function executed by a thread contains an array variable

    • Function executed by a thread is recursive

  • How to change the stack size in attr: 

      pthread_attr_t attr;      

  pthread_attr_init(&attr);                   // Init to default values

  pthread_attr_setstacksize(&attr, 10000000); // New size = 10000000 bytes

  • Create the thread with the new stack size: 

        pthread_create(&tid, &attr, worker, NULL)...

DEMO: demo/pthread/stack-size.c