Thread Life Cycle#

Thread Life Cycle#

  • pthread_create() creates new thread

  • Start function is called

  • Thread terminates

  • pthread_join() synchronizes with termination (fetches exit status)

No parent/child relationship ⟶ anybody can join

 ../../../../../../../_images/pthread-life-joinable-return.svg

Thread Creation#

man pthread_create#
int pthread_create(
    pthread_t *thread, const pthread_attr_t *attr,
    void *(*start_routine) (void *), void *arg);

  • thread: ID of the new thread (output parameter)

  • attr ⟶ see later (NULL ⟶ default attribute)

  • start_routine: thread start function, void*/void*

  • arg: parameter of the start function

Thread Termination (1)#

Thread termination alternatives:

  • Return from start function

  • pthread_exit() from somewhere inside the thread (cf. exit() from a process)

  • pthread_cancel() from outside (cf. kill())

  • exit() of the entire process ⟶ all contained threads are terminated

Danger

Don’t use pthread_cancel() unless you know what you are doing!

Thread Termination (2)#

Without any further ado: the manual …

man 3 pthread_exit#
void pthread_exit(void *retval);
man 3 pthread_cancel#
int pthread_cancel(pthread_t thread);

Exit Status, pthread_join()#

A thread’s “exit status”:

  • void*, just like the start parameter ⟶ more flexible than a process’s int.

  • Parameter to pthread_exit()

  • Return type of the start function

man 3 pthread_join#
int pthread_join(pthread_t thread, void **retval);

Detached Threads#

Sometimes one does not want to use pthread_join()

  • Rather, run a thread in the “background”.

  • “Detached” thread

  • ⟶ Thread attribute

man 3 pthread_attr_setdetachstate#
int pthread_attr_setdetachstate(
    pthread_attr_t *attr, int detachstate);
PTHREAD_CREATE_DETACHED
  Threads that are created using attr will be created in a
  detached state.

  • Detaching at runtime …

man 3 pthread_detach#
int pthread_detach(pthread_t thread);

Thread ID#

  • pthread_create() returns pthread_t to the caller

  • Thread ID of calling thread: pthread_self()

  • Compare using pthread_equal()

man 3 pthread_self#
pthread_t pthread_self(void);
man 3 pthread_equal#
int pthread_equal(pthread_t t1, pthread_t t2);

Scheduled Entities (1)#

Kernel maintains scheduled entities (Process IDs, “1:1 scheduling”)

Threads inside firefox#
$ ps -eLf|grep firefox
$ ls -1 /proc/30650/task/
13960
13961
... (many  more) ...

Scheduled Entities (2)#

Too bad:

  • Scheduled entity’s ID is not the same as pthread_t

  • Correlation of OS threads and POSIX thread is Linux specific

man 2 gettid#
pid_t gettid(void);