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

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);