Session 4061 19 (3/27)
Today
Posix Threads Howto
Todays Objectives
Write code using the POSIX Thread API, including
Create threads Exit threads Join and detach threads Declare cleanup routines Use thread-local variables
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Admin
HW3 deadline: Thursday Read: Chapter 12
Creating a Thread
pthread_create(...) Analogue of the fork()/exec() combo The new thread is automatically put into runnable state Parameters:
Thread ID attributes (e.g. detach state, stack config, scheduling config) Routine to run A single argument thats passed to the new thread
Creating: thread_id
Threads have an identifier thats set upon calling pthread_create Other functions:
pthread_self(void) pthread_equal(pthread_t t1, pthread_t t2)
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Creating: start_routine and arg
What does the thread do? You pass it a pointer to a function to run.
void *thread_fn(void *arg) { ... }
Void *
both the return value and the arg are void * - cast them however you wish How do you pass or return multiple values?
The new function may or may not have started when pthread_create returns Can the argument be a local variable in the function that started the thread?
Terminating a Thread
When you return from start_routine, the thread exits. You can also exit explicitly
pthread_exit(void *return_value)
Return type of void *... but you cannot return an automatic variable Wait...where does the return value go?
We get it when we wait for (join) the thread Processes arent cleaned up until theyre waited on. Threads arent cleaned up until they are detached or joined
Joining
Joining a thread is analogous to waiting for a process
int pthread_join(pthread_t thread, void **value_ptr);
Note: there is no parent-child relationship. Threads can join freely. Threads can only be joined if they are not detached...
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Detaching
A detached thread cannot be joined, and will immediately go away when it exits
int pthread_detach(pthread_t thread);
Cancelling a Thread
Signals be can sent
pthread_kill(pthread_t thread)
But there is also a call that cancels threads:
pthread_cancel(pthread_t thread)
Sets a cancellation request Threads handle or ignore cancellation requests
using pthread_testcancel two functions (pthread_setcancelstate and pthread_setcanceltype) which control how cancel requests behave
Controlling Cancellation
pthread_setcancelstate
Enable or disable cancel handling
pthread_setcanceltype
Deferred (explicit) or asynchronous (immediate)
pthread_testcancel
(if enabled and deferred default) Creates a cancellation point Run cleanup routines and exit
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Cleanup Routines
When a thread exits, it runs cleanup routines, if any are defined
void pthread_cleanup_push(void (*routine)(void*), void *arg); void pthread_cleanup_pop(int execute);
Push installs handlers, pop removes them, and optionally executes them Its a stack...LIFO order of execution
Thread Safety
In a multi-threaded environment, all threads see the same globals
Functions that are safe for use in signal handlers (reentrant) are safe for multithreaded programming, too
What about other functions?
#define _REENTRANT to cause many library func...