[PDF] Threads: POSIX API Pthreads PDF 14-Threads

POSIX and threads ▫ Early on ○ Each OS had its own thread library/API ○ Difficult to write multithreaded programs ▫ Learn a new API with each new OS

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Copyright ©: University of Illinois CS 241 Staff

Threads: POSIX API 'Pthreads'

Creating a Thread

n When a new thread is created it runs concurrently with the creating thread. n When creating a thread you indicate which function the thread should execute. Copyright ©: University of Illinois CS 241 Staff

Compare: Normal function call (one thread)

Compare: Threaded function call

Creating program Created thread

processfd(); processfd() { }

Calling program Called function

Thread creation Thread of execution

Threads vs. Processes

n Process ¡ fork is expensive (time & memory) ¡ each process has its own virtual addr. space n Thread

¡ Lightweight process ¡ Shared virtual address space ¡ Does not require lots of memory or startup time

Design choices: Processes versus Threads

Thread-Specific Resources

n Each thread has its own ¡ pthread_t identifier ¡ Stack, Registers state,

Program Counter

n Threads within the same

process can communicate using shared memory ¡ Must be done carefully! ¡ Virtual memory is shared

Process and Threads

n Each process can include many threads n All threads of a process share:

¡ Process ID ¡ Virtual Memory (program code and global data) ¡ Open file/socket descriptors ¡ Semaphores ¡ Signal handlers ¡ Working environment (current directory, user ID,

Threads and address space

From: https://computing.llnl.gov/tutorials/pthreads/images/thread.gif

Process Creation vs. Thread Creation

n http://www.llnl.gov/computing/tutorials/pthreads.

n Timings reflect 50,000 process/thread. n Creations, were performed with the time utility, and units are

Platform fork() pthread_create() real user sys real user sys AMD 2.3 GHz Opteron (16 cpus) 12.5 1.0 12.5 1.2 0.2 1.3 AMD 2.4 GHz Opteron (8 cpus) 17.6 2.2 15.7 1.4 0.3 1.3 IBM 4.0 GHz POWER6 (8 cpus) 9.5 0.6 8.8 1.6 0.1 0.4 IBM 1.9 GHz POWER5 p5-575 (8 cpus) 64.2 30.7 27.6 1.7 0.6 1.1 IBM 1.5 GHz POWER4 (8 cpus) 104.5 48.6 47.2 2.1 1.0 1.5 INTEL 2.4 GHz Xeon (2 cpus) 54.9 1.5 20.8 1.6 0.7 0.9 INTEL 1.4 GHz Itanium2 (4 cpus) 54.5 1.1 22.2 2.0 1.2 0.6

POSIX and threads

n Early on ¡ Each OS had its own thread library/API ¡ Difficult to write multithreaded programs n Learn a new API with each new OS n Modify code with each port to a new OS n So

¡ POSIX (IEEE 1003.1c-1995) provided a

Pthread Operations

POSIX function description SWKUHDGBFUHDWH create a thread SWKUHDGBGHWDFK set thread to release resources SWKUHDGBHTXDO test two thread IDs for equality SWKUHDGBH[LW exit a thread without exiting process SWKUHDGBNLOO send a signal to a thread SWKUHDGBMRLQ wait for a thread SWKUHDGBVHOI find out own thread ID

Creating a Thread

int pthread_create (pthread_t* tid, pthread_attr_t* attr, void*(child_main)(void*), void* arg); n creates a new posix thread n Parameters:

¡ tid:

n Unique thread identifier returned from call

¡ attr:

n Attributes structure used to define new thread n Use NULL for default values

¡ child_main:

n Main routine for child thread n Takes a pointer (void*), returns a pointer (void*)

¡ arg:

Creating a Thread

n pthread_create() takes a pointer to a function as one of its arguments ¡ child_main is called with the argument specified by arg ¡ child_main can only have one parameter of type void * ¡ Complex parameters can be passed by creating a structure

and passing the address of the structure

¡ The structure can't be a local variable ¡ By default, a new thread is created in a joinable state

Exiting a thread

n Question:

¡ If a thread calls exit(), what about other

Exiting a thread

n Question: ¡ If a thread calls exit(), what about other threads in the same process? n A multithreaded process terminates when:

¡ one of its threads calls exit ¡ it returns from main() ¡ it receives a termination signal ¡ all threads have called pthread_exit

Terminating Threads: pthread_exit()

void pthread_exit(void * retval); n Terminate the calling thread n Makes the value retval available to any successful join with the terminating thread n Returns

¡ pthread_exit() cannot return to its caller

n Parameters

¡ retval:

n Pointer to data returned to joining thread n Pass a pointer to heap not to the stack n Note ¡ If main() exits by calling pthread_exit() before its threads, the other threads continue to execute. Otherwise, they will be terminated when main() finishes. Copyright ©: University of Illinois CS 241 Staff

Detaching Threads: pthread_detach()

int pthread_detach(pthread_t thread);

n Thread resources can be reclaimed on termination n Return results of a detached thread are unneeded

n Returns ¡ 0 on success ¡ Error code on failure n Parameters

¡ thread:

n Target thread identifier n Notes ¡ pthread_detach() can be used to explicitly detach a thread even though it was created as joinable

¡ There is no converse routine

Detached Threads

Master Thread Worker Thread Worker Thread

pthread_create() pthread_exit()

Worker Thread

pthread_exit() pthread_exit()

Waiting for Threads: pthread_join()

int pthread_join(pthread_t thread, void** retval); n Suspends execution of the calling thread until the target thread terminates, unless the target thread has already terminated. n Returns ¡ 0 on success ¡ Error code on failure n Parameters

¡ thread:

n Target thread identifier

¡ retval:

Joined Threads

Master Thread Worker Thread

pthread_create() pthread_join() pthread_exit() suspends calling thread, retrieves void* retval

Example 1

int x = 0;

char *p; void *thread(void *th){ x = x + 10; strcat(p, "Hello from thread!"); printf("thread: my x is %d. Bye from thread!\n", x); pthread_exit((void *) p+5); } int main() { pthread_t tid; char *p_char; p_char = p = malloc(25 * sizeof(char)); // data allocated on heap strcpy (p, "main-thread:"); pthread_create(&tid, NULL, thread, NULL); pthread_join(tid, (void **) &p_char); printf("%s\n", p_char); printf("main: my x is %d; Bye from main!\n", x); }

Example 2

int x = 0;

What is the output?

Valid outputs for example 2 (non-deterministic)

Output #1

--------------------------------------- main-thread:Hello from thread! thread: my x is 10. Bye from thread! main: my x is 10; Bye from main!

Output #2

--------------------------------------- main-thread: thread: my x is 10. Bye from thread! main: my x is 10; Bye from main!

Output #3

--------------------------------------- main-thread: main: my x is 10; Bye from main! Copyright ©: University of Illinois CS 241 Staff pthread Error Handling n pthread functions do not follow the usual Unix conventions

¡ Similarity

n Returns 0 on success

¡ Differences

n Returns error code on failure n Does not set errno

¡ What about errno?

Threads vs processes

n Threads are similar to concurrent processes ¡ Pros: thread creation is faster; data sharing among threads is fast and easy ¡ Cons: application is less robust; data sharing requires synchronization to avoid race conditions n If a thread misbehaves, it can corrupt data of other threads within same process n If a thread crashes, the entire process crashes Copyright ©: University of Illinois CS 241 Staff

Threads vs. Processes

Property Processes created with fork Threads of a process Ordinary function calls variables Get copies of all variables Share global variables Share global variables IDs Get new process IDs Share the same process ID but have unique thread ID Share the same process ID (and thread ID) Data/control Must communicate explicitly, e.g., use pipes, shared memory, msg. passing. May communicate with return value or carefully shared variables May communicate with return value or shared variables Parallelism (one CPU) Concurrent Concurrent Sequential Parallelism (multiple CPUs) May be executed simultaneously May be executed simultaneously Sequential

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[PDF] [PDF] Threads: POSIX API Pthreads

Threads: POSIX API 'Pthreads'

Creating a Thread

Compare: Normal function call (one thread)

Compare: Threaded function call

Creating program Created thread

Calling program Called function

Thread creation Thread of execution

Threads vs. Processes

Design choices: Processes versus Threads

Thread-Specific Resources

Program Counter

Process and Threads

Threads and address space

Process Creation vs. Thread Creation

POSIX and threads

¡ POSIX (IEEE 1003.1c-1995) provided a

Pthread Operations

Creating a Thread

¡ tid:

¡ attr:

¡ child_main:

¡ arg:

Creating a Thread

Exiting a thread

¡ If a thread calls exit(), what about other

Exiting a thread

Terminating Threads: pthread_exit()

¡ pthread_exit() cannot return to its caller

¡ retval:

Detaching Threads: pthread_detach()

¡ thread:

¡ There is no converse routine

Detached Threads

Master Thread Worker Thread Worker Thread

Worker Thread

Waiting for Threads: pthread_join()

¡ thread:

¡ retval:

Joined Threads

Master Thread Worker Thread

Example 1

Example 2

What is the output?

Valid outputs for example 2 (non-deterministic)

Output #1

Output #2

Output #3

¡ Similarity

¡ Differences

¡ What about errno?

Threads vs processes

Threads vs. Processes