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}
void *server(void *arg) {
int k=(int)arg;
// perform service
return(0);
}
stack frame of server()
stack frame of start_servers()
stack frame of main()
Operating Systems – CSCI 402
Creating a POSIX Thread
…
arg
thread, i 0
…
argc, argv
start_servers( ) {
pthread_t thread;
int i;
for (i = 0; i < 100; i++) pthread_create(&thread, 0, server, (void*)i); every thread needs a separate stack first stack frame in every child thread corresponds to server() one arg in each of these stack frames 321 0 stack space 1 Copyright ý . Systems - CSCI 402 Creating a POSIX Thread ... arg arg 0 thread, i 1 ... argc, argv start_servers( ) { pthread_t thread; int i; for (i = 0; i < 100; i++) pthread_create(&thread, 0, server, (void*)i); } void *server(void *arg) { int k=(int)arg; // perform service return(0); } stack frame of server() stack frame of server() stack frame of start_servers() stack frame of main() every thread needs a separate stack first stack frame in every child thread corresponds to server() one arg in each of these stack frames 321 0 stack space 2 Copyright ý . Systems - CSCI 402 Creating a POSIX Thread ... arg arg thread, i ? ... argc, argv start_servers( ) { pthread_t thread; int i; for (i = 0; i < 100; i++) pthread_create(&thread, 0, server, &i); } void *server(void *arg) { int *iptr=(int*)arg; // perform service return(0); } stack frame of server() stack frame of server() every thread needs a separate stack first stack frame in every child thread corresponds to server() one arg in each of these stack frames 321 0 stack frame of start_servers() stack frame of main() stack space 3 Copyright ý . Systems - CSCI 402 Creating a POSIX Thread These are the same: keep thread handle in the stack pthread_t thread; pthread_create(&thread, ...); keep thread handle in the heap pthread_t *thread_ptr = (pthread_t*)malloc(sizeof(pthread_t)); pthread_create(thread_ptr, ...); need to make sure that eventually you will call the following to not leak memory free(thread_ptr); 4 321 0 Copyright ý . Cheng } DWORD WINAPI server(void *arg) { // perform service return(0); } Operating Systems - CSCI 402 Creating a Win32 Thread start_servers( ) { HANDLE thread; DWORD id; int i; for (i = 0; i < 100; i++) thread = CreateThread( 0, // security attributes 0, // default # of stack pages allocated server, // first procedure arg, // argument 0, // default attributes 0, // creation flags &id); // thread ID We won¡¯t talk about Win32 much 321 0 5 Copyright ý . Systems - CSCI 402 Complications rlogind(int r_in, int r_out, int l_in, int l_out) { pthread_t in_thread, out_thread; pthread_create(&in_thread, 0, incoming, r_in, l_out); // Cannot do this ... pthread_create(&out_thread, 0, outgoing, l_in, r_out); // Cannot do this ... /* How do we wait till they are done? */ } 6 321 0 Copyright ý . Systems - CSCI 402 Multiple Arguments typedef struct { int first, second; } two_ints_t; rlogind(int r_in, int r_out, int l_in, int l_out) { pthread_t in_thread, out_thread; two_ints_t in={r_in, l_out}, out={l_in, r_out}; pthread_create(&in_thread, 0, incoming, &in); ... /* How do we wait till they are done? */ } void *incoming(void *arg) { two_ints_t *p=(two_ints_t*)arg; ... p->first …
return NULL;
}
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7
Copyright ý . Systems – CSCI 402
Multiple Arguments
in out
…
typedef struct {
int first, second;
} two_ints_t;
rlogind()
main()
rlogind(int r_in, int r_out, int l_in, int l_out) {
pthread_t in_thread, out_thread;
two_ints_t in={r_in, l_out}, out={l_in, r_out};
pthread_create(&in_thread,
arg p
…
0,
incoming,
&in);
incoming()
/* How do we wait till they are done? */
}
void *incoming(void *arg) {
two_ints_t *p=(two_ints_t*)arg;
… p->first …
return NULL;
}
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Copyright ý . Cheng

ask yourself, “How can I be sure?”
if the answer is, “I hope it works”, then you need a different solution
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Operating Systems – CSCI 402
Multiple Arguments
Need to be careful how to pass argument to new thread when you call pthread_create()
passing address of a local variable (like the previous example) only works if we are certain the this storage doesn¡¯t go out of scope until the thread is done with it
passing address of a static or a global variable only works if we are certain that only one thread at a time is using the storage passing address of a dynamically allocated storage only works if we can free the storage when, and only when, the thread is finished with it
this would not be a problem if the language supports garbage collection
Memory corruption happens when memory is re-used unexpectedly
9
Copyright ý . Systems – CSCI 402
When Is The Child Thread Done?
In our example, what would happen if we return from rlogind() right after the child threads are created?
in out
…
typedef struct {
int first, second;
} two_ints_t;
rlogind()
main()
rlogind(int r_in, int r_out, int l_in, int l_out) {
pthread_t in_thread, out_thread;
two_ints_t in={r_in, l_out}, out={l_in, r_out};
pthread_create(&in_thread,
void *incoming(void *arg) {
two_ints_t *p=(two_ints_t*)arg;
… p->first …
return NULL;
}
321 0
0,
incoming,
&in);
incoming()
arg p
}
10
Copyright ý . Systems – CSCI 402
When Is The Child Thread Done?
In our example, what would happen if we return from rlogind() right after the child threads are created?
in out
…
typedef struct {
int first, second;
} two_ints_t;
rlogind()
main()
rlogind(int r_in, int r_out, int l_in, int l_out) {
pthread_t in_thread, out_thread;
two_ints_t in={r_in, l_out}, out={l_in, r_out};
pthread_create(&in_thread,
void *incoming(void *arg) {
two_ints_t *p=(two_ints_t*)arg;
… p->first …
return NULL;
}
321 0
0,
incoming,
&in);
incoming()
arg p
}
11
Copyright ý . Systems – CSCI 402
When Is The Child Thread Done?
To wait for a child thread to die, use pthread_join()
int pthread_join(thread_t thread,
(void **)ret_value);
rlogind(int r_in, int r_out, int l_in, int l_out) {
pthread_t in_thread, out_thread;
two_ints_t in={r_in, l_out}, out={l_in, r_out};
pthread_create(&in_thread, 0, incoming, &in);
pthread_create(&out_thread, 0, outgoing, &out);
/* if not interested in thread return values */
pthread_join(in_thread, 0);
pthread_join(out_thread, 0);
}
(void**) is the address of a variable of type (void*) pthread_join() is a blocking call
can only return if the specified thread has terminated
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Copyright ý . Systems – CSCI 402
Thread Termination
Thread return values
which threads receive these values how do they do it?
clearly, receiving thread must wait until the producer thread produced it, i.e., producer thread has terminated so we must have a way for one thread to wait for another thread to terminate
must have a way to say which thread you are waiting for
need a unique identifier tricky if it can be reused
int pthread_join(thread_t thread,
(void **)ret_value);
13
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Copyright ý . Systems – CSCI 402
result
…
Thread Termination
How does a thread self-terminate?
1) return from its “first procedure”
return a value of type (void*) 2) callpthread_exit(ret_value)
ret_value is of type (void*)
child() parent() main()
parent() {
pthread_t thread;
void *result=(void*)0;
pthread_create(&thread,
void *child(void *arg) {
…
if (terminate_now) {
pthread_exit((void*)1);
}
return((void*)2);
}
Thread Control Block
0, child, 0);
pthread_join(thread,
(void**)&result);
switch ((int)result) {
case 1: …
case 2: …
}
…
}
TID
Exit/Return Code
…
14
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Copyright ý . Termination
Difference between pthread_exit() and exit() pthread_exit() terminates only the calling thread exit() terminates the process, including all threads running in it
it will not wait for any thread to terminate what will this code do?
int main(int argc, char *argv[]) {
// create all the threads
return(0);
as a result, none of the created
child threads may get a chance to run
main() is called by a “startup routine”: exit(main(argc,argv))
main()
startup()
Operating Systems – CSCI 402
argc, argv
}
when main() returns, exit() will be called
15
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Copyright ý . Cheng

as it turns out, this special case is taken care of in the pthread library implemetation
You should use pthread_join() unless you are absolutely
sure what you are doing
321 0
Operating Systems – CSCI 402
Thread Termination
Difference between pthread_exit() and exit() pthread_exit() terminates only the calling thread exit() terminates the process, including all threads running in it
it will not wait for any thread to terminate what about this code?
int main(int argc, char *argv[]) {
// create all the threads
pthread_exit(0); // exit the main thread
return(0);
}
here, pthread_exit() will terminate the main thread, so exit() is never called
16
Copyright ý . Cheng

it also goes into a zombie state
all the thread related information is freed up, except for the thread ID, return code, and stack space
a running thread cannot delete its own stack!
What if two threads want to join with the same thread?
after the first thread joins, the thread ID and return code are freed up and the thread ID may get reused
so don¡¯t do this!
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Operating Systems – CSCI 402
Thread Termination
Calling pthread_exit() is the only way a thread can self-terminate (without affecting other threads)
Any thread can join with any other thread
there¡¯s no parent/child relationships among threads unlike process termination and wait()
What happens if a thread terminates and no other thread wants to join with this thread?
17
Copyright ý . Systems – CSCI 402
Detached Threads
What if you have a thread that you don¡¯t want any thread to join with it?
you can detach the thread after you have created it
start_servers( ) {
pthread_t thread;
int i;
for (i = 0; i < 100; i++) { pthread_create(&thread, 0, server, 0); pthread_detach(thread); } ... } server( ) { ... } 18 321 0 Copyright ý . Cheng pthread_create(&tid, 0, (void *(*)(void *))func, (void *)1); int func = 4; // func definition 1 void func(int i) { // func definition 2 ... } void *func(void *arg) { // func definition 3 int i = (int)arg; ... return(0); } Types a function is just an address (of something in the text/code segment) Operating Systems - CSCI 402 19 321 0 Copyright ý . Systems - CSCI 402 Thread Attributes pthread_t thread; pthread_attr_t thr_attr; pthread_attr_init(&thr_attr); /* establish some attributes */ ... pthread_create(&thread, &thr_attr, startroutine, arg); pthread_attr_destroy(&thr_attr); ... thread attribute only needs to be valid when a thread is created therefore, it can be destroyed as soon as the thread is created 20 321 0 Copyright ý . Systems - CSCI 402 Stack Size pthread_t thread; pthread_attr_t thr_attr; pthread_attr_init(&thr_attr); pthread_attr_setstacksize(&thr_attr, 20*1024*1024); ... pthread_create(&thread, &thr_attr, startroutine, arg); pthread_attr_destroy(&thr_attr); the above code set the stack size to 20MB the default stack size is not small default stack size is probably around 1MB in Solaris and 8MB in some Linux implementations if you need to create a lot of threads, you may want to have smaller stack size if you have very deep recursion in your code, you may want a bigger stack size 321 0 21 Copyright ý . Cheng ... // create the worker threads for (i=0; i
#include #include
#define M 3
#define N 4
#define P 5
int A[M][N];
int B[N][P];
int C[M][P];
void *matmult(void *arg) {
int row = (int)arg, col;
int i, t;
for (col=0; col < P; col++) { t = 0; for (i=0; i