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I am new to pthreads, and I am trying to understand it. I saw some examples like the following.
I could see that the main() is blocked by the API pthread_exit(), and I have seen examples where the main function is blocked by the API pthread_join(). I am not able to understand when to use what?
I am referring to the following site - https://computing.llnl.gov/tutorials/pthreads/. I am not able to get the concept of when to use pthread_join() and when to use pthread_exit().
Can somebody please explain? Also, a good tutorial link for pthreads will be appreciated.
#include <pthread.h>
#include <stdio.h>
#define NUM_THREADS 5
void *PrintHello(void *threadid)
{
long tid;
tid = (long)threadid;
printf("Hello World! It's me, thread #%ld!\n", tid);
pthread_exit(NULL);
}
int main (int argc, char *argv[])
{
pthread_t threads[NUM_THREADS];
int rc;
long t;
for(t=0; t<NUM_THREADS; t++){
printf("In main: creating thread %ld\n", t);
rc = pthread_create(&threads[t], NULL, PrintHello, (void *)t);
if (rc){
printf("ERROR; return code from pthread_create() is %d\n", rc);
exit(-1);
}
}
/* Last thing that main() should do */
pthread_exit(NULL);
Realized one more thing i.e.
pthread_cancel(thread);
pthread_join(thread, NULL);
Sometimes, you want to cancel the thread while it is executing.
You could do this using pthread_cancel(thread);.
However, remember that you need to enable pthread cancel support.
Also, a clean up code upon cancellation.
thread_cleanup_push(my_thread_cleanup_handler, resources);
pthread_setcancelstate(PTHREAD_CANCEL_ENABLE, 0);
static void my_thread_cleanup_handler(void *arg)
{
// free
// close, fclose
}
As explained in the openpub documentations,
pthread_exit() will exit the thread that calls it.
In your case since the main calls it, main thread will terminate whereas your spawned threads will continue to execute. This is mostly used in cases where the
main thread is only required to spawn threads and leave the threads to do their job
pthread_join
will suspend execution of the thread that has called it unless the target thread terminates
This is useful in cases when you want to wait for thread/s to terminate before further
processing in main thread.
pthread_exit terminates the calling thread while pthread_join suspends execution of calling thread until target threads completes execution.
They are pretty much well explained in detail in the open group documentation:
pthread_exit
pthread_join
Both methods ensure that your process doesn't end before all of your threads have ended.
The join method has your thread of the main function explicitly wait for all threads that are to be "joined".
The pthread_exit method terminates your main function and thread in a controlled way. main has the particularity that ending main otherwise would be terminating your whole process including all other threads.
For this to work, you have to be sure that none of your threads is using local variables that are declared inside them main function. The advantage of that method is that your main doesn't have to know all threads that have been started in your process, e.g because other threads have themselves created new threads that main doesn't know anything about.
The pthread_exit() API
as has been already remarked, is used for the calling thread termination.
After a call to that function a complicating clean up mechanism is started.
When it completes the thread is terminated.
The pthread_exit() API is also called implicitly when a call to the return() routine occurs in a thread created by pthread_create().
Actually, a call to return() and a call to pthread_exit() have the same impact, being called from a thread created by pthread_create().
It is very important to distinguish the initial thread, implicitly created when the main() function starts, and threads created by pthread_create().
A call to the return() routine from the main() function implicitly invokes the exit() system call and the entire process terminates.
No thread clean up mechanism is started.
A call to the pthread_exit() from the main() function causes the clean up mechanism to start and when it finishes its work the initial thread terminates.
What happens to the entire process (and to other threads) when pthread_exit() is called from the main() function depends on the PTHREAD implementation.
For example, on IBM OS/400 implementation the entire process is terminated, including other threads, when pthread_exit() is called from the main() function.
Other systems may behave differently.
On most modern Linux machines a call to pthread_exit() from the initial thread does not terminate the entire process until all threads termination.
Be careful using pthread_exit() from main(), if you want to write a portable application.
The pthread_join() API
is a convenient way to wait for a thread termination.
You may write your own function that waits for a thread termination, perhaps more suitable to your application, instead of using pthread_join().
For example, it can be a function based on waiting on conditional variables.
I would recommend for reading a book of David R. Butenhof “Programming with POSIX Threads”.
It explains the discussed topics (and more complicated things) very well (although some implementation details, such as pthread_exit usage in the main function, not always reflected in the book).
You don't need any calls to pthread_exit(3) in your particular code.
In general, the main thread should not call pthread_exit, but should often call pthread_join(3) to wait for some other thread to finish.
In your PrintHello function, you don't need to call pthread_exit because it is implicit after returning from it.
So your code should rather be:
void *PrintHello(void *threadid) {
long tid = (long)threadid;
printf("Hello World! It's me, thread #%ld!\n", tid);
return threadid;
}
int main (int argc, char *argv[]) {
pthread_t threads[NUM_THREADS];
int rc;
intptr_t t;
// create all the threads
for(t=0; t<NUM_THREADS; t++){
printf("In main: creating thread %ld\n", (long) t);
rc = pthread_create(&threads[t], NULL, PrintHello, (void *)t);
if (rc) { fprintf(stderr, "failed to create thread #%ld - %s\n",
(long)t, strerror(rc));
exit(EXIT_FAILURE);
};
}
pthread_yield(); // useful to give other threads more chance to run
// join all the threads
for(t=0; t<NUM_THREADS; t++){
printf("In main: joining thread #%ld\n", (long) t);
rc = pthread_join(&threads[t], NULL);
if (rc) { fprintf(stderr, "failed to join thread #%ld - %s\n",
(long)t, strerror(rc));
exit(EXIT_FAILURE);
}
}
}
pthread_exit() will terminate the calling thread and exit from that(but resources used by calling thread is not released to operating system if it is not detached from main thread.)
pthrade_join() will wait or block the calling thread until target thread is not terminated.
In simple word it will wait for to exit the target thread.
In your code, if you put sleep(or delay) in PrintHello function before pthread_exit(), then main thread may be exit and terminate full process, Although your PrintHello function is not completed it will terminate. If you use pthrade_join() function in main before calling pthread_exit() from main it will block main thread and wait to complete your calling thread (PrintHello).
Hmm.
POSIX pthread_exit description from http://pubs.opengroup.org/onlinepubs/009604599/functions/pthread_exit.html:
After a thread has terminated, the result of access to local (auto) variables of the thread is
undefined. Thus, references to local variables of the exiting thread should not be used for
the pthread_exit() value_ptr parameter value.
Which seems contrary to the idea that local main() thread variables will remain accessible.
Using pthread_exit in the main thread(in place of pthread_join), will leave the main thread in defunct(zombie) state. Since not using pthread_join, other joinable threads which are terminated will also remain in the zombie state and cause resource leakage.
Failure to join with a thread that is joinable (i.e., one that is
not detached), produces a "zombie thread". Avoid doing this, since
each zombie thread consumes some system resources, and when enough
zombie threads have accumulated, it will no longer be possible to
create new threads (or processes).
Another point is keeping the main thread in the defunct state, while other threads are running may cause implementation dependent issues in various conditions like if resources are allocated in main thread or variables which are local to the main thread are used in other threads.
Also, all the shared resources are released only when the process exits, it's not saving any resources. So, I think using pthread_exit in place of pthread_join should be avoided.
When pthread_exit() is called, the calling threads stack is no longer addressable as "active" memory for any other thread. The .data, .text and .bss parts of "static" memory allocations are still available to all other threads. Thus, if you need to pass some memory value into pthread_exit() for some other pthread_join() caller to see, it needs to be "available" for the thread calling pthread_join() to use. It should be allocated with malloc()/new, allocated on the pthread_join threads stack, 1) a stack value which the pthread_join caller passed to pthread_create or otherwise made available to the thread calling pthread_exit(), or 2) a static .bss allocated value.
It's vital to understand how memory is managed between a threads stack, and values store in .data/.bss memory sections which are used to store process wide values.
#include<stdio.h>
#include<pthread.h>
#include<semaphore.h>
sem_t st;
void *fun_t(void *arg);
void *fun_t(void *arg)
{
printf("Linux\n");
sem_post(&st);
//pthread_exit("Bye");
while(1);
pthread_exit("Bye");
}
int main()
{
pthread_t pt;
void *res_t;
if(pthread_create(&pt,NULL,fun_t,NULL) == -1)
perror("pthread_create");
if(sem_init(&st,0,0) != 0)
perror("sem_init");
if(sem_wait(&st) != 0)
perror("sem_wait");
printf("Sanoundry\n");
//Try commenting out join here.
if(pthread_join(pt,&res_t) == -1)
perror("pthread_join");
if(sem_destroy(&st) != 0)
perror("sem_destroy");
return 0;
}
Copy and paste this code on a gdb. Onlinegdb would work and see for yourself.
Make sure you understand once you have created a thread, the process run along with main together at the same time.
Without the join, main thread continue to run and return 0
With the join, main thread would be stuck in the while loop because it waits for the thread to be done executing.
With the join and delete the commented out pthread_exit, the thread will terminate before running the while loop and main would continue
Practical usage of pthread_exit can be used as an if conditions or case statements to ensure 1 version of some code runs before exiting.
void *fun_t(void *arg)
{
printf("Linux\n");
sem_post(&st);
if(2-1 == 1)
pthread_exit("Bye");
else
{
printf("We have a problem. Computer is bugged");
pthread_exit("Bye"); //This is redundant since the thread will exit at the end
//of scope. But there are instances where you have a bunch
//of else if here.
}
}
I would want to demonstrate how sometimes you would need to have a segment of code running first using semaphore in this example.
#include<stdio.h>
#include<pthread.h>
#include<semaphore.h>
sem_t st;
void* fun_t (void* arg)
{
printf("I'm thread\n");
sem_post(&st);
}
int main()
{
pthread_t pt;
pthread_create(&pt,NULL,fun_t,NULL);
sem_init(&st,0,0);
sem_wait(&st);
printf("before_thread\n");
pthread_join(pt,NULL);
printf("After_thread\n");
}
Noticed how fun_t is being ran after "before thread" The expected output if it is linear from top to bottom would be before thread, I'm thread, after thread. But under this circumstance, we block the main from running any further until the semaphore is released by func_t. The result can be verified with https://www.onlinegdb.com/
I am testing pthread_cancel() functionality with my test code in C. This is not the exact code I am using but a simplified form for explanation.
int main(argc, char *argvp[])
{
while(1) {
start_thread();
sleep(5);
end_thread();
}
}
void my_thread(void *arg)
{
printf("in my thread\n");
pthread_setcanceltype(PTHREAD_CANCEL_ASYNCHRONOUS, NULL);
return 0;
}
void start_thread()
{
pthread_create(&my_id, NULL, &my_thread, NULL);
}
void end_thread()
{
pthread_cancel(my_id);
}
If I do ps aux on my machine, I see that for my above process the RSS section is always increasing.But if I replace pthread_cancel() with pthread_join(), RSS for my process doesn't increase.
I am not allocating any resource in my thread so it seems like pthread_cancel() is not cleaning up some pthread related stuff behind the scenes?
I know pthread_cancel() is killing the thread because my thread count stays stable at 2 (checked with top ... main process + my_thread).
I looked over the pthread_cleanup_pop/push funcs but they look like they are used to clean user allocated memory.
Do I need to use them every time with pthread_cancel()?
Edit: RSS increases in the amount of 4kBytes every once loop.
Calling pthread_detach() after pthread_cancel() will free the resources which were held/used by pthread "itself.
Of-course the user must make sure on his own that his/her own allocated resources inside the now cancelled thread are also free'ed up.
I cant terminate the thread, it keeps sending things even after I close the terminal...
void *RTPfun(void * client_addr);
int main(int argc, char *argv[])
{
pthread_t RTPthread;
pthread_create(&RTPthread, NULL, &RTPfun, (void*)client_addr);
...
...
pthread_exit(&RTPfun);
return 0;
}
void *RTPfun(void * client_addr)
{
...
...
return 0;
}
Can someone tell me what am I doing wrong?
Thanks!
pthread_exit kills your current thread.
Notice, that if you kill the main thread as you do, it does not terminate the process. Other threads keep running.
You probably want to use pthread_cancel.
More generally though, killing threads is a bad idea. Correct way is to ask your threads politely to terminate and wait till they do.
If you call exit() from main, it will terminate main thread with all other thread.
If you call the method pthread_exit() from your main it will terminate main thread and let other thread will run continuously.
In your case you are calling pthread_exit() from main so your main thread get terminated, and other thread running until thread gets finish the job.
To cancel thread Add Below in RTPfun and add pthread_cancel in main.
/* call this when you are not ready to cancel the thread */
pthread_setcancelstate(PTHREAD_CANCEL_DISABLE, NULL);
/* call this when you are ready to cancel the thread */
pthread_setcancelstate(PTHREAD_CANCEL_ENABLE, NULL);
Working sample code:
#include <pthread.h>
#include <stdio.h>
#include <unistd.h>
void *RTPfun(void * client_addr);
int main(int argc, char *argv[])
{
pthread_t RTPthread;
int client_addr;
pthread_create(&RTPthread, NULL, &RTPfun, (void*)client_addr);
sleep(2);
pthread_cancel(RTPthread);
pthread_join(RTPthread, NULL);
return 0;
}
void *RTPfun(void * client_addr)
{
int count = 0;
pthread_setcancelstate(PTHREAD_CANCEL_DISABLE, NULL);
while(1) {
if(count > 10) {
printf("thread set for cancel\n");
pthread_setcancelstate(PTHREAD_CANCEL_ENABLE, NULL);
}
sleep(1);
printf("count:%d\n", count);
count ++;
}
return 0;
}
Used sleep in code for just understanding.
My understanding:
From "man pthread_exit" clearly talks about the description and rational behavior.
we will need to clean-up all respected resource that are been used in the thread created.
" The pthread_exit() function shall terminate the calling thread and make the value value_ptr available to any successful join with the terminating thread."
we shall pass "value_ptr" to exit(value_ptr) --> this will help to analyse what was the results of exit process.
obviously exit() - call exiting from the process, will release resources that used for.
In other way, you can create pthread in detached state, this attribute will reclaim the resources implicitly when pthread_exit . Refer "man pthread_detach".
We don't need to use pthread_join .. either go for pthread_detach again its simple go ahead set attribute .
/* set the thread detach state */
ret = pthread_attr_setdetachstate(&tattr,PTHREAD_CREATE_DETACHED);
Thanks.
Sankar
Your pthread_exit() exits the current thread. The parameter is used to pass a return value to any thread that then wants to join with it - and not to specify which thread to exit, as your code is implying.
The nicest way to do what you want is to use pthread_cancel() from your main thread. It takes the thread to cancel as a parameter, and then sends a cancellation request to that thread. Notice though, that by default cancellation is deferred, so your thread will keep on running until it hits a function that is a cancellation point - if you don't use any of those functions, you can insert an explicit cancellation point with a call to pthread_testcancel().
If you need to do some cleanup (for instance to free allocated memory, unlock mutexes, etc), you can insert a cleanup handler that is automatically called when canceling the thread. Have a look at pthread_cleanup_push() for this.
You can also set your thread to use asynchronous cancellation - your thread can then be canceled immediately, without hitting a cancellation point. However, asynchronous cancellation should only be used if you aren't using any system calls at all (that is, it's okay if you're purely doing calculations on already available data - but not if you're for instance using printf, file I/O, socket communication or similar), as otherwise you'll risk your system ending up in an inconsistent state.
After calling pthread_cancel(), your main thread should call pthread_join() on the canceled thread, to make sure all thread resources are cleaned up (unless you create the thread as detached).
You can of course also just have a shared doExit flag between the two threads, that the main thread can set, and which the other thread looks at from time to time. It's basically a manual way of using pthread_cancel().
I am new to pthreads, and I am trying to understand it. I saw some examples like the following.
I could see that the main() is blocked by the API pthread_exit(), and I have seen examples where the main function is blocked by the API pthread_join(). I am not able to understand when to use what?
I am referring to the following site - https://computing.llnl.gov/tutorials/pthreads/. I am not able to get the concept of when to use pthread_join() and when to use pthread_exit().
Can somebody please explain? Also, a good tutorial link for pthreads will be appreciated.
#include <pthread.h>
#include <stdio.h>
#define NUM_THREADS 5
void *PrintHello(void *threadid)
{
long tid;
tid = (long)threadid;
printf("Hello World! It's me, thread #%ld!\n", tid);
pthread_exit(NULL);
}
int main (int argc, char *argv[])
{
pthread_t threads[NUM_THREADS];
int rc;
long t;
for(t=0; t<NUM_THREADS; t++){
printf("In main: creating thread %ld\n", t);
rc = pthread_create(&threads[t], NULL, PrintHello, (void *)t);
if (rc){
printf("ERROR; return code from pthread_create() is %d\n", rc);
exit(-1);
}
}
/* Last thing that main() should do */
pthread_exit(NULL);
Realized one more thing i.e.
pthread_cancel(thread);
pthread_join(thread, NULL);
Sometimes, you want to cancel the thread while it is executing.
You could do this using pthread_cancel(thread);.
However, remember that you need to enable pthread cancel support.
Also, a clean up code upon cancellation.
thread_cleanup_push(my_thread_cleanup_handler, resources);
pthread_setcancelstate(PTHREAD_CANCEL_ENABLE, 0);
static void my_thread_cleanup_handler(void *arg)
{
// free
// close, fclose
}
As explained in the openpub documentations,
pthread_exit() will exit the thread that calls it.
In your case since the main calls it, main thread will terminate whereas your spawned threads will continue to execute. This is mostly used in cases where the
main thread is only required to spawn threads and leave the threads to do their job
pthread_join
will suspend execution of the thread that has called it unless the target thread terminates
This is useful in cases when you want to wait for thread/s to terminate before further
processing in main thread.
pthread_exit terminates the calling thread while pthread_join suspends execution of calling thread until target threads completes execution.
They are pretty much well explained in detail in the open group documentation:
pthread_exit
pthread_join
Both methods ensure that your process doesn't end before all of your threads have ended.
The join method has your thread of the main function explicitly wait for all threads that are to be "joined".
The pthread_exit method terminates your main function and thread in a controlled way. main has the particularity that ending main otherwise would be terminating your whole process including all other threads.
For this to work, you have to be sure that none of your threads is using local variables that are declared inside them main function. The advantage of that method is that your main doesn't have to know all threads that have been started in your process, e.g because other threads have themselves created new threads that main doesn't know anything about.
The pthread_exit() API
as has been already remarked, is used for the calling thread termination.
After a call to that function a complicating clean up mechanism is started.
When it completes the thread is terminated.
The pthread_exit() API is also called implicitly when a call to the return() routine occurs in a thread created by pthread_create().
Actually, a call to return() and a call to pthread_exit() have the same impact, being called from a thread created by pthread_create().
It is very important to distinguish the initial thread, implicitly created when the main() function starts, and threads created by pthread_create().
A call to the return() routine from the main() function implicitly invokes the exit() system call and the entire process terminates.
No thread clean up mechanism is started.
A call to the pthread_exit() from the main() function causes the clean up mechanism to start and when it finishes its work the initial thread terminates.
What happens to the entire process (and to other threads) when pthread_exit() is called from the main() function depends on the PTHREAD implementation.
For example, on IBM OS/400 implementation the entire process is terminated, including other threads, when pthread_exit() is called from the main() function.
Other systems may behave differently.
On most modern Linux machines a call to pthread_exit() from the initial thread does not terminate the entire process until all threads termination.
Be careful using pthread_exit() from main(), if you want to write a portable application.
The pthread_join() API
is a convenient way to wait for a thread termination.
You may write your own function that waits for a thread termination, perhaps more suitable to your application, instead of using pthread_join().
For example, it can be a function based on waiting on conditional variables.
I would recommend for reading a book of David R. Butenhof “Programming with POSIX Threads”.
It explains the discussed topics (and more complicated things) very well (although some implementation details, such as pthread_exit usage in the main function, not always reflected in the book).
You don't need any calls to pthread_exit(3) in your particular code.
In general, the main thread should not call pthread_exit, but should often call pthread_join(3) to wait for some other thread to finish.
In your PrintHello function, you don't need to call pthread_exit because it is implicit after returning from it.
So your code should rather be:
void *PrintHello(void *threadid) {
long tid = (long)threadid;
printf("Hello World! It's me, thread #%ld!\n", tid);
return threadid;
}
int main (int argc, char *argv[]) {
pthread_t threads[NUM_THREADS];
int rc;
intptr_t t;
// create all the threads
for(t=0; t<NUM_THREADS; t++){
printf("In main: creating thread %ld\n", (long) t);
rc = pthread_create(&threads[t], NULL, PrintHello, (void *)t);
if (rc) { fprintf(stderr, "failed to create thread #%ld - %s\n",
(long)t, strerror(rc));
exit(EXIT_FAILURE);
};
}
pthread_yield(); // useful to give other threads more chance to run
// join all the threads
for(t=0; t<NUM_THREADS; t++){
printf("In main: joining thread #%ld\n", (long) t);
rc = pthread_join(&threads[t], NULL);
if (rc) { fprintf(stderr, "failed to join thread #%ld - %s\n",
(long)t, strerror(rc));
exit(EXIT_FAILURE);
}
}
}
pthread_exit() will terminate the calling thread and exit from that(but resources used by calling thread is not released to operating system if it is not detached from main thread.)
pthrade_join() will wait or block the calling thread until target thread is not terminated.
In simple word it will wait for to exit the target thread.
In your code, if you put sleep(or delay) in PrintHello function before pthread_exit(), then main thread may be exit and terminate full process, Although your PrintHello function is not completed it will terminate. If you use pthrade_join() function in main before calling pthread_exit() from main it will block main thread and wait to complete your calling thread (PrintHello).
Hmm.
POSIX pthread_exit description from http://pubs.opengroup.org/onlinepubs/009604599/functions/pthread_exit.html:
After a thread has terminated, the result of access to local (auto) variables of the thread is
undefined. Thus, references to local variables of the exiting thread should not be used for
the pthread_exit() value_ptr parameter value.
Which seems contrary to the idea that local main() thread variables will remain accessible.
Using pthread_exit in the main thread(in place of pthread_join), will leave the main thread in defunct(zombie) state. Since not using pthread_join, other joinable threads which are terminated will also remain in the zombie state and cause resource leakage.
Failure to join with a thread that is joinable (i.e., one that is
not detached), produces a "zombie thread". Avoid doing this, since
each zombie thread consumes some system resources, and when enough
zombie threads have accumulated, it will no longer be possible to
create new threads (or processes).
Another point is keeping the main thread in the defunct state, while other threads are running may cause implementation dependent issues in various conditions like if resources are allocated in main thread or variables which are local to the main thread are used in other threads.
Also, all the shared resources are released only when the process exits, it's not saving any resources. So, I think using pthread_exit in place of pthread_join should be avoided.
When pthread_exit() is called, the calling threads stack is no longer addressable as "active" memory for any other thread. The .data, .text and .bss parts of "static" memory allocations are still available to all other threads. Thus, if you need to pass some memory value into pthread_exit() for some other pthread_join() caller to see, it needs to be "available" for the thread calling pthread_join() to use. It should be allocated with malloc()/new, allocated on the pthread_join threads stack, 1) a stack value which the pthread_join caller passed to pthread_create or otherwise made available to the thread calling pthread_exit(), or 2) a static .bss allocated value.
It's vital to understand how memory is managed between a threads stack, and values store in .data/.bss memory sections which are used to store process wide values.
#include<stdio.h>
#include<pthread.h>
#include<semaphore.h>
sem_t st;
void *fun_t(void *arg);
void *fun_t(void *arg)
{
printf("Linux\n");
sem_post(&st);
//pthread_exit("Bye");
while(1);
pthread_exit("Bye");
}
int main()
{
pthread_t pt;
void *res_t;
if(pthread_create(&pt,NULL,fun_t,NULL) == -1)
perror("pthread_create");
if(sem_init(&st,0,0) != 0)
perror("sem_init");
if(sem_wait(&st) != 0)
perror("sem_wait");
printf("Sanoundry\n");
//Try commenting out join here.
if(pthread_join(pt,&res_t) == -1)
perror("pthread_join");
if(sem_destroy(&st) != 0)
perror("sem_destroy");
return 0;
}
Copy and paste this code on a gdb. Onlinegdb would work and see for yourself.
Make sure you understand once you have created a thread, the process run along with main together at the same time.
Without the join, main thread continue to run and return 0
With the join, main thread would be stuck in the while loop because it waits for the thread to be done executing.
With the join and delete the commented out pthread_exit, the thread will terminate before running the while loop and main would continue
Practical usage of pthread_exit can be used as an if conditions or case statements to ensure 1 version of some code runs before exiting.
void *fun_t(void *arg)
{
printf("Linux\n");
sem_post(&st);
if(2-1 == 1)
pthread_exit("Bye");
else
{
printf("We have a problem. Computer is bugged");
pthread_exit("Bye"); //This is redundant since the thread will exit at the end
//of scope. But there are instances where you have a bunch
//of else if here.
}
}
I would want to demonstrate how sometimes you would need to have a segment of code running first using semaphore in this example.
#include<stdio.h>
#include<pthread.h>
#include<semaphore.h>
sem_t st;
void* fun_t (void* arg)
{
printf("I'm thread\n");
sem_post(&st);
}
int main()
{
pthread_t pt;
pthread_create(&pt,NULL,fun_t,NULL);
sem_init(&st,0,0);
sem_wait(&st);
printf("before_thread\n");
pthread_join(pt,NULL);
printf("After_thread\n");
}
Noticed how fun_t is being ran after "before thread" The expected output if it is linear from top to bottom would be before thread, I'm thread, after thread. But under this circumstance, we block the main from running any further until the semaphore is released by func_t. The result can be verified with https://www.onlinegdb.com/
gcc 4.4.3 c89 pthreads
I use valgrind for checking memory errors.
I am just wondering if there is any tool for linux that can detect running threads that haven't been terminated after the program finishes.
I am running a multi-thread application and need a tool to make sure all threads have finished.
Many thanks for any suggestions,
If the program has terminated (because the initial thread returned from main(), some thread called exit(), or a fatal signal was recieved by the process) then you are guaranteed that all threads have been terminated with extreme prejudice.
If you want to write your program so that it ensures that all its threads have exited before main() exits, then you need to loop over all your threads at the end of main(), calling pthread_join() on each one. (This also means that you shouldn't create your threads detached, or detach them).
A Tool Approach
You can use Valgrind to help with this (via it's Helgrind tool), but it requires minor modification of the code. For each thread, you make the thread lock a unique mutex when the thread is created, and release the mutex when the thread exits. Then, when run under Helgrind, you will get a warning if the thread hasn't exited when the program terminates because the thread will still be holding the lock to the mutex. Consider this example thread start routine:
void * thread_start (void *arg)
{
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
pthread_mutex_lock(&mutex);
// ...
// Here the thread does whatever it normally does
// ...
// Unlock the mutex before exiting
pthread_mutex_unlock(&mutex);
}
Simply run the program using Valgrind's Helgrind tool like so:
$ valgrind --tool=helgrind ./<program-name>
If the thread didn't exit when the program terminated, then Helgrind produces a warning like this:
==2203== Thread #2 was created
==2203== at 0x31C96D3CDE: clone (in /lib64/libc-2.5.so)
==2203== by 0x31CA206D87: pthread_create##GLIBC_2.2.5 (in /lib64/libpthread-2.5.so)
==2203== by 0x4A0B206: pthread_create_WRK (hg_intercepts.c:229)
==2203== by 0x4A0B2AD: pthread_create#* (hg_intercepts.c:256)
==2203== by 0x40060A: main (main.c:26)
==2203==
==2203== Thread #2: Exiting thread still holds 1 lock
==2203== at 0x4005DD: thread_start (main.c:13)
==2203== by 0x4A0B330: mythread_wrapper (hg_intercepts.c:201)
==2203== by 0x31CA20673C: start_thread (in /lib64/libpthread-2.5.so)
==2203== by 0x31C96D3D1C: clone (in /lib64/libc-2.5.so)
You will get false positives using this method if you don't add the mutex unlock code anywhere the thread may exit (e.g. using pthread_exit), but fixing such a false-positive is easy once it is identified.
An Alternative Approach (Recommended)
Having said all of the above, that's probably not the approach I myself would take. Instead, I would write the program such that it cannot terminate until all threads have exited. The simplest way to achieve this is to call pthread_exit from the main thread before returning from main. Doing so will mean that the process will stay alive so long as any other thread is still running.
If you take this approach, and the process doesn't quit when you expect it to, then you know that a thread is still running. You can then attach a debugger to the process to determine which threads are still running and what they are doing.
If you plan to use Boost.Threads library, then you can use the .join() method.
For example:
#include <boost/thread/thread.hpp>
#include <iostream>
void hello()
{
std::cout <<
"Hello world, I'm a thread!"
<< std::endl;
}
int main(int argc, char* argv[])
{
boost::thread thrd(&hello);
thrd.join();
return 0;
}
There is a simple trick in this similar question: Multiple threads in C program
If you call pthread_exit from main, your process will not exit until all the other threads complete.
Original answer was updated to address pthread_exit() scenario.
Assuming you want to tell whether all threads were pthread_join()-ed properly before you return from main(), there are a few ways:
Run it under the gdb and break on the last line of main(), then look at the output of "threads" command. There should only be main thread.
Make a shared library that overrides pthread_create with a wrapper that keeps a counter of how many threads are started. Thread wrapper increments a counter and calls the actual thread function, and a function registered with pthread_create_key() will decrement it when a thread returns or exits. Library destructor will check if the counter is zero, which means that all of them were terminated. Use it with your executable with LD_PRELOAD=checker.so ./your_executable (no code modification necessary).
Tested on Debian 5.0.5.
checker.c
#define _GNU_SOURCE
#include <pthread.h>
#include <stdio.h>
#include <dlfcn.h>
#include <stdlib.h>
/* thread-local storage key */
static pthread_key_t tls_key = 0;
static int counter = 0;
static pthread_mutex_t g_mutex;
/* TLS destructor prototype */
void on_thread_end(void*);
void __attribute__ ((constructor))
init_checker()
{
pthread_mutexattr_t attr;
pthread_mutexattr_init(&attr);
pthread_mutex_init(&g_mutex, &attr);
pthread_mutexattr_destroy(&attr);
pthread_key_create(&tls_key, &on_thread_end);
}
void __attribute__ ((destructor))
finalize_checker()
{
int remain;
pthread_mutex_lock(&g_mutex);
remain = counter;
pthread_mutex_unlock(&g_mutex);
pthread_mutex_destroy(&g_mutex);
if (remain)
fprintf(stderr, "Warning: %d threads not terminated\n", remain);
pthread_key_delete(tls_key);
}
/* thread function signature */
typedef void* (*ThreadFn)(void*);
struct wrapper_arg
{
ThreadFn fn;
void* arg;
};
/* TLS destructor: called for every thread we created
when it exits */
void
on_thread_end(void *arg)
{
free(arg);
pthread_mutex_lock(&g_mutex);
--counter;
pthread_mutex_unlock(&g_mutex);
}
static void*
thread_wrapper(void *arg)
{
void *ret;
struct wrapper_arg *warg;
warg = (struct wrapper_arg*)arg;
/* Thread started, increment count. */
pthread_mutex_lock(&g_mutex);
++counter;
pthread_mutex_unlock(&g_mutex);
/* set thread-specific data to avoid leaks
* when thread exits
*/
pthread_setspecific(tls_key, arg);
/* Run the actual function. */
ret = (*warg->fn)(warg->arg);
/* Thread finishes, TLS destructor will be called. */
return ret;
}
/* pthread_create signature */
typedef int (*CreateFn)(pthread_t*,const pthread_attr_t*,ThreadFn,void*);
/* Overriding phtread_create */
int
pthread_create(
pthread_t *thread,
const pthread_attr_t *attr,
ThreadFn start_routine,
void *arg)
{
CreateFn libc_pthread_create;
struct wrapper_arg *warg;
/* Get a handle to the real function. */
libc_pthread_create
= (CreateFn)dlsym(RTLD_NEXT, "pthread_create");
if (!libc_pthread_create)
return -1;
/* Wrap user function. */
warg = malloc(sizeof(struct wrapper_arg));
if (!warg)
return -1;
warg->fn = start_routine;
warg->arg = arg;
/* Create a thread with a wrapper. */
return libc_pthread_create(thread, attr, &thread_wrapper, warg);
}
Makefile
CFLAGS+=-fpic -O3
checker.so: checker.o
gcc -shared -Wl,-soname,$# -o $# $^ -ldl -lpthread
Correct me if wrong, but a program is not finished until all running threads have ended.
You don't need any external tool for this: i would track the threads by using a simple semaphore instead.
1) set it up so that its initial count is the same as the number of your threads:
sem_init( &semThreadCount, 0, threadCount );
2) Modify your threads to "notify" they are exiting gracefully:
sem_wait( &semThreadCount );
3) You can either quit whenever the threads are finished or when the semaphore is 0, or just print the remaining semaphore value and quit, that will be the number of still-running threads:
int v;
sem_getvalue( &semThreadCount, &v );
This way you can both ensure no threads are still running if your exit or, with some logging, being able to know which ones are still running after you quit.
Remember to sem_destroy the sempahore as well.
If you can not use C++ and therefore KMan's answer, then you can also join detached pthreads using the "C" API. (Joining means to wait for the detached threads to finish their work.)
See the pthread tutorial.
The existance of the process, that is if there is any thread still running, can be checked with waitpid.
If you just want your process to continue with all the threads, but you don't need the one of main anymore you can end that thread by pthread_exit. Other than an explicit exit or a simple return this will not terminate your other threads.
Such tools already exists. On Linux you can use ps or top. On Windows, good ole Task Manager does the job:. Just check whether your process still exists:
if the process still exists, it means that one or more threads in it are running.
if there are no more threads running, the process is terminated.
If they're threads (rather than processes) then you just need to check for your process stll running because threads run inside a process.
You can check if a process is running with ps -ef then pipe the result into grep to find your specific process.
If you want an external means to observe the threads in execution for your process, on Linux you can look in /proc/(pid)/task. That's the method tools like ps(1) or top(1) use.
See http://linux.die.net/man/5/proc
You're missing out on the important part:
A program cannot exit unless all its threads are terminated.
What you should do, however, is pthread_join() on all the threads before exiting. This ensures that all threads terminated, and will allow you to free() all their respective pthread_ts, so that you do not leak memory from them.
Have that said, valgrind can give you a comprehensive view on threads you haven't cleaned up after. Run it with --leakcheck=full and make sure you are not leaving various structs behind you. Those will indicate there is a thread you haven't completely terminated properly.