I am working on Ubuntu 12.04.2 LTS. I have a strange problem with pthread_kill(). The following program ends after writing only "Create thread 0!" to standard output. The program ends with exit status 138.
If I uncomment "usleep(1000);" everything executes properly. Why would this happen?
#include <nslib.h>
void *testthread(void *arg);
int main() {
pthread_t tid[10];
int i;
for(i = 0; i < 10; ++i) {
printf("Create thread %d!\n", i);
Pthread_create(&tid[i], testthread, NULL);
//usleep(1000);
Pthread_kill(tid[i], SIGUSR1);
printf("Joining thread %d!\n", i);
Pthread_join(tid[i]);
printf("Joined %d!", i);
}
return 0;
}
void sighandlertest(int sig) {
printf("print\n");
pthread_exit();
//return NULL;
}
void* testthread(void *arg) {
struct sigaction saction;
memset(&saction, 0, sizeof(struct sigaction));
saction.sa_handler = &sighandlertest;
if(sigaction(SIGUSR1, &saction, NULL) != 0 ) {
fprintf(stderr, "Sigaction failed!\n");
}
printf("Starting while...\n");
while(true) {
}
return 0;
}
If the main thread does not sleep a bit before raising the SIGUSR1, the signal handler for the thread created most propably had not been set up, so the default action for receiving the signal applies, which is ending the process.
Using sleep()s to synchronise threads is not recommended as not guaranteed to be reliable. Use other mechanics here. A condition/mutex pair would be suitable.
Declare a global state variable int signalhandlersetup = 0, protect access to it by a mutex, create the thread, make the main thread wait using pthread_cond_wait(), let the created thread set up the signal handle for SIGUSR1, set signalhandlersetup = 0 and then signal the condition the main thread is waiting on using pthread_signal_cond(). Finally let the main thread call pthread_kill() as by your posting.
Related
i have a problem with the pthread library in a C-Application for Linux.
In my Application a Thread is started over and over again.
But I allways wait until the Thread is finished before starting it.
At some point the thread doesn't start anymore and I get an out of memory error.
The solution I found is to do a pthread_join after the thread has finished.
Can anyone tell me why the Thread doesn't end correctly?
Here is an Example Code, that causes the same Problem.
If the pthread_join isn't called the Process stops at about 380 calls of the Thread:
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <pthread.h>
#include <unistd.h>
volatile uint8_t check_p1 = 0;
uint32_t stack_start;
void *thread1(void *ch)
{
static int counter = 0;
int i;
int s[100000];
char stack_end;
srand(time(NULL) + counter);
for (i = 0; i < (sizeof (s)/sizeof(int)); i++) //do something
{
s[i] = rand();
}
counter++;
printf("Thread %i finished. Stacksize: %u\n", counter, ((uint32_t) (stack_start)-(uint32_t) (&stack_end)));
check_p1 = 1; // Mark Thread as finished
return 0;
}
int main(int argc, char *argv[])
{
pthread_t p1;
int counter = 0;
stack_start = (uint32_t)&counter; // save the Address of counter
while (1)
{
counter++;
check_p1 = 0;
printf("Start Thread %i\n", counter);
pthread_create(&p1, NULL, thread1, 0);
while (!check_p1) // wait until thread has finished
{
usleep(100);
}
usleep(1000); // wait a little bit to be really sure that the thread is finished
//pthread_join(p1,0); // crash without pthread_join
}
return 0;
}
The solution I found is to do a pthread_join after the thread has finished.
That is the correct solution. You must do that, or you leak thread resources.
Can anyone tell me why the Thread doesn't end correctly?
It does end correctly, but you must join it in order for the thread library to know: "yes, he is really done with this thread; no need to hold resources any longer".
This is exactly the same reason you must use wait (or waitpid, etc.) in this loop:
while (1) {
int status;
pid_t p = fork();
if (p == 0) exit(0); // child
// parent
wait(&status); // without this wait, you will run out of OS resources.
}
The following code is taken from this site and it shows how to use mutexes. It implements both pthread_join and pthread_mutex_lock:
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
void *functionC();
pthread_mutex_t mutex1 = PTHREAD_MUTEX_INITIALIZER;
int counter = 0;
main()
{
int rc1, rc2;
pthread_t thread1, thread2;
/* Create independent threads each of which will execute functionC */
if( (rc1=pthread_create( &thread1, NULL, &functionC, NULL)) )
{
printf("Thread creation failed: %d\n", rc1);
}
if( (rc2=pthread_create( &thread2, NULL, &functionC, NULL)) )
{
printf("Thread creation failed: %d\n", rc2);
}
/* Wait till threads are complete before main continues. Unless we */
/* wait we run the risk of executing an exit which will terminate */
/* the process and all threads before the threads have completed. */
pthread_join( thread1, NULL);
pthread_join( thread2, NULL);
exit(EXIT_SUCCESS);
}
void *functionC()
{
pthread_mutex_lock( &mutex1 );
counter++;
printf("Counter value: %d\n",counter);
pthread_mutex_unlock( &mutex1 );
}
I ran the code as given above as it is and it produced following result:
Counter value: 1
Counter value: 2
But in the second run i removed "pthread_mutex_lock( &mutex1 );" and "pthread_mutex_unlock( &mutex1 );" . I compiled and ran the code, it again produced the same result.
Now the thing that confuses me is why mutex lock is used in above code when same thing can be done without it (using pthread_join)? If pthread_join prevents another thread from running untill the first one has finished then i think it would already prevent the other thread from accessing the counter value. Whats the purpose of pthread_mutex_lock?
The join prevents the starting thread from running (and thus terminating the process) until thread1 and thread2 finish. It doesn't provide any synchronization between thread1 and thread2. The mutex prevents thread1 from reading the counter while thread2 is modifying it, or vice versa.
Without the mutex, the most obvious thing that could go wrong is that thread1 and thread2 run in perfect synch. They each read zero from the counter, each add one to it, and each output "Counter value: 1".
I've been creating programs exemplifying concurrency bugs using POSIX threads.
The overall question I have is what is the difference between the main() thread and one created by pthread_create(). My original understanding was that they are pretty much the same but, I'm getting different results from the two programs below.
To expand before showing the code I've written, what I am wondering is: Is there a difference between the following.
int main() {
...
pthread_create(&t1, NULL, worker, NULL);
pthread_create(&t2, NULL, worker, NULL);
...
}
and
int main() {
...
pthread_create(&t1, NULL, worker, NULL);
worker();
...
}
To expand using a full example program. I've made two versions of the same program. They both have the same function worker()
void *worker(void *arg) {
printf("Entered worker function\n");
int myid;
int data = 999;
pthread_mutex_lock(&gidLock);
myid = gid;
gid++;
printf("myid == %d\n", myid);
pthread_mutex_unlock(&gidLock);
if (myid == 0) {
printf("Sleeping since myid == 0\n");
sleep(1);
result = data;
printf("Result updated\n");
}
return NULL;
}
gid and data are globals initialized to 0.
What's the difference between the following main() functions
int main_1() {
pthread_t t1, t2;
int tmp;
/* initialize globals */
gid = 0;
result = 0;
pthread_create(&t1, NULL, worker, NULL);
pthread_create(&t2, NULL, worker, NULL);
pthread_join(t2, NULL);
printf("Parent thread exited worker function\n");
tmp = result;
printf("%d\n", tmp);
pthread_exit((void *) 0);
}
and
int main_2() {
pthread_t t1;
int tmp;
/* initialize globals */
gid = 0;
result = 0;
pthread_create(&t1, NULL, worker, NULL);
worker(NULL);
printf("Parent thread exited worker function\n");
tmp = result;
printf("%d\n", tmp);
pthread_exit((void *) 0);
}
Sample output for main_1()
Entered worker function
myid == 0
Sleeping since myid == 0
Entered worker function
myid == 1
Parent thread exited worker function
0
Result Updated
Sample output for main_2()
Entered worker function
myid == 0
Sleeping since myid == 0
Entered worker function
myid == 1
/* program waits here */
Result updated
Parent thread exited worker function
999
Edit: The program intentionally has a concurrency bug (an atomicity violation). Delays were added by calling sleep() to attempt to force the buggy interleaving to occur. The intention of the program is to be used to test software automatically detecting concurrency bugs.
I would think that main_1() and main_2() are essentially the same program which should result in the same interleaving when run on the same system (or largely the same interleaving; it is indeterminate, but running the same program on the same system only tend explores a small section of the potential scheduling and rarely deviates [1]).
The "desired" output is is that from main_1()
I'm not sure why the thread with myid == 1 stalls and does not return in main_2(). If I had to guess
Thanks for reading this far, and if anyone needs more information I'd be happy to oblige. Here are links to the full source code.
main_1(): https://gist.github.com/2942372
main_2(): https://gist.github.com/2942375
I've been compiling with gcc -pthread -lpthread
Thanks again.
[1] S. Park, S. Lu, Y. Zhou. "CTrigger: Exposing Atomicity Violation Bugs from Their Hiding Places"
I used Eclispse Indigo + CDT 8.0.2 + cygwin to develope a multi-thread systerm, the code is below:
pthread_mutex_t mutexCmd = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t signalCmd = PTHREAD_COND_INITIALIZER;
void * Func(void * arg)
{
int iStatus;
while (1)
{
int a = 1;
pthread_cleanup_push(pthread_mutex_unlock, &mutexCmd);
pthread_mutex_lock(&mutexCmd);
iStatus = pthread_cond_wait(&signalCmd, &mutexCmd);
if (iStatus) {
err_abort(iStatus, "signalCmd status error");
}
if(arg->Cmd != navi_Go) //Just a command tag;
{
pthread_mutex_unlock(&(pNaviCtrl->mutexCmd));
continue;
}
//do some work
//.....
pthread_mutex_unlock(&mutexCmd);
pthread_cleanup_pop(1);
}
//pthread_detach(pthread_self());
return NULL;
}
int main()
{
int iStatus = 0;
pthread = tid;
iStatus = pthread_create(&tid;NULL, Func, NULL);
if(iStatus)
{
err_abort(iStatus, "Start pthread error");
}
// do some work
...
//Cancel thread
void * retval;
iStatus = pthread_cancel(tid)
iStatus = pthread_join(tid; &retval);
if(iStatus){
err_abort(iStatus,"Stop thread error");
}
return iStatus;
}
where program run, it stop at "iStatus = pthread_join(tid1; &retval);" couldn't go forward anymore, I think the thread could be happed to deadlock, but can't find the reason. I supposed after call pthread_cancel(), the thread will exit and return to the pthread_join(),
who can tell me what's wrong with my code?
Don't put cleanup_push and _pop inside the while loop. Don't call them more than once. If you look at them, they are macros that wrap the code between them in { }. They setup a longjump that is used when you call pthread_cancel.
pthread_cleanup_pop(1) tells the pthread library to not only pop the cleanup entry off the stack, but to also execute it. So that call will also implicitly call:
pthread_mutex_unlock(&mutexCmd);
Since you've already unlocked the mutex, that call has undefined behavior (assuming the mutex type is PTHREAD_MUTEX_NORMAL). I imagine that call is just never returning or something.
Note that your code has other problems handing the cleanup - if you execute the continue for the loop, you'll call pthread_cleanup_push() a second time (or more), which will add another cleanup context.
There may be other problems (I'm not very familiar with pthread_cancel()).
My pthread_detach calls fail with a "Bad file descriptor" error. The calls are in the destructor for my class and look like this -
if(pthread_detach(get_sensors) != 0)
printf("\ndetach on get_sensors failed with error %m", errno);
if(pthread_detach(get_real_velocity) != 0)
printf("\ndetach on get_real_velocity failed with error %m", errno);
I have only ever dealt with this error when using sockets. What could be causing this to happen in a pthread_detach call that I should look for? Or is it likely something in the thread callback that could be causing it? Just in case, the callbacks look like this -
void* Robot::get_real_velocity_thread(void* threadid) {
Robot* r = (Robot*)threadid;
r->get_real_velocity_thread_i();
}
inline void Robot::get_real_velocity_thread_i() {
while(1) {
usleep(14500);
sensor_packet temp = get_sensor_value(REQUESTED_VELOCITY);
real_velocity = temp.values[0];
if(temp.values[1] != -1)
real_velocity += temp.values[1];
} //end while
}
/*Callback for get sensors thread*/
void* Robot::get_sensors_thread(void* threadid) {
Robot* r = (Robot*)threadid;
r->get_sensors_thread_i();
} //END GETSENSORS_THREAD
inline void Robot::get_sensors_thread_i() {
while(1) {
usleep(14500);
if(sensorsstreaming) {
unsigned char receive;
int read = 0;
read = connection.PollComport(port, &receive, sizeof(unsigned char));
if((int)receive == 19) {
read = connection.PollComport(port, &receive, sizeof(unsigned char));
unsigned char rest[54];
read = connection.PollComport(port, rest, 54);
/* ***SET SENSOR VALUES*** */
//bump + wheel drop
sensor_values[0] = (int)rest[1];
sensor_values[1] = -1;
//wall
sensor_values[2] = (int)rest[2];
sensor_values[3] = -1;
...
...
lots more setting just like the two above
} //end if header == 19
} //end if sensors streaming
} //end while
} //END GET_SENSORS_THREAD_I
Thank you for any help.
The pthread_* functions return an error code; they do not set errno. (Well, they may of course, but not in any way that is documented.)
Your code should print the value returned by pthread_detach and print that.
Single Unix Spec documents two return values for this function: ESRCH (no thread by that ID was found) and EINVAL (the thread is not joinable).
Detaching threads in the destructor of an object seems silly. Firstly, if they are going to be detached eventually, why not just create them that way?
If there is any risk that the threads can use the object that is being destroyed, they need to be stopped, not detached. I.e. you somehow indicate to the threads that they should shut down, and then wait for them to reach some safe place after which they will not touch the object any more. pthread_join is useful for this.
Also, it is a little late to be doing that from the destructor. A destructor should only be run when the thread executing it is the only thread with a reference to that object. If threads are still using the object, then you're destroying it from under them.