I am writing a multi threads c++ program, follows a simple function I used for tests. If I comment out the sleep() there, the program will work. However if I put the sleep() in the loop, there will be no output on screen, it seems like the sleep() kills the whole execution. Please help. Will the sleep function kill execution of a pthread program? How can I sleep the threads?
void *shop(void *array){
int second = difftime( time(0), start );
// init shopping list with characters
string *info = (string *)array;
int size = stoi(info[0]);
string character = info[1];
string career = info[2];
if ( career == "Auror" ) {
int process = 3;
while (process < size+1) {
int shop = stoi(info[process]);
pthread_mutex_lock(&counter);
cout << process << endl;
sleep(1);
pthread_mutex_unlock(&counter);
process++;
}
}
The output without sleep is:
3
4
5
6
3
4
5
6
The output with sleep() is like:
3
3
It breaks the while loop.
Let the main thread wait for the child thread to finish up its task through pthread_join.
The pthread_join() function waits for the thread specified by thread to terminate. If that thread has already terminated, then pthread_join() returns immediately. The thread specified by thread must be joinable.
Yes, if the main thread dies, your app dies including all "Worker Threads".
Add a getchar() or a system("pause") or whatever in your main thread.
Related
using experimental/filesystem and std::async is there a way to ensure filewrites finish before exiting the main thread?
code used to call the async function
std::vector<std::future<void>> img_Futures;
for (int i = 0; i < parsedResult.size(); i++)
{
img_Futures.push_back(std::async(std::launch::async, downloadImage, parsedResult[i], baseFolderPath, i));
}
What ends up happening is it writes most of the files, but then there are some files that get left at 0 bytes because (I think) the main thread exits before they have time to write to them.
You can call std::future::wait() on each of the futures. This will block until the future's result is available, i.e. until your downloadImage() function has finished.
I'm trying to understand how to better use condition variables, and I have the following code.
Behavior.
The expected behavior of the code is that:
Each thread prints "thread n waiting"
The program waits until the user presses enter
When the user presses enter, notify_one is called once for each thread
All the threads print "thread n ready.", and exit
The observed behavior of the code is that:
Each thread prints "thread n waiting" (Expected)
The program waits until the user presses enter (Expected)
When the user presses enter, notify_one is called once for each thread (Expected)
One of the threads prints "thread n ready", but then the code hangs. (???)
Question.
Why does the code hang? And how can I have multiple threads wait on the same condition variable?
Code
#include <condition_variable>
#include <iostream>
#include <string>
#include <vector>
#include <thread>
int main() {
using namespace std::literals::string_literals;
auto m = std::mutex();
auto lock = std::unique_lock(m);
auto cv = std::condition_variable();
auto wait_then_print =[&](int id) {
return [&, id]() {
auto id_str = std::to_string(id);
std::cout << ("thread " + id_str + " waiting.\n");
cv.wait(lock);
// If I add this line in, the code gives me a system error:
// lock.unlock();
std::cout << ("thread " + id_str + " ready.\n");
};
};
auto threads = std::vector<std::thread>(16);
int counter = 0;
for(auto& t : threads)
t = std::thread(wait_then_print(counter++));
std::cout << "Press enter to continue.\n";
std::getchar();
for(int i = 0; i < counter; i++) {
cv.notify_one();
std::cout << "Notified one.\n";
}
for(auto& t : threads)
t.join();
}
Output
thread 1 waiting.
thread 0 waiting.
thread 2 waiting.
thread 3 waiting.
thread 4 waiting.
thread 5 waiting.
thread 6 waiting.
thread 7 waiting.
thread 8 waiting.
thread 9 waiting.
thread 11 waiting.
thread 10 waiting.
thread 12 waiting.
thread 13 waiting.
thread 14 waiting.
thread 15 waiting.
Press enter to continue.
Notified one.
Notified one.
thread 1 ready.
Notified one.
Notified one.
Notified one.
Notified one.
Notified one.
Notified one.
Notified one.
Notified one.
Notified one.
Notified one.
Notified one.
Notified one.
Notified one.
Notified one.
This is undefined behavior.
In order to wait on a condition variable, the condition variable must be waited on by the same exact thread that originally locked the mutex. You cannot lock the mutex in one execution thread, and then wait on the condition variable in another thread.
auto lock = std::unique_lock(m);
This lock is obtained in the main execution thread. Afterwards, the main execution thread creates all these multiple execution threads. Each one of these execution threads executes the following:
cv.wait(lock)
The mutex lock was not acquired by the execution thread that calls wait() here, therefore this is undefined behavior.
A more closer look at what you are attempting to do here suggests that you will likely get your intended results if you simply move
auto lock = std::unique_lock(m);
inside the lambda that gets executed by each new execution thread.
You also need to simply use notify_all() instead of calling notify_one() multiple times, due to various race conditions. Remember that wait() automatically unlocks the mutex and waits on the condition variable, and wait() returns only after the thread successfully relocked the mutex after being notified by the condition variable.
I want to write a program in which many functions run simultaneously. I already figured out that in order to do that I need to use threads.
In my program a routine of heating and rotating an object with different temperatures and velocities has to run for a determined period of time in a loop. Once the time has passed I want the process to continue with my next heating/rotating (...). My idea was to write a "timer thread" that is able to end the current routine in some way, and skip to the next one. Is this possible?
I suppose most ways to do this are going to involve a shared flag between the working thread and the thread that signals it to stop working.
So you might have something along these lines:
// Use a std::atomic_bool to signal the thread safely
void process_stuff(std::atomic_bool& stop_processing)
{
while(!stop_processing) // do we keep going?
{
// do some measurements or action
std::this_thread::sleep_for(std::chrono::milliseconds(1)); // wait for next action
}
}
Elsewhere in another thread ...
std::atomic_bool stop_processing{false};
// start thread (use std::ref() to pass by reference)
std::thread proc_thread(process_stuff, std::ref(stop_processing));
// wait for some time...
std::this_thread::sleep_for(std::chrono::seconds(3));
stop_processing = true; // signal end
proc_thread.join(); // wait for end to happen
// now create a new thread...
In the initiating thread, by changing the value of the variable stop_processing you signal the running thread to stop looping, in which case it ends gracefully.
Check this:
int main() {
// first thread
auto thread1 = std::make_unique<std::thread>([]() {
std::this_thread::sleep_for(std::chrono::seconds(1));
std::cout << "over\n";
});
// disposing to second thread
std::thread([thread2 = std::move(thread1)](){
thread2->join();
}).detach();
//spinning a new thread
thread1.reset(new std::thread([]() {
std::this_thread::sleep_for(std::chrono::seconds(1));
std::cout << "next over\n";
}));
thread1->join();
return 0;
}
I am trying to create a number of different threads that are required to wait for all of the threads to be created before they can perform any actions. This is a smaller part of a large program, I am just trying to take it in steps. As each thread is created it is immediately blocked by a semaphore. After all of the threads have been created, I loop through and release all the threads. I then wish each thread to print out its thread number to verify that they all waited. I only allow one thread to print at a time using another semaphore.
The issue I'm having is that although I create threads #1-10, a thread prints that it is #11. Also, a few threads say they have the same number as another one. Is the error in my passing the threadID or is it in my synchronization somehow?
Here is relevant code:
//Initialize semaphore to 0. Then each time a thread is spawned it will call
//semWait() making the value negative and blocking that thread. Once all of the
//threads are created, semSignal() will be called to release each of the threads
sem_init(&threadCreation,0,0);
sem_init(&tester,0,1);
//Spawn all of the opener threads, 1 for each valve
pthread_t threads[T_Valve_Numbers];
int check;
//Loop starts at 1 instead of the standard 0 so that numbering of valves
//is somewhat more logical.
for(int i =1; i <= T_Valve_Numbers;i++)
{
cout<<"Creating thread: "<<i<<endl;
check=pthread_create(&threads[i], NULL, Valve_Handler,(void*)&i);
if(check)
{
cout <<"Couldn't create thread "<<i<<" Error: "<<check<<endl;
exit(-1);
}
}
//Release all of the blocked threads now that they have all been created
for(int i =1; i<=T_Valve_Numbers;i++)
{
sem_post(&threadCreation);
}
//Make the main process wait for all the threads before terminating
for(int i =1; i<=T_Valve_Numbers;i++)
{
pthread_join(threads[i],NULL);
}
return 0;
}
void* Valve_Handler(void* threadNumArg)
{
int threadNum = *((int *)threadNumArg);
sem_wait(&threadCreation);//Blocks the thread until all are spawned
sem_wait(&tester);
cout<<"I'm thread "<<threadNum<<endl;
sem_post(&tester);
}
When T_Valve_Numbers = 10, some sample output is:
Creating thread: 1
Creating thread: 2
Creating thread: 3
Creating thread: 4
Creating thread: 5
Creating thread: 6
Creating thread: 7
Creating thread: 8
Creating thread: 9
Creating thread: 10
I'm thread 11 //Where is 11 coming from?
I'm thread 8
I'm thread 3
I'm thread 4
I'm thread 10
I'm thread 9
I'm thread 7
I'm thread 3
I'm thread 6
I'm thread 6 //How do I have 2 6's?
OR
Creating thread: 1
Creating thread: 2
Creating thread: 3
Creating thread: 4
Creating thread: 5
Creating thread: 6
Creating thread: 7
Creating thread: 8
Creating thread: 9
Creating thread: 10
I'm thread 11
I'm thread 8
I'm thread 8
I'm thread 4
I'm thread 4
I'm thread 8
I'm thread 10
I'm thread 3
I'm thread 9
I'm thread 8 //Now '8' showed up 3 times
"I'm thread..." is printing 10 times so it appears like my semaphore is letting all of the threads through. I'm just not sure why their thread number is messed up.
check=pthread_create(&threads[i], NULL, Valve_Handler,(void*)&i);
^^
You're passing the thread start function the address of i. i is changing all the time in the main loop, unsynchronized with the thread functions. You have no idea what the value of i will be once the thread function gets around to actually dereferencing that pointer.
Pass in an actual integer rather than a pointer to the local variable if that's the only thing you'll ever need to pass. Otherwise, create a simple struct with all the parameters, build an array of those (one for each thread) and pass each thread a pointer to its own element.
Example: (assuming your thread index never overflows an int)
#include <stdint.h> // for intptr_t
...
check = pthread_create(..., (void*)(intptr_t)i);
...
int threadNum = (intptr_t)threadNumArg;
Better/more flexible/doesn't require intprt_t that might not exist example:
struct thread_args {
int thread_index;
int thread_color;
// ...
}
// ...
struct thread_args args[T_Valve_Numbers];
for (int i=0; i<T_Valve_Numbers; i++) {
args[i].thread_index = i;
args[i].thread_color = ...;
}
// ...
check = pthread_create(..., &(args[i-1])); // or loop from 0, less surprising
A word of caution about this though: that array of thread arguments needs to stay alive at least as long as the threads will use it. In some situations, you might be better of with a dynamic allocation for each structure, passing that pointer (and its ownership) to the thread function (especially if you're going to detach the threads rather than joining them).
If you're going to join the threads at some point, keep those arguments around the same way you keep your pthread_t structures around. (And if you're creating and joining in the same function, the stack is usually fine for that.)
For example I want each thread to not start running until the previous one has completed, is there a flag, something like thread.isRunning()?
#include <iostream>
#include <vector>
#include <thread>
using namespace std;
void hello() {
cout << "thread id: " << this_thread::get_id() << endl;
}
int main() {
vector<thread> threads;
for (int i = 0; i < 5; ++i)
threads.push_back(thread(hello));
for (thread& thr : threads)
thr.join();
cin.get();
return 0;
}
I know that the threads are meant to run concurrently, but what if I want to control the order?
There is no thread.isRunning(). You need some synchronization primitive to do it.
Consider std::condition_variable for example.
One approachable way is to use std::async. With the current definition of std::async is that the associated state of an operation launched by std::async can cause the returned std::future's destructor to block until the operation is complete. This can limit composability and result in code that appears to run in parallel but in reality runs sequentially.
{
std::async(std::launch::async, []{ hello(); });
std::async(std::launch::async, []{ hello(); }); // does not run until hello() completes
}
If we need the second thread start to run after the first one is completed, is a thread really needed?
For solution I think try to set a global flag, the set the value in the first thread, and when start the second thread, check the flag first should work.
You can't simply control the order like saying "First, thread 1, then thread 2,..." you will need to make use of synchronization (i.e. std::mutex and condition-variables std::condition_variable_any).
You can create events so as to block one thread until a certain event happend.
See cppreference for an overview of threading-mechanisms in C++-11.
You will need to use semaphore or lock.
If you initialize semaphore to value 0:
Call wait after thread.start() and call signal/ release in the end of thread execution function (e.g. run funcition in java, OnExit function etc...)
So the main thread will keep waiting until the thread in loop has completed its execution.
Task-based parallelism can achieve this, but C++ does not currently offer task model as part of it's threading libraries. If you have TBB or PPL you can use their task-based facilities.
I think you can achieve this by using std::mutex and std::condition_variable from C++11. To be able to run threads sequentially array of booleans in used, when thread is done doing some work it writes true in specific index of the array.
For example:
mutex mtx;
condition_variable cv;
int ids[10] = { false };
void shared_method(int id) {
unique_lock<mutex> lock(mtx);
if (id != 0) {
while (!ids[id - 1]) {
cv.wait(lock);
}
}
int delay = rand() % 4;
cout << "Thread " << id << " will finish in " << delay << " seconds." << endl;
this_thread::sleep_for(chrono::seconds(delay));
ids[id] = true;
cv.notify_all();
}
void test_condition_variable() {
thread threads[10];
for (int i = 0; i < 10; ++i) {
threads[i] = thread(shared_method, i);
}
for (thread &t : threads) {
t.join();
}
}
Output:
Thread 0 will finish in 3 seconds.
Thread 1 will finish in 1 seconds.
Thread 2 will finish in 1 seconds.
Thread 3 will finish in 2 seconds.
Thread 4 will finish in 2 seconds.
Thread 5 will finish in 0 seconds.
Thread 6 will finish in 0 seconds.
Thread 7 will finish in 2 seconds.
Thread 8 will finish in 3 seconds.
Thread 9 will finish in 1 seconds.