I'm new to programming/C++ and I'm experimenting with simple multithreading. I have tried the following codes:
Example 1
#include <iostream>
#include <thread>
void printFunc() {
while(1) {
std::cout << "threadOne Running..." << std::endl;
}
}
int main() {
std::thread threadOne(printFunc);
threadOne.detach();
while(1) {
std::cout << "main running..." << std::endl;
}
return 0;
}
Example 2
#include <iostream>
#include <thread>
void printFunc() {
while(1) {
std::cout << "threadOne running..." << std::endl;
}
}
void initThread() {
std::thread threadOne(printFunc);
threadOne.detach();
}
int main() {
initThread();
while(1) {
std::cout << "main running..." << std::endl;
}
return 0;
}
When I run example 1 using Visual Studio in debug & release mode, it prints "main running..." most of the time and prints "threadOne running..." once in a while. But when I run example 2, it prints both of them (jumps between two prints "equally").
Edit:
Execution of example 1
Execution of example 2
Possible reason for what you're seeing;
Because you did not specify which version of C++ you're using, I'll assume its C++11;
As per Is cout thread-safe
Concurrent access to a synchronized (§27.5.3.4) standard iostream object’s formatted and unformatted input (§27.7.2.1) and output (§27.7.3.1) functions or a standard C stream by multiple threads shall not result in a data race (§1.10). [ Note: Users must still synchronize concurrent use of these objects and streams by multiple threads if they wish to avoid interleaved characters. — end note ]
Meaning that you still have to synchronize both cout streams.
One way of doing that would be to wrap cout in your own class and assign it a mutex.
Related
I'm trying to switch an application over from using boost::interprocess::named_mutex to boost::interprocess::file_lock for interprocess synchronization, but when I did so I noticed that my condition variables were never being woken up.
I've created two examples that demonstrate the types of changes I made and the issues I'm seeing. In both examples the same application should periodically send notifications if invoked with any arguments, or wait for notifications if invoked with no arguments
Originally my application used name_mutex and named_condition. The below example using name_mutex and named_condition works as expected: every time the "sender" application prints out "Notifying" the "receiver" application prints out "Notified!" (provided I manually clean out /dev/shm/ between runs).
#include <iostream>
#include <boost/interprocess/sync/scoped_lock.hpp>
#include <boost/interprocess/sync/named_condition.hpp>
#include <boost/interprocess/sync/named_mutex.hpp>
#include <boost/thread.hpp>
int main(int argc, char** argv)
{
boost::interprocess::named_mutex mutex(boost::interprocess::open_or_create,
"mutex");
// Create condition variable
boost::interprocess::named_condition cond(boost::interprocess::open_or_create, "cond");
while(true)
{
if(argc > 1)
{// Sender
std::cout << "Notifying" << std::endl;
cond.notify_all();
boost::this_thread::sleep_for(boost::chrono::seconds(1));
}
else
{// Receiver
std::cout << "Acquiring lock..." << std::endl;
boost::interprocess::scoped_lock<boost::interprocess::named_mutex> lock(mutex);
std::cout << "Locked. Waiting for notification..." << std::endl;
cond.wait(lock);
std::cout << "Notified!" << std::endl;
}
}
return 0;
}
The following code represents my attempt to change the working code above from using name_mutex and named_condition to using file_lock and named_condition_any
#include <iostream>
#include <boost/interprocess/sync/scoped_lock.hpp>
#include <boost/interprocess/sync/named_condition_any.hpp>
#include <boost/interprocess/sync/file_lock.hpp>
#include <boost/thread.hpp>
int main(int argc, char** argv)
{
// Second option for locking
boost::interprocess::file_lock flock("/tmp/flock");
// Create condition variable
boost::interprocess::named_condition_any cond(boost::interprocess::open_or_create,
"cond_any");
while(true)
{
if(argc > 1)
{// Sender
std::cout << "Notifying" << std::endl;
cond.notify_all();
boost::this_thread::sleep_for(boost::chrono::seconds(1));
}
else
{// Receiver
std::cout << "Acquiring lock..." << std::endl;
boost::interprocess::scoped_lock<boost::interprocess::file_lock> lock(flock);
std::cout << "Locked. Waiting for notification..." << std::endl;
cond.wait(lock);
std::cout << "Notified!" << std::endl;
}
}
return 0;
}
However I can't seem to get the "receiver" application to wake up when notified. The "sender" happily prints "Notifying" at ~1Hz, but the "receiver" hangs after printing "Locked. Waiting for notification..." once.
What am I doing wrong with my file_lock/named_condition_any implementation?
This appears to be caused by a bug in the implementation of boost::interprocess::named_condition_any.
boost::interprocess::named_condition_any is implemented using an instance of boost::interprocess::ipcdetail::shm_named_condition_any. boost::interprocess::ipcdetail::shm_named_condition_any has all of the member variables associated with its implementation aggregated into a class called internal_condition_members. When shm_named_condition_any is constructed it either creates or opens shared memory. If it creates the shared memory it also instantiates an internal_condition_members object in that shared memory.
The problem is that shm_named_condition_any also maintains a "local" (i.e. just on the stack, not in shared memory) member instance of an internal_condition_members object, and its wait, timed_wait, notify_one, and notify_all functions are all implemented using the local internal_condition_members member instead of the internal_condition_members from shared memory.
I was able to get the expected behavior from my example by editing boost/interprocess/sync/shm/named_condition_any.hpp and changing the implementation of the shm_named_condition_any class as follows:
typedef ipcdetail::condition_any_wrapper<internal_condition_members> internal_condition;
internal_condition m_cond;
to
typedef ipcdetail::condition_any_wrapper<internal_condition_members> internal_condition;
internal_condition &internal_cond()
{ return *static_cast<internal_condition*>(m_shmem.get_user_address()); }
and changing all usages of m_cond to this->internal_cond(). This is analogous to how the shm_named_condition class is implemented.
I have a "watch thread" which checks whether other threads are running and calculates some data. If these threads end I want to finish my watch thread, too. How can I do it?
#include <iostream>
#include <thread>
using namespace std;
void f1() {
cout << "thread t1" << endl;
for (int i=0; i<1000; ++i) {
cout << "t1: " << i << endl;
}
}
void f2() {
cout << "thread t2" << endl;
while (T1_IS_RUNNING) {
cout << "t1 still running" << endl;
}
}
int main() {
thread t1(f1);
thread t2(f2);
t1.join();
t2.join();
return 0;
}
In the example above I need to implement T1_IS_RUNNING. Any ideas how to do it? My guess is to get number of running threads but I haven't found any related method in STL.
There is a How to check if a std::thread is still running? already, but I think they use too complicated solutions for my case. Isn't a simple thread counter (std::atomic) good enough?
You can just use a flag for it (running example):
#include <iostream>
#include <thread>
using namespace std;
bool T1_IS_RUNNING = true;
void f1() {
cout << "thread t1" << endl;
for (int i=0; i<1000; ++i) {
cout << "t1: " << i << endl;
}
T1_IS_RUNNING = false;
cout << "thread t1 finish" << endl;
}
void f2() {
cout << "thread t2" << endl;
while (T1_IS_RUNNING) {
cout << "t1 still running" << endl;
}
cout << "thread t2 finish" << endl;
}
int main() {
thread t1(f1);
thread t2(f2);
t1.join();
t2.join();
return 0;
}
This is safe as long as only one of them writes the flag and the other reads it, otherwise you need to use an atomic flag, a mutex or a semaphore.
With atomic_int:
int main(){
std::atomic_int poor_man_semaphore{0};
poor_man_semaphore++;
std::thread t1([&]()
{
std::this_thread::sleep_for(std::chrono::seconds(100));
poor_man_semaphore--;
});
poor_man_semaphore++;
std::thread t2([&]()
{
std::this_thread::sleep_for(std::chrono::seconds(1));
poor_man_semaphore--;
});
poor_man_semaphore++;
std::thread t3([&]()
{
std::this_thread::sleep_for(std::chrono::seconds(1));
poor_man_semaphore--;
});
t2.join();
t3.join();
while ( poor_man_semaphore > 0 )
{
std::this_thread::sleep_for(std::chrono::seconds(1));
}
t1.join();
return 0;
}
Let me give a quick fix to the code, as there is already a detailed post, this will not be long.
This answer exists because there are many wrong answers here.
My interpretation of your problem is you want a "watch thread" to do work while other threads are still alive, but stop whenever others stop.
#include <fstream>
#include <thread>
#include <atomic> // this is REQUIRED, NOT OPTIONAL
using namespace std;
atomic_int count(1); // REQUIRED to be atomic
void f1() {
ofstream f1out{"f1out.txt"};
f1out << "thread t1" << endl;
for (int i=0; i<1000; ++i) {
f1out << "t1: " << i << endl;
}
count--;
}
void f2() {
ofstream f2out{"f2out.txt"};
f2out << "thread t2" << endl;
while (count > 0) {
f2out << "t1 still running" << endl;
}
}
int main() {
thread t1(f1);
thread t2(f2);
t1.join();
t2.join();
}
Notes on atomic
The syntax of atomic_int might look like an int but they are different and failing to use atomic_int is undefined behaviour.
From [intro.races], emphasis mine
Two expression evaluations conflict if one of them modifies a memory location and the other one reads or modifies the same memory location. [...]
The execution of a program contains a data race if it contains two potentially concurrent conflicting actions, at least one of which is not atomic, and neither happens before the other [...] . Any such data race results in undefined behavior.
Notes on cout
Likewise, it is a data race if the threads use cout concurrently, I can't find a simple replacement to preserve the meaning and effect. I opt into using ofstream in the end.
For people concerned
Yes, the atomic operations need not be sequentially consistent but that really doesn't help with clarity.
This link might help you.
Amongst a lot of solutions, one seems quite easy to implement :
An easy solution is to have a boolean variable that the thread sets to true on regular intervals, and that is checked and set to false by the thread wanting to know the status. If the variable is false for to long then the thread is no longer considered active.
A more thread-safe way is to have a counter that is increased by the child thread, and the main thread compares the counter to a stored value and if the same after too long time then the child thread is considered not active.
May be you could set an array of boolean, one by thread you run, and then check it whenever you want to know if other threads are running ?
I am converting a previous thread wrapper around pthreads to std::thread.
However c++11 does not have any way to cancel the thread. I REQUIRE, nonetheless, to cancel threads since they may be performing a very lengthy task inside an external library.
I was considering using the native_handle that gives me pthread_id in my platform. I'm using gcc 4.7 in Linux (Ubuntu 12.10). The idea would be:
#include <iostream>
#include <thread>
#include <chrono>
using namespace std;
int main(int argc, char **argv) {
cout << "Hello, world!" << endl;
auto lambda = []() {
cout << "ID: "<<pthread_self() <<endl;
while (true) {
cout << "Hello" << endl;
this_thread::sleep_for(chrono::seconds(2));
}
};
pthread_t id;
{
std::thread th(lambda);
this_thread::sleep_for(chrono::seconds(1));
id = th.native_handle();
cout << id << endl;
th.detach();
}
cout << "cancelling ID: "<< id << endl;
pthread_cancel(id);
cout << "cancelled: "<< id << endl;
return 0;
}
The thread is canceled by an exception thrown by pthreads.
My question is:
Will there be any problem with this approach (besides not being portable)?
No, I don't think that you will not have additional problems than:
not being portable
having to program _very_very_ carefully that all objects of the cancelled thread are destroyed...
For example, the Standard says that when a thread ends variables will be destroyed. If you cancel a thread this will be much harder for the compiler, if not impossible.
I would, therefore recommend not to cancel a thread if you can somehow avoid it. Write a standard polling-loop, use a condition variable, listen on a signal to interrupt reads and so on -- and end the thread regularly.
I have the book "beyond the C++ standard library" and there are no examples of multithreading using boost. Would somebody be kind enough to show me a simple example where two threads are executed using boost- lets say asynchronously?
This is my minimal Boost threading example.
#include <boost/thread.hpp>
#include <iostream>
using namespace std;
void ThreadFunction()
{
int counter = 0;
for(;;)
{
cout << "thread iteration " << ++counter << " Press Enter to stop" << endl;
try
{
// Sleep and check for interrupt.
// To check for interrupt without sleep,
// use boost::this_thread::interruption_point()
// which also throws boost::thread_interrupted
boost::this_thread::sleep(boost::posix_time::milliseconds(500));
}
catch(boost::thread_interrupted&)
{
cout << "Thread is stopped" << endl;
return;
}
}
}
int main()
{
// Start thread
boost::thread t(&ThreadFunction);
// Wait for Enter
char ch;
cin.get(ch);
// Ask thread to stop
t.interrupt();
// Join - wait when thread actually exits
t.join();
cout << "main: thread ended" << endl;
return 0;
}
I am trying to make use of boost::thread to perform "n" similar jobs. Of course, "n" in general could be exorbitantly high and so I want to restrict the number of simultaneously running threads to some small number m (say 8). I wrote something like the following, where I open 11 text files, four at a time using four threads.
I have a small class parallel (which upon invoking run() method would open an output file and write a line to it, taking in a int variable. The compilation goes smoothly and the program runs without any warning. The result however is not as expected. The files are created, but they are not always 11 in number. Does anyone know what's the mistake I am making?
Here's parallel.hpp:
#include <fstream>
#include <iostream>
#include <boost/thread.hpp>
class parallel{
public:
int m_start;
parallel()
{ }
// member function
void run(int start=2);
};
The parallel.cpp implementation file is
#include "parallel.hpp"
void parallel::run(int start){
m_start = start;
std::cout << "I am " << m_start << "! Thread # "
<< boost::this_thread::get_id()
<< " work started!" << std::endl;
std::string fname("test-");
std::ostringstream buffer;
buffer << m_start << ".txt";
fname.append(buffer.str());
std::fstream output;
output.open(fname.c_str(), std::ios::out);
output << "Hi, I am " << m_start << std::endl;
output.close();
std::cout << "Thread # "
<< boost::this_thread::get_id()
<< " work finished!" << std::endl;
}
And the main.cpp:
#include <iostream>
#include <fstream>
#include <string>
#include <boost/thread.hpp>
#include <boost/shared_ptr.hpp>
#include "parallel.hpp"
int main(int argc, char* argv[]){
std::cout << "main: startup!" << std::endl;
std::cout << boost::thread::hardware_concurrency() << std::endl;
parallel p;
int populationSize(11), concurrency(3);
// define concurrent thread group
std::vector<boost::shared_ptr<boost::thread> > threads;
// population one-by-one
while(populationSize >= 0) {
// concurrent threads
for(int i = 0; i < concurrency; i++){
// create a thread
boost::shared_ptr<boost::thread>
thread(new boost::thread(¶llel::run, &p, populationSize--));
threads.push_back(thread);
}
// run the threads
for(int i =0; i < concurrency; i++)
threads[i]->join();
threads.clear();
}
return 0;
}
You have a single parallel object with a single m_start member variable, which all threads access without any synchronization.
Update
This race condition seems to be a consequence of a design problem. It is unclear what an object of type parallel is meant to represent.
If it is meant to represent a thread, then one object should be allocated for each thread created. The program as posted has a single object and many threads.
If it is meant to represent a group of threads, then it should not keep data that belongs to individual threads.