So I have a thread that I was running using .join() but I needed an interactive user interface while running the thread so I stopped using join because it halted the program while it ran. The ui has a stop button to kill the thread and now I need a way to stop the thread without killing the whole program because I can't use .detach(). Thanks!
There is no safe way to unilaterally terminate a thread. Instead, the thread's code must periodically check whether the GUI thread has requested that it exit.
I'm not familiar with the new C++ library functions, but I believe you can do this with atomic_bool, e.g., see this question.
you could pass a reference to a bool variable to the thread and check if it is true. if it is, return from the thread.
Example:
bool terminate = false;
std::mutex m;
std::thread t([&terminate,&m] {
std::unique_lock<std::mutex> lm{m,std::defer_lock}; //don't lock yet
int i = 0;
while (true) {
lm.lock(); //protect terminate -> no race conditions
if (terminate)
return;
lm.unlock(); //release lock for terminate
//do what your thread should do
std::cout << i++ << std::endl;
}
});
/*
do something else here
*/
m.lock();
terminate = true;
m.unlock();
t.join();
I need a code construction for my project which waits for some time, but when there is an interrupt (e.g. incoming udp packets) it leaves this loop, does something, and after this restart the waiting.
How can I implement this? My first idea is using while(wait(2000)), but wait is a void construct...
Thank you!
I would put the loop inside a function
void awesomeFunction() {
bool loop = true;
while (loop) {
wait(2000);
...
...
if (conditionMet)
loop = false;
}
}
Then i would put this function inside another loop
while (programRunning) {
awesomeFunction();
/* Loop ended, do stuff... */
}
There are a few things I am not clear about from the question. Is this a multi-threaded application, where one thread handles (say) the UDP packets, and the other waits for the event, or is this single-threaded? You also didn't mention what operating system this is, which is relevant. So I am going to assume Linux, or something that supports the poll API, or something similar (like select).
Let's assume a single threaded application that waits for UDP packets. The main idea is that once you have the socket's file descriptor, you have an infinite loop on a call to poll. For instance:
#include <poll.h>
// ...
void handle_packets() {
// m_fd was created with `socket` and `bind` or `connect`.
struct pollfd pfd = {.fd = m_fd, .events = POLLIN};
int timeout;
timeout = -1; // Wait indefinitely
// timeout = 2000; // Wait for 2 seconds
while (true) {
pfd.revents = 0;
poll(&pfd, 1, timeout);
if ((pfd.revents & POLLIN) != 0) {
handle_single_packet(); // Method to actually read and handle the packet
}
if ((pfd.revents & (POLLERR | POLLHUP)) != 0) {
break; // return on error or hangup
}
}
}
A simple example of select can be found here.
If you are looking at a multi-threaded application, trying to communicate between the two threads, then there are several options. Two of which are:
Use the same mechanism above. The file descriptor is the result of a call to pipe. The thread sleeping gets the read end of the pipe. The thread waking get the write end, and writes a character when it's time to wake up.
Use C++'s std::condition_variable. It is documented here, with a complete example. This solution depends on your context, e.g., whether you have a variable that you can wait on, or what has to be done.
Other interrupts can also be caught in this way. Signals, for instance, have a signalfd. Timer events have timerfd. This depends a lot on what you need, and in what environment you are running. For instance, timerfd is Linux-specific.
I keep running into this problem of trying to run a thread with the following properties:
runs in an infinite loop, checking some external resource, e.g. data from the network or a device,
gets updates from its resource promptly,
exits promptly when asked to,
uses the CPU efficiently.
First approach
One solution I have seen for this is something like the following:
void class::run()
{
while(!exit_flag)
{
if (resource_ready)
use_resource();
}
}
This satisfies points 1, 2 and 3, but being a busy waiting loop, uses 100% CPU.
Second approach
A potential fix for this is to put a sleep statement in:
void class::run()
{
while(!exit_flag)
{
if (resource_ready)
use_resource();
else
sleep(a_short_while);
}
}
We now don't hammer the CPU, so we address 1 and 4, but we could wait up to a_short_while unnecessarily when the resource is ready or we are asked to quit.
Third approach
A third option is to do a blocking read on the resource:
void class::run()
{
while(!exit_flag)
{
obtain_resource();
use_resource();
}
}
This will satisfy 1, 2, and 4 elegantly, but now we can't ask the thread to quit if the resource does not become available.
Question
The best approach seems to be the second one, with a short sleep, so long as the tradeoff between CPU usage and responsiveness can be achieved.
However, this still seems suboptimal, and inelegant to me. This seems like it would be a common problem to solve. Is there a more elegant way to solve it? Is there an approach which can address all four of those requirements?
This depends on the specifics of the resources the thread is accessing, but basically to do it efficiently with minimal latency, the resources need to provide an API for either doing an interruptible blocking wait.
On POSIX systems, you can use the select(2) or poll(2) system calls to do that, if the resources you're using are files or file descriptors (including sockets). To allow the wait to be preempted, you also create a dummy pipe which you can write to.
For example, here's how you might wait for a file descriptor or socket to become ready or for the code to be interrupted:
// Dummy pipe used for sending interrupt message
int interrupt_pipe[2];
int should_exit = 0;
void class::run()
{
// Set up the interrupt pipe
if (pipe(interrupt_pipe) != 0)
; // Handle error
int fd = ...; // File descriptor or socket etc.
while (!should_exit)
{
// Set up a file descriptor set with fd and the read end of the dummy
// pipe in it
fd_set fds;
FD_CLR(&fds);
FD_SET(fd, &fds);
FD_SET(interrupt_pipe[1], &fds);
int maxfd = max(fd, interrupt_pipe[1]);
// Wait until one of the file descriptors is ready to be read
int num_ready = select(maxfd + 1, &fds, NULL, NULL, NULL);
if (num_ready == -1)
; // Handle error
if (FD_ISSET(fd, &fds))
{
// fd can now be read/recv'ed from without blocking
read(fd, ...);
}
}
}
void class::interrupt()
{
should_exit = 1;
// Send a dummy message to the pipe to wake up the select() call
char msg = 0;
write(interrupt_pipe[0], &msg, 1);
}
class::~class()
{
// Clean up pipe etc.
close(interrupt_pipe[0]);
close(interrupt_pipe[1]);
}
If you're on Windows, the select() function still works for sockets, but only for sockets, so you should install use WaitForMultipleObjects to wait on a resource handle and an event handle. For example:
// Event used for sending interrupt message
HANDLE interrupt_event;
int should_exit = 0;
void class::run()
{
// Set up the interrupt event as an auto-reset event
interrupt_event = CreateEvent(NULL, FALSE, FALSE, NULL);
if (interrupt_event == NULL)
; // Handle error
HANDLE resource = ...; // File or resource handle etc.
while (!should_exit)
{
// Wait until one of the handles becomes signaled
HANDLE handles[2] = {resource, interrupt_event};
int which_ready = WaitForMultipleObjects(2, handles, FALSE, INFINITE);
if (which_ready == WAIT_FAILED)
; // Handle error
else if (which_ready == WAIT_OBJECT_0))
{
// resource can now be read from without blocking
ReadFile(resource, ...);
}
}
}
void class::interrupt()
{
// Signal the event to wake up the waiting thread
should_exit = 1;
SetEvent(interrupt_event);
}
class::~class()
{
// Clean up event etc.
CloseHandle(interrupt_event);
}
You get a efficient solution if your obtain_ressource() function supports a timeout value:
while(!exit_flag)
{
obtain_resource_with_timeout(a_short_while);
if (resource_ready)
use_resource();
}
This effectively combines the sleep() with the obtain_ressurce() call.
Check out the manpage for nanosleep:
If the nanosleep() function returns because it has been interrupted by a signal, the function returns a value of -1 and sets errno to indicate the interruption.
In other words, you can interrupt sleeping threads by sending a signal (the sleep manpage says something similar). This means you can use your 2nd approach, and use an interrupt to immediately wake the thread if it's sleeping.
Use the Gang of Four Observer Pattern:
http://home.comcast.net/~codewrangler/tech_info/patterns_code.html#Observer
Callback, don't block.
Self-Pipe trick can be used here.
http://cr.yp.to/docs/selfpipe.html
Assuming that you are reading the data from file descriptor.
Create a pipe and select() for readability on the pipe input as well as on the resource you are interested.
Then when data comes on resource, the thread wakes up and does the processing. Else it sleeps.
To terminate the thread send it a signal and in signal handler, write something on the pipe (I would say something which will never come from the resource you are interested in, something like NULL for illustrating the point). The select call returns and thread on reading the input knows that it got the poison pill and it is time to exit and calls pthread_exit().
EDIT: Better way will be just to see that the data came on the pipe and hence just exit rather than checking the value which came on that pipe.
The Win32 API uses more or less this approach:
someThreadLoop( ... )
{
MSG msg;
int retVal;
while( (retVal = ::GetMessage( &msg, TaskContext::winHandle_, 0, 0 )) > 0 )
{
::TranslateMessage( &msg );
::DispatchMessage( &msg );
}
}
GetMessage itself blocks until any type of message is received therefore not using any processing (refer). If a WM_QUIT is received, it returns false, exiting the thread function gracefully. This is a variant of the producer/consumer mentioned elsewhere.
You can use any variant of a producer/consumer, and the pattern is often similar. One could argue that one would want to split the responsibility concerning quitting and obtaining of a resource, but OTOH quitting could depend on obtaining a resource too (or could be regarded as one of the resources - but a special one). I would at least abstract the producer consumer pattern and have various implementations thereof.
Therefore:
AbstractConsumer:
void AbstractConsumer::threadHandler()
{
do
{
try
{
process( dequeNextCommand() );
}
catch( const base_except& ex )
{
log( ex );
if( ex.isCritical() ){ throw; }
//else we don't want loop to exit...
}
catch( const std::exception& ex )
{
log( ex );
throw;
}
}
while( !terminated() );
}
virtual void /*AbstractConsumer::*/process( std::unique_ptr<Command>&& command ) = 0;
//Note:
// Either may or may not block until resource arrives, but typically blocks on
// a queue that is signalled as soon as a resource is available.
virtual std::unique_ptr<Command> /*AbstractConsumer::*/dequeNextCommand() = 0;
virtual bool /*AbstractConsumer::*/terminated() const = 0;
I usually encapsulate command to execute a function in the context of the consumer, but the pattern in the consumer is always the same.
Any (welln at least, most) approaches mentioned above will do the following: thread is created, then it's blocked wwiting for resource, then it's deleted.
If you're worried about efficiency, this is not a best approach when waiting for IO. On Windows at least, you'll allocate around 1mb of memory in user mode, some in kernel for just one additional thread. What if you have many such resources? Having many waiting threads will also increase context switches and slow down your program. What if resource takes longer to be available and many requests are made? You may end up with tons of waiting threads.
Now, the solution to it (again, on Windows, but I'm sure there should be something similar on other OSes) is using threadpool (the one provided by Windows). On Windows this will not only create limited amount of threads, it'll be able to detect when thread is waiting for IO and will stwal thread from there and reuse it for other operations while waitting.
See http://msdn.microsoft.com/en-us/library/windows/desktop/ms686766(v=vs.85).aspx
Also, for more fine-grained control bit still having ability give up thread when waiting for IO, see IO completion ports (I think they'll anyway use threadpool inside): http://msdn.microsoft.com/en-us/library/windows/desktop/aa365198(v=vs.85).aspx
I wish to create a thread in a child process before the respective child process changes it's image using exec system call. However, seemingly, the pthread_create call is being overlooked.
pthread_t thread;
pthread_attr_t attribute;
pthread_attr_init(&attribute);
pthread_attr_setdetachstate(&attribute, PTHREAD_CREATE_DETACHED);
pid_t cid = fork();
if(cid == 0) //CHILD Process
{
switch(x->option)
{
case 1: pthread_create(&thread, &attribute, compressShow, NULL);
execl("/home/aamir/Lab/ass3/compression", "compression", source, destination, NULL);
cout<<"Execution failed."<<endl; break; //This segment will execute if exec fails.
}
else //PARENT Process
{
wait(0); //Prevents termination of original main until forked exec completes execution
pthread_cancel(thread);
}
The thread is basically just a progress display that is intended to output '.' (dots) in concurrence with the forked child.
If I remove the exec call the thread does execute. I've searched on google and read somewhere that you cannot use pthread_create between a fork and exec, something to do with async safe functions. Can you please help?
The exec bit zapps everything including threads and just starts a new process. That includes memory etc.
The program might (and usually) does not get to the bit to fire up the thread.
I made a multithread application that generates/destroy 100 threads continuously:
//Here is the thread class (one by every thread
struct s_control
{
data_in[D_BUFFER_SIZE];//data in to thread
data_out[D_BUFFER_SIZE];//data generated by the thread
//I use volatile in order to status data is avaiable in and out of the thread:
volatile __int16 status;//thread state 0=empty,1=full,2=filling (thread running)
}*control;
//Here is the thread main function
static void* F_pull(void* vv)//=pull_one_curl()
{
s_control* cc = (s_control* ) vv;
//use of cc->data_in and filling of cc->data out
cc->status=1; //Here advises that thread is finished and data out is filled
return NULL;
}
void main()
{
initialization();
control=new s_control[D_TAREAS];
pthread_t *tid=new pthread_t[D_TAREAS];
for (th=0;th<D_TAREAS;th++)
{ //Access to status of thread at the beginning
//(to avoid if it changes in the middle):
long status1=control[th].status
if (status1==0) //Thread finished and data_out of thread is empty
{ control[i2].status=2; //Filling in (thread initiated)status LLENANDO
error = pthread_create(&tid[th],NULL,F_pull,(void *) &control[th]);
}
else if (status1==1) //Thread finished and data_out of thread is full
{
//do things with control[th].data_out;
//and fill in control[th].data_in with data to pass to next thread
control[th].status=0; //Thread is finished and now its data_out is empty
}
else
{
//printf("\nThread#%li:filling",i2);
}
}while(!_kbhit());
finish();
}
Then as you can see, at the end of the thread, I used the variable volatile to advise that thread is about to exit:
begin of thread{ ....
cc->status=1; //Here advises that thread is finished and data out is filled
return NULL;
}//END OF THREAD
But after cc->status is set to 1 thread is not finished yet (it exist one more line)
So I do not like set status inside the thread.
I tried pthread_kill, but it didn´t work, because it does not work until thread is alive, as can be seen at:
pthread_kill
I am not sure if this answers your question, but you can use pthread_join() to wait for a thread to terminate. In conjunction with some (properly synchronized) status variables, you should be able to achieve what you need.