C++ Semaphore Confusion? - c++

Question

So, I'm writing a sort of oscilloscope-esque program that reads the the serial port on the computer and performs an fft on this data to convert it to the frequency spectrum. I ran into an issue though with the layout of my program which is broken up into a SerialHandler class (utilizing boost::Asio), an FFTHandler class, and a main function. The SerialHandler class uses the boost::Asio`` async_read_some function to read from the port and raise an event called HandleOnPortReceive which then reads the data itself.
The issue was that I couldn't find a way to pass that data from the event handler, being raised by an io_service object on another thread, to the FFTHandler class, which is on yet another thread. I was recommended to use semaphores to solve my problem, but I have next to no knowledge on semaphore.h usage, so my implementation is now rather broken and doesn't do much of anything it's supposed to.
Here's some code if that makes it a little clearer:
using namespace Foo;
//main function
int main(void){
SerialHandler serialHandler;
FFTHandler fftHandler;
sem_t *qSem_ptr = &qSem;
sem_init(qSem_ptr, 1, 0);
//create separate threads for both the io_service and the AppendIn so that neither will block the user input statement following
serialHandler.StartConnection(tempInt, tempString); //these args are defined, but for brevity's sake, I ommitted the declaration
t2= new boost::thread(boost::bind(&FFTHandler::AppendIn, &fftHandler, q, qSem));
//allow the user to stop the program and avoid the problem of an infinite loop blocking the program
char inChar = getchar();
if (inChar) {...some logic to stop reading}
}
namespace Foo{
boost::thread *t1;
boost::thread *t2;
sem_t qSem;
std::queue<double> q;
boost::mutex mutex_;
class SerialHandler{
private:
char *rawBuffer; //array to hold incoming data
boost::asio::io_service ioService;
boost::asio::serial_port_ptr serialPort;
public:
void SerialHandler::StartConnection(int _baudRate, string _comPort){
//some functionality to open the port that is irrelevant to the question goes here
AsyncReadSome(); //starts the read loop
//create thread for io_service object and let function go out of scope
t1 = new boost::thread(boost::bind(&boost::asio::io_service::run, &ioService));
}
void SerialHandler::AsyncReadSome(){
//there's some other stuff here for error_catching, but this is the only important part
serialPort->async_read_some (
boost::asio::buffer(rawBuffer, SERIAL_PORT_READ_BUF_SIZE),
boost::bind(
&SerialHandler::HandlePortOnReceive,
this, boost::asio::placeholders::error,
boost::asio::placeholders::bytes_transferred, q));
}
void SerialHandler::HandlePortOnReceive(const boost::system::error_code& error, size_t bytes_transferred, std::queue<double>& q){
boost::mutex::scoped_lock lock(mutex_);
//more error checking goes here, but I've made sure they aren't returning and are not the issue
for (unsigned int i =0; i<bytes_transferred; i++){
unsigned char c = rawBuffer[i];
double d = (double) c; //loop through buffer and read
if (c==endOfLineChar){
} else //if not delimiting char, push into queue and post semaphore
{
q.push(d);
//cout << d << endl;
sem_post(&qSem);
cout << q.front() << endl;
cout << "size is: " << q.size() << endl;
}
}
//loop back on itself and start the next read
AsyncReadSome();
}
}
class FFTHandler{
private:
double *in; //array to hold inputs
fftw_complex *out; //holds outputs
int currentIndex;
bool filled;
const int N;
public:
void AppendIn(std::queue<double> &q, sem_t &qSem){
while(1){ //this is supposed to stop thread from exiting and going out of scope...it doesn't do that at all effectively...
cout << "test" << endl;
sem_wait(&_qSem); //wait for data...this is blocking but I don't know why
double d = _q.front();
_q.pop();
in[currentIndex]=d; //read queue, pop, then append in array
currentIndex++;
if (currentIndex == N){ //run FFT if full and reset index
currentIndex = N-overlap-1;
filled = true;
RunFFT();
}
}
}
}
}
That debug line in FFTHandler::AppendIn(..) is indeed firing, so the thread is being created, but it's immediateley going out of scope it seems and destructing the thread, because it seems I've set up the while to respond incorrectly to the semaphore.
TLDR: That was a long explanation to simply say, "I don't understand semaphores but need to somehow implement them. I tried, failed, so now I'm coming here to hopefully receive help on this code from somebody more knowledgeable than me.
UPDATE: So after playing around with some debug statements, it seems that the issue is that the while(1){...} statement is indeed firing, but, the sem_wait(&_qSem); is causing it to block. For whatever reason it is waiting indefinitely and despite the fact that the semaphore is being posted, it continues to wait and never progress beyond that line.

Answer 1

Since you're already using boost::mutex and its scoped lock type, I suggest you use boost::condition_variable instead of a POSIX semaphore. Otherwise you're mixing C++11-style synchronisation with POSIX synchronisation.
You lock the mutex when adding to the queue, but I don't see anything locking the mutex to read from the queue. It also looks like you're looping back to call AsyncReadSome while the mutex is still locked.
Pick a single form of synchronisation, and then use it correctly.

Answer 2

The initial value of the semaphore is 0 which is valid for this case. So it needs a sem_post for FFTHandler::AppendIn() to be unblocked. But I dont see the code that invokes SerialHandler::AsyncReadSome() for the first time for the serial port to be read and the push to happen into the queue. If you fix that part of the code, I think sem_post would happen and the FFTHandler thread would run. As the first step you can have debug prints one after the sem_wait and one inside AsyncReadSome() function, and my guess is that both wont get executed.
So, essentially you would want to ensure that 'reading' gets initiated and is kept alive as part of the main thread or a different thread.