How to read for X seconds maximum in C++? - c++

Question

I want my program to wait for something to read in a FIFO, but if the read (I use std::fstream) lasts more than 5 seconds, I want it to exit.
Is it possible or do I have to use alarm absolutely?
Thank you.

Answer 1

I do not believe there is a clean way to accomplish this that is portable C++ only solution. Your best option is to use poll or select on *nix based systems and WaitForSingleObject or WaitForMultipleObjects on Windows.
You can do this transparently by creating a proxy streambuffer class that forwards calls to a real streambuffer object. This will allow you to call the appropriate wait function before doing the actual read. It might look something like this...
class MyStreamBuffer : public std::basic_streambuf<char>
{
public:
MyStreamBuffer(std::fstream& streamBuffer, int timeoutValue)
: timeoutValue_(timeoutvalue),
streamBuffer_(streamBuffer)
{
}
protected:
virtual std::streamsize xsgetn( char_type* s, std::streamsize count )
{
if(!wait(timeoutValue_))
{
return 0;
}
return streamBuffer_.xsgetn(s, count);
}
private:
bool wait() const
{
// Not entirely complete but you get the idea
return (WAIT_OBJECT_0 == WaitForSingleObject(...));
}
const int timeoutValue_;
std::fstream& streamBuffer_;
};
You would need to do this on every call through. It might get a little tedious but would provide a transparent solution for providing timeouts even where they might not be explicitly supported in client code.

Answer 2

For the one insterested by the way I resolved my problem, here is my function reading from my stream. I couldn't finally use std::fstream so I replaced it by C system calls.
std::string
NamedPipe::readForSeconds(int seconds)
{
fd_set readfs;
struct timeval t = { seconds, 0 };
FD_ZERO(&readfs);
FD_SET(this->_stream, &readfs);
if (select(this->_stream + 1, &readfs, NULL, NULL, &t) < 0)
throw std::runtime_error("Invalid select");
if (FD_ISSET(this->_stream, &readfs))
return this->read();
throw NamedPipe::timeoutException();
}