I'm building a small io service that checks read and write availability of some fds.
To do that, I have a thread dedicated to the select without any timeout so that the select only wakes up when a fd becomes available.
However, I sometimes want to force select to be woken up on specific events. To do so, I simply use a pipe, watch for its read availability and write on it when I want to wake up the select call.
This works most of the time, but it sometimes happens that nothing happen when I write to the pipe. So the select call remains blocked indefinitely.
Here is a part of the code I use:
Select thread:
FD_ZERO(&rd_set);
//! set some other fds...
FD_SET(m_notif_pipe_fds[0], &rd_set);
select(max_fd + 1, &rd_set, &wr_set, nullptr, nullptr);
if (FD_ISSET(m_notif_pipe_fds[0], &rd_set)) {
char buf[1024];
read(m_notif_pipe_fds[0], buf, 1024);
}
Notify thread:
write(m_notif_pipe_fds[1], "a", 1);
The max_fd variable has effectively been set to the highest fd value (not the number of fd to watch which is a common error).
Any idea?
I'd suggest you to make your pipe non-blocking
int flags = fcntl(m_notif_pipe_fd[1], F_GETFL, 0);
assert(flags != -1);
fcntl(m_notif_pipe_fd[1], F_SETFL, flags | O_NONBLOCK);
and set pipe buffer size to 1
int pipe_sz = fcntl(m_notif_pipe_fd[1], F_SETPIPE_SZ, 1);
See this question
Related
In my code I open a FIFO (created with mkfifo) and then I proceed to use a QSocketNotifier to receive notifications of incoming data, to read it while it arrives.
// create the FIFO
if(!mkfifo(SERIAL_FIFO, 0600)) {
// nonblocking open (even open itself would block until a first write)
in_fifo = ::open(SERIAL_FIFO, O_RDONLY | O_NONBLOCK);
if(in_fifo >= 0) {
// create notifier
in_fifo_notifier = new QSocketNotifier(in_fifo, QSocketNotifier::Read, this);
connect(&*in_fifo_notifier, &QSocketNotifier::activated,
this, [this](QSocketDescriptor /*socket*/, QSocketNotifier::Type /*type*/){
// copy all the available data
char buf[4096];
for(;;) {
ssize_t rl = ::read(in_fifo, buf, sizeof(buf));
if(rl <= 0) break;
::write(out_fd, buf, rl);
}
});
}
The problem is that, whenever someone writes on the other end of the pipe, the signal keeps getting activated (with associated 100% CPU usage), even though every time I read all the data. Where's the problem?
Ultimately, this is just a variation over the problem described here, as Qt under the hood uses select/epoll machinery to implement QSocketNotifier. Opening the FIFO as O_RDWR fixes the problem.
I'm reading CAN-BUS traffic under SocketCAN and C++ in GNU/Linux. I've found that the read call is blocking, and I'm struggling to figure out how to stop my program properly when I don't want to keep reading.
Of course, I could hit Ctrl+C if I've invoked the program from the terminal, but the point is to find a way to do it programmatically when some condition is met (e.g., record for 5 seconds, or when some event happens, like a flag is raised). A timeout could work, or something like a signal, but I don't know how to do it properly.
// Read (blocking)
nbytes = read(s, &frame, sizeof(struct can_frame));
You don't.
Use a method like select or epoll to determine whether the socket has activity before beginning the read. Then it will not actually block.
The select/epoll call is itself blocking, but can be given a timeout so that you always have an escape route (or, in the case of epoll, the lovely epollfd for immediate triggering of a breakout).
Read is always blocking... you want to only read if data is waiting... so consider doing a poll on the socket first to see if data is available and if so THEN read it. You can loop over doing the poll until you no longer want to read anymore...
bool pollIn(int fd)
{
bool returnValue{false};
struct pollfd *pfd;
pfd = calloc(1, sizeof(struct pollfd));
pfd.fd = fd;
pfd.events = POLLIN;
int pollReturn{-1};
pollReturn = poll(pfd, 1, 0);
if (pollReturn > 0)
{
if (pfd.revents & POLLIN)
{
returnValue = true;
}
}
free(pfd);
return(returnValue);
}
The above should return if there is data waiting at the socket file descriptor.
while(!exitCondition)
{
if(pollIn(fd))
{
nbytes = read(fd, &frame, sizeof(struct can_frame));
// other stuff you need to do with your read
}
}
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 searched but did not get a relevant answer to this question, i am working on a linux machine, i wanted to check if the standard input stream contains any character, without removing the characters from the stream.
You might want to try select() function, and wait for having data into the input stream.
Description:
select() and pselect() allow a program to monitor multiple file
descriptors, waiting until one or more of the file descriptors become
"ready" for some class of I/O operation (e.g., input possible). A file
descriptor is considered ready if it is possible to perform the
corresponding I/O operation (e.g., read(2)) without blocking.
In your case, the file descriptor will be stdin
void yourFunction(){
fd_set fds;
struct timeval timeout;
int selectRetVal;
/* Set time limit you want to WAIT for the fdescriptor to have data,
or not( you can set it to ZERO if you want) */
timeout.tv_sec = 0;
timeout.tv_usec = 1;
/* Create a descriptor set containing our remote socket
(the one that connects with the remote troll at the client side). */
FD_ZERO(&fds);
FD_SET(stdin, &fds);
selectRetVal = select(sizeof(fds)*8, &fds, NULL, NULL, &timeout);
if (selectRetVal == -1) {
/* error occurred in select(), */
printf("select failed()\n");
} else if (selectRetVal == 0) {
printf("Timeout occurred!!! No data to fetch().\n");
//do some other stuff
} else {
/* The descriptor has data, fetch it. */
if (FD_ISSET(stdin, &fds)) {
//do whatever you want with the data
}
}
}
Hope it helps.
cacho was on the right path, however select is only necessary if you're dealing with more than one file descriptor, and stdin is not a POSIX file descriptor (int); It's a FILE *. You'd want to use STDIN_FILENO, if you go that route.
It's not a very clean route to take, either. I'd prefer to use poll. By specifying 0 as the timeout, poll will return immediately.
If none of the defined events have occurred on any selected file
descriptor, poll() shall wait at least timeout milliseconds for an
event to occur on any of the selected file descriptors. If the value
of timeout is 0, poll() shall return immediately. If the value of
timeout is -1, poll() shall block until a requested event occurs or
until the call is interrupted.
struct pollfd stdin_poll = { .fd = STDIN_FILENO
, .events = POLLIN | POLLRDBAND | POLLRDNORM | POLLPRI };
if (poll(&stdin_poll, 1, 0) == 1) {
/* Data waiting on stdin. Process it. */
}
/* Do other processing. */
I have a piece of code that accepts 2 connections, creates a file descriptor set with their respective sockets, and passes it to select. But when select returns, the number of file descriptors in the file descriptor set was reduced to 1, and select can just detect received data for the first socket in the fd_array array.
Any ideas where I should look at?
Thanks in advance,
Andre
fd_set mSockets;
/* At this point
mSockets.fd_count = 2
mSockets.fd_array[0] = 3765
mSockets.fd_array[1] = 2436
*/
select(0, & mSockets, 0, 0, 0);
/* At this point
mSockets.fd_count = 1
mSockets.fd_array[0] = 3765
mSockets.fd_array[1] = 2436
*/
That is by design the readfds, writefds and exceptfds paramters of the select functions are in/out paramters.
You should initialize the fd_set before each call to select:
SOCKET s1;
SOCKET s2;
// open sockets s1 and s2
// prepare select call
FD_ZERO(&mSockets);
FD_SET(s1, &mSockets);
FD_SET(s2, &mSockets);
select(0, &mSockets, 0, 0, 0);
// evaluate select results
if (FD_ISSET(s1, &mSockets))
{
// process s1 traffic
}
if (FD_ISSET(s2, &mSockets))
{
// process s2 traffic
}
Additionally cou can check the return value of select. It indicates invalid if you can opertate with the sockets at all. I.e. a zero return indicates, that all FD_ISSET amcros will return 0.
EDIT:
Since readfds, writefds and exceptfds are also out paramters of the select functions, they are modified. The fd_count member indicates how many fd_array members are valid. You should not evaluate fd_array[1] if fd_count is less than 2.