I am writing code on ucLinux for socket communication. I use select() for reading the data on sockets. I also have a 20 msec timer (created using setitimer) running in the same thread for performing a parallel operation. My select function gets blocked each time saying "Interrupted by system call", since it receives the SIGALRM signal issue by the timer on overflow, every 20 msec. I tried restarting the system when EINTR is issued, and run select() again. but this wont help, since i will always receive the SIGALRM by timer every 20 msec. I dont want to ignore this signal since it is used for performing other tasks in the system, but i want to use select without being affected by this signal. Is there any way to handle this? I cannot use functions like timer_create() as these are not supported on the platform I am using. So, I am stuck up with using setitimerfor timer creation. Is there any way I can run both together independently in my code?
What you're doing is pretty weird. Let's face it: timers are an ancient and mostly-obsolete mechanism for doing work. Pretty much everyone these days avoids signals like the plague. There's essentially nothing useful you can do in a signal callback (you certainly can't call anything complicated like malloc for example), so you must have some way to get the timer notification back from the SIGALRM handler to the main thread already -- you're not actually doing the work in the signal handler are you?
So you have two tactics: use the standard self-pipe trick to turn the signal into an event on an fd, the "normal" way to handle things like SIGTERM, SIGINT and so on. You call socketpair or pipe to make a pipe, then write a byte into the pipe from the signal handler. You read the byte back from you select loop. You commonly write the value of the signal as the data, but you could write anything really.
The other tactic (much more sane) is to avoid the mess with signals and setitimer completely. setitimer is seriously legacy and causes problems for all sorts of things (eg. it can cause functions like getaddrinfo to hang, a bug that still hasn't been fixed in glibc (http://www.cygwin.org/frysk/bugzilla/show_bug.cgi?id=15819). Signals are bad for your health. So the "normal" tactic is to use the timeout argument to select. You have a linked list of timers, objects you use to manager periodic events in your code. When you call select, you use as the timeout the shortest of your remaining timers. When the select call returns, you check if any timers are expired and call the timer handler as well as the handlers for your fd events. That's a standard application event loop. This way your loop code so you can listen for timer-driven events as well as fd-driven events. Pretty much every application on your system uses some variant on this mechanism.
Is an option for you doing something like this?
While(1) {
int rc = select(nfds, &readfds, &writefds, &exceptfds, &timeout);
if ((rc < 0) && (errno == EINTR) )
continue;
else {
// some instructions
}
}
If this is not an option for you you can probably use pselect which adds a parameter to the end (sigmask) which specifies a set of signals that should be blocked during the pselect(), see here
Related
I have a C application using poll to wait for some data.
Currently I am implementing the rest of my application into this one and I use time based interrupts (SIGRTMIN). As expected poll() returns if one of my other timers call back.
How can I stop poll from doing that? I am reading a lot about ppoll(), but not sure how to use that... Can I use this to stop this function from returning when a timer event is fired?
(I do not have any problems with the poll being delayed a few ms)
If a thread / process blocking in poll() receives an unblocked signal then poll() will be interrupted. If you don't want that to happen then you can block the desired signal before calling poll(), and then unblock it after poll() returns (see sigprocmask()). Note, however, that that won't cause poll() to be delayed -- quite the opposite. If anything, it will cause receipt of the signal to be delayed. If poll() blocks long enough then it could cause multiple RT signals to queue up, so that after you unblock that signal you receive it multiple times in quick succession.
You should consider instead checking poll()'s return value (which you should always do anyway) and retrying if it is EINTR.
I am trying to write a benchmark that receives a signal from the kernel telling it to adjust its parameters. I'm trying to study whether a proactive or reactive approach works best.
In the proactive approach, I use setitimer to set an alarm periodically and force the benchmark to look at its performance thus far and re-tune itself.
In the reactive approach, the kernel periodically monitors the process and signals it if it is performing poorly.
Since I've been using the setitimer functionality, and since setitimer causes SIGALRM, I have asked the kernel to throw a SIGALRM in the reactive approach. This has been working fine. However, now I need to use SIGALRM to run the benchmark for a specific duration of time.
Is there a way to multiplex SIGALRM to serve both purposes - to do a timed run and terminate and to re-tune. Is there a function/syscall similar to setitimer that allows the user to set an alarm but with a custom signal?
Yes. You want to look at the timer_create / timer_settime etc., family of calls.
The 2nd parameter of timer_create is a struct sigevent. The field within that, sigev_signo can be set to send a specific signal number on timer expiration.
I'm writing an application that uses timer to do some data acquisition and processing at a fix sample rate (200Hz).
The application acts like a server and run in background. It should be controllable from other processes or other machines from UDP.
To do so, I use the timer_create() API to generate SIGUSR1 periodically and call an handler that do the acquisition and the processing.
The code to configure the timer is as follow (minus error check for clarity):
sa.sa_flags = SA_SIGINFO;
sa.sa_sigaction = handler;
sigemptyset(&sa.sa_mask);
sigaction(SIGUSR1, &sa, NULL);
sev.sigev_notify = SIGEV_SIGNAL;
sev.sigev_signo = SIGUSR1;
sev.sigev_value.sival_ptr = &timerid;
timer_create(CLOCK_REALTIME, &sev, &timerid);
timer_settime(...)
The code above is called when a 'start' command is received from UDP. To check for command I have an infinite loop in my main program that call recvfrom() syscall.
The problem is, when a 'start' command is received, and then, the timer is properly started and running (using the code above), I get an 'interrupted system calls' error (EINTR) due the SIGUSR1 signal sent by the timer interrupting the recvfrom() call. If I check for this particular error code and ignore it, I finally get a 'connection refused' error when calling recvfrom().
So here my questions:
How to solve this 'interrupted system calls' error as it seems to
ignore it and re-do the recvfrom() doesn't work?
Why do I get the 'connection refused' error after about twenty tries?
I have the feeling that using SIGEV_THREAD could be a solution, as I understand it, create a new thread (like phread_create) without generate a signal. Am I right?
Is the signal number important here? Is there any plus to use real time signal?
Is there any other way to do what I intent to do: having a background loop checking for command from UDP and real-time periodic task?
And here the bonus question:
Is it safe to do the data acquisition and the processing in the handler or should I use a semaphore mechanism to wake up a thread that do it?
Solution:
As suggest in an answer and in the comments, using SA_RESTART seems to fix the main issue.
Solution 2:
Using SIGEV_THREAD over SIGEV_SIGNAL works too. I've read somewhere that using SIGEV_THREAD could require more ressources than SIGEV_SIGNAL. However I have not seen significant difference regarding the timing of the task.
Timers tend to be implemented using SIGALARM.
Signal receipt, including SIGALARM, tends to cause long running system calls to return early with EINTR in errno.
SA_RESTART is one way around this, so system calls interrupted by receipt of a signal, will be automatically restarted. Another is to check for EINTR from your system calls' errno's and restart them when you receive EINTR.
With read() and write() of course, you can't just restart, you need to pick up where you left off. That's why these return the length of data transmitted.
Given that you're using Linux, I would opt for using timerfd_create instead.
That way you can just select(2), poll(2) or epoll(7) instead and handle timer events without the difficulty of signal handlers in your main loop.
As for EINTR (Interrupted System Call), those are properly handled by just restarting the specific system call that got interrupted.
Restarting the interrupted system call is the correct response to EINTR. You "Connection Refused" problem is an unrelated error - on a UDP socket, it indicates that a previous packet sent on that socket was rejected by the destination (notified through an ICMP message).
Question 5: Your use of a message and real-time periodic thread is perfectly fine. However, I would suggest you avoid using timers altogether, precisely because they use signals. I've run into this problem myself and eventually replaced the timer with a simple clock_nanosleep() that uses TIMER_ABSTIME with time updated to maintain the desired rate (i.e. add the period to the absolute time). The result was simpler code, no more problems with signals, and a more accurate timer than the signal-based timer. BTW, you should measure your timer's period in the handler to make sure it is accurate enough. My experience with timers was 8 years ago, so the problem with accuracy might be fixed. However, the other problems with signals are inherent to signals themselves and thus can't be "solved" -- only worked around.
Also, I see no problem with doing data acquisition from the handler, it should certainly reduce latency in retrieving the data.
This is a question similar to Proper way to close a blocking UDP socket. I have a thread in C which is reading from a UDP socket. The read is blocking. I would like to know if it is possible to be able to exit the thread, without relying on the recv() returning? For example can I close the socket from another thread and safely expect the socket read thread to exit? Didn't see any high voted answer on that thread, thats why I am asking it again.
This really depends on what system you're running under. For example, if you're running under a POSIX-compliant system and your thread is cancelable, the recv() call will be interrupted when you cancel the thread since it's a cancel point.
If you're using an older socket implementation, you could set a signal handler for your thread for something like SIGUSR1 and hope nobody else wanted it and signal, since recv() will interrupt on a signal. Your best option is not to block, if at all possible.
I don't think closing a socket involved in a blocking operation is a safe guaranteed way of terminating the operation. For instance, kernel.org warns darkly:
It is probably unwise to close file descriptors while they may be in
use by system calls in other threads in the same process. Since a
file descriptor may be reused, there are some obscure race conditions
that may cause unintended side effects.
Instead you could use a signal and make recv fail with EINTR
(make sure SA_RESTART is not enabled). You can send a signal to a
specific thread with pthread_kill
You could enable SO_RCVTIMEO on the socket before starting the recv
call
Personally I usually try to stay clear of all the signal nastiness but it's a viable option.
You've got a couple of options for that. A signal will interrupt the read operation, so all you need to do is make sure a signal goes off. The recv operation should fail with error number EINTR.
The simplest option is to set up a timer to interrupt your own process after some timeout e.g. 30 seconds:
itimerval timer
timeval time;
time.tv_sec = 30;
time.tv_usec = 0;
timer.it_value = time;
if( setitimer( ITIMER_REAL, &timer, NULL ) != 0 )
printf( "failed to start timer\n" );
You'll get a SIGALRM after the specified time, which will interrupt your blocking operation, and give you the chance to repeat the operation or quit.
You cannot deallocate a shared resource while another thread is or might be using it. In practice, you will find that you cannot even write code to do what you suggest.
Think about it. When you go to call close, how can you possibly know that the other thread is actually blocked in recv? What if it's about to call recv, but then another thread calls socket and gets the descriptor you just closed? Now, not only will that thread not detect any error, but it will be calling recv on the wrong socket!
There is probably a good way to solve your outer problem, the reason you need to exit from a blocking UDP socket read. There are also several ugly hacks available. The basic approach is to make the socket non-blocking and instead of making a blocking UDP socket read, fake a blocking read with select or poll. You can then abort this loop several ways:
One way is to have select time out and check an 'abort' flag when select returns.
Another way is to also select on the read end of a pipe. Send a single byte to the pipe to abort the select.
If posix complient system, you can try to monitor your thread:
pthread_create with a function that makes your recv and pthread_cond_signal just after, then returns.
The calling thread makes a pthread_cond_timedwait with the desired timeout and terminates the called thread if timed_out.
Right now I have a function connected to SIGARLM that goes off after 1 second and will re-alarm itself to go off in another second everytime. There's a test in the logic of the SIGALRM function I wrote to see if a certain timeout has been reached and when it does I need it to kill a function that's running. Does anybody know how I can do this?
I forgot to mention: in the function that needs to be killed it waits on scanf() and the function needs to die even if scanf() hasn't returned yet.
One approach that might be worth looking into is using select to poll stdin and see if any data is ready. select lets you wait for some period of time on a file descriptor, controlling when you can be interrupted and by what, and seems like it's perfect here. You could just sit in a loop waiting for up to a second, then failing gracefully if no data is available. That way, SIGALRM wouldn't need to kill the function; it would take care of that all by itself.
Not sure exactly what you're asking or what the structure of the program is. If I understand correctly: some function is running and you want to terminate it if it's been running for X time. You have a SIGALARM wake up every second and that will check the running time of the other function and do the terminate.
How do you plan to kill the function? Is it a function in the same process, or is it a separate process. Is your question how to terminate it or how to tell when it needs to be terminated?
I've done something which I believe is similar. I had a multi-threaded application with a structure which contained information about the threads I wished to monitor. The structure contained a member variable "startTime". My monitoring (SIGALARM) function had access to a list of threads. When the monitor woke up it would traverse the list, compare current time to each thread startTime and send a message to the function if it had exceeded it's allowed runtime.
Does this help at all?
You could use a (global) variable to communicate between the signal handler and the function that should be stopped. The function then would check that variable to see if it should still continue running or if it should exit.
Something line this:
volatile int worker_expired = 0;
void worker() {
while (!worker_expired) {
// ...
}
}
void sig_alrm() {
worker_expired = 1;
}
If you want the signal to terminate IO operations, you need to make sure it's an interrupting signal handler. On modern systems, system calls interrupted by signals automatically restart unless you specify otherwise. Use the sigaction function rather than the signal function to setup your signal handlers if you want control over things like this. With sigaction, unless you specify SA_RESTART, signal handlers can interrupt.
If you're using file-descriptor IO functions like read, you should now get the effects you want.
If you're using stdio functions like fscanf, getting interrupted by a signal will put the FILE into an error state that can only be cleared by clearerr, and will lose any partial input in the buffer. Interrupting signals do not mix very well with stdio unless you just want to abort all operations on the file and close it when a signal is received.
So ... to restate slightly: it isn't so much that you want to kill the function as that you want any pending i/o to terminate and the function to exit.
I would either:
use select() to periodically wake up and check a flag set by the signal handler. if the flag isn't set and there's no input pending then loop and call select() again.
i suspect that your SIGALARM handler is doing more than just checking this one timer, and so using pselect() to check for i/o OR SIGALARM is probably not an option for you. i wonder if you could grab a user defined signal, and pass that in pselect. then your alarm handler would send that user defined signal.
Regarding choice 1, if SIGALARM is waking every second then you can adjust the time that select() sleeps to be within your maximum error latency. In other words assume that the timeout occurs immediately after the call to select(), then it will take until select() wakes up to detect the flag set by the SIGALARM handler. So if select() wakes up 10 times per second then it could take up to 1/10 second to detect the setting of the "give up" flag (set by the SIGALARM handler).