How to interrupt a fread call?

Question

I have the following situation:

There is a thread that reads from a device with a fread call. This call is blocking as long as there is no data send from the device. When I stop this thread it remains hanging inside this thread.

Now I found the following inside the man page of fread:

ERRORS

On all systems that conform to the Single UNIX Specification, the fread() function sets errno as listed for the following conditions:

[EINTR] The read operation was terminated due to the receipt of a signal, and no data was transferred.

That would mean there is a way to interrupt the call from a different thread. But I have no idea how. Can someone tell me how to send a signal to interrupt the fread call? And what signal I need to send?

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Update 08-10-10 09:25

I still haven't got it to work. I tryed the kill() and pthread_kill() with different signals. But nothing seems to interrupt the fread() call. The only thing that I got working is killing the entire application, but that's not what I want.

Answer 1

Take a look at man 2 kill. (Or see here)

I get the feeling that you don't want to do this, though--most of the time people ignore errno EINTR and read again. You might want to look into non-blocking reads instead.

Answer 2

You can use the kill() syscall.

UPDATE

Turns out I misread your question. As R. pointed out below, kill() is only for killing processes, not threads.

Answer 3

You really want to read about the select(2) system call, which will allow you to find out whether there is data available on that file descriptor, without blocking at all or without blocking on only that device.

Answer 4

In the thread, instead of blocking with fread, block with select. When select returns, check an "am I done" variable. If not done, you can call fread to get the data.

From the other thread -- that wants to stop the fread thread -- you can set the "am I done" variable and then close the fd so that the fread thread will wake up from select immediately.

If your context prohibits you from closing the fd (you mention you're reading from a device, but say you had a socket you wanted kept open), you could open a second fd that you write to from the other thread to wake up select.

As suggested in the comments below, closing the fd to wake up select may not be portable. You can use the second-fd strategy mentioned above to achieve this more portably.

Answer 5

1. Signals:

Using signals, as many others pointed out, would work. However, as many others also pointed out, the approach has its disadvantages.

2. Select():

Using select() (or other multiplexing function), you can block waiting for data to arrive from more than one file descriptor, and specify a timeout.

Use the timeout to your advantage. Whenever select() returns, check a global variable to see if you must terminate. If you want immediate reaction, keep reading.

3. Select() and pipes:

Multiple fds means you can wait for data arriving through the device you mentioned and, say, a pipe.

Before you create the thread, create a pipe, and then have the thread block on select() monitoring both the device and the pipe. Whenever you want to unblock select whether the device has new data or not, send a byte down the pipe.

If select() tells you it unblocked due to data arriving through the pipe, you can clean up and terminate. Note this method is much more flexible than the signaling method, since you can, besides just using the pipe as a wake-up method, use it to pass useful information or commands.

4. Select(), pipes and signals:

If you are using multiple processes and don't want to/can't pass around a pipe, you can combine both solutions. Create a pipe and install a signal handler for, say, SIGUSR1. In the signal handler, send a byte down the pipe.

Whenever a process sends SIGUSR1, the handler will be called and unblock select(). By examining the fdsets, you will know it was for no other reason than your own program signaling itself.