interruption of blocked I/O functions

Question

I have a thread, that reads serial device (/dev/ttyUSB0), and writes data to stdout :

void io_thread_func () {

    int serial_port = open(settings.port, O_RDWR);

    while (1) {

        char buf[100];

        int n = read(serial_port, buf, 100);

        fwrite(buf, 1, n, stdout);

    }

}

How I can interrupt "read(...)" syscall from another thread ? Can I call "close(serial_port)" from another thread to interrupt all I/O blocking functions ? (I need to interrupt I/O blocking functions to close the thread correctly)

Gleb: I'd hazard that your question is practically identical to https://stackoverflow.com/questions/3882379/how-to-interrupt-a-fread-call - would you agree? — Tony Delroy, Dec 24 '20 at 05:46
@TonyDelroy It's not clear how useful any of the answers there are. — Barmar, Dec 24 '20 at 06:12
Similar question, check this one https://stackoverflow.com/questions/6910335/interrupting-syscalls-in-threads-on-linux — Krishna Kanth Yenumula, Dec 24 '20 at 06:16
`int pthread_kill(pthread_t tid, int sig);` sends a signal `sig` to another thread, within the same process — Krishna Kanth Yenumula, Dec 24 '20 at 06:19
@Barmar: if that's the case, Gleb should cite the other question and say how the answers there were tried and failed for his purpose (or offer a bounty for better solutions to the existing question, but Gleb can't do that as a new contributor - none of them internet point thingies). — Tony Delroy, Dec 24 '20 at 06:28
is there a syscall "interrupt_all_io_functions(int fd)" ? or not ? — gleb zheglov, Dec 24 '20 at 06:36

score 2 · Accepted Answer · answered Dec 24 '20 at 06:14

How i can interrupt "read(...)" syscall from another thread ? Can i call "close(serial_port)" from another thread to interrupt all I/O blocking functions ? (i need to interrupt I/O blocking functions to close the thread correctly)

You can send a signal to the thread. That will interrupt the read function.

You absolutely cannot close the serial port from another thread. That can cause all kinds of disasters. Consider:

This thread is about to call read but hasn't called it yet.
Another thread calls close on the serial_port.
Before the first thread can call read another thread accepts an incoming socket connection for a critical internal system purpose.
That thread happens to get the same descriptor as serial_port.
This thread calls read then fwrite, dumping the data received on the critical socket to stdout.

Good thing it's stdout. Because a bug once caused that exact same scenario -- except sensitive data was sent to random inbound connections from the Internet rather than stdout.

is there a syscall "interrupt_all_io_functions(int fd)" ? or not ? — gleb zheglov, Dec 24 '20 at 06:39
@glebzheglov Signals is one way. There are lots of other ways. Ideally, you just wouldn't block in a function you didn't want to wait forever in. For example, you can `poll` both the serial port and a local socket. To unblock the call to `poll`, you can just send a dummy byte on the local socket. There are lots of ways to do this. — David Schwartz, Dec 24 '20 at 22:24

interruption of blocked I/O functions

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