[erlang-questions] Port Driver. erl_drv_thread_join and ErlDrvTid reuse.

Zhemzhitsky Sergey <>
Tue Oct 2 15:55:58 CEST 2012


Hello erlang gurus,

Currently I’ve faced with the following issue with termination of threads created in port drivers.

I’m creating a separate thread in the erlang port driver that sends terms to some erlang process.

--------------------------------------
The thread is created like this:

ErlDrvTid threadId;
if(erl_drv_thread_create(“thread name”, &threadId, threadFunc, threadData, NULL)) {
    // log error & exit
}
driverData->tid = threadId;

Driver data looks like this:
struct DriverData {
    ErlDrvPort port;
    ErlDrvTid tid;
    int isRunning;
}
--------------------------------------

Thread function looks like this
DriverData* data = (DriverData*) threadData
data. isRunning = 1;
while(data.isRunning) {
    doSomething();
}
--------------------------------------

When I’d like to stop this thread from one of the driver callbacks I do something like this:

void handleInput(ErlDrvData handle, char * buf, ErlDrvSizeT len) {
    DriverData* data = (DriverData*) handle;

    int command = parseBuf(buf, len);
    if (command == STOP) {
        data.isRunning = 0;
        if(erl_drv_thread_join(data->tid, NULL)) { // here is the place where EDEADLK happens
            // report error
        }
    } else {
        …
    }
}

>From time to time erl_drv_thread_join returns error EDEADLK=35, i.e. the current thread (scheduler thread) tries to join itself.

According to the documentation “A Thread identifier may be reused very quickly after a thread has terminated. Therefore, if a thread corresponding to one of the involved thread identifiers has terminated since the thread identifier was saved, the result of erl_drv_equal_tids() might not give the expected result.”

I suppose that thread terminates earlier then erl_drv_thread_join call happens, so ErlDrvTid is already reused.

So the question is how to use erl_drv_thread_join properly and how to guarantee that the saved ErlDrvTid value points to the same data that was returned from erl_drv_thread_create?

Best Regards,
Sergey


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