[FFmpeg-devel] [PATCH 10/11] avformat/fifo: Add AVFMT_FLAG_NONBLOCK support

Jan Sebechlebsky sebechlebskyjan at gmail.com
Thu Aug 4 12:28:38 EEST 2016

On 08/03/2016 03:41 PM, Nicolas George wrote:
> Le sextidi 16 thermidor, an CCXXIV, sebechlebskyjan at gmail.com a écrit :
>> +    if (ctx->termination_requested)
>> +        return 1;
> This is a common misconception: volatile is not enough for thread
> communication. IIUC, volatile forces the compiler to issue actual store and
> fetch operations to memory, but not a memory barrier. That means it can be
> used for communication with a signal handler (provided the object is small
> enough so that operations are atomic, cf. sig_atomic_t) or a hardware
> device. But for inter-thread communication on SMP systems, the lack of
> memory barrier means that the change may be limited to the processor's cache
> and only reach the central shared memory and the other core's cache at some
> indeterminate point later.
> For inter-thread communication, you either need a mutex or a special atomic
> and synchronized operation. The second solution is more efficient, but
> currently less portable. You can have a look at libavutil/atomic.h, and also
> at Anton's efforts to replace it with a more standard and modern API, it
> happened just last week on the fork's mailing-list.
This was kind of intentional, since the flag is uint8_t and only thing 
what happens is
that it is set to 1(from initial 0) at some point of time I think the 
worst scenario is that flag will be detected later (but that also 
happens when the lock is used), there is no risk that corrupted value 
will be read.
     But I agree this is probably not a good practice, so I will change 
the flag to be int and use atomic get/set.
>> +
>> +    return ff_check_interrupt(&ctx->orig_interrupt_callback);
> I am not sure if this part is worth your efforts: if the application wants
> to cancel a write, it uses the FIFO muxer APIs, it does not need to set up
> an interrupt callback. Do you see a case where it would be useful?
I think there is no reason to disable custom interrupt callback here, 
there is almost no
overhead if it is used, user can choose to ignore it, but when he 
decides to use it it will work as expected.
>> +        ret = pthread_tryjoin_np( fifo->writer_thread, NULL);
> This function is a GNU extension (np means nonportable), this is the reason
> for the build failure reported by Michael; I am rather surprised you did not
> get it too since the build options should hide nonportable functions.
> Anyway, this functions is rather unnecessary, you can just set a flag just
> before the thread terminates and check the flag before pthread_join().
Yes, this was a mistake, I've changed that to use flag.



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