30 Nov
2015
30 Nov
'15
6 a.m.
El 30/11/15 a les 14:27, Robin Gareus ha escrit:
On 11/30/2015 12:11 PM, Xavier Mendez wrote:
This is going to run on UNIX-like systems, mostly Linux, and I'm
programming in C/C++.
Hello,
I'm wondering whether it's safe to do non-blocking reads or writes from
inside the process callback.
From what I've seen, non-blocking I/O doesn't cause the process to go
into blocked state, and the realtime scheduler should not switch to
another process. But the documentation doesn't seem to allow them:
On which platform? [...] it cannot call functions that might block
for a long time. This
includes all I/O functions (disk, TTY, network), [...]
So, is it safe to use non-blocking I/O in the process callback?
The short answer is:
"If you don’t know how long it will take, don't do it." [1]
All i/o involve syscalls, and then it depends what the kernel does for
the specific system call(s). Asynchronous I/O usually involves signals
at some point which makes it not safe to use.
I wasn't very specific, let me clarify: I'm only going to do some
read(2) or write(2) syscalls on an FD which has O_NONBLOCK set.
AFAIK these don't involve signals...
Some implementation also
involve mutexes to avoid resource conflicts. You'll have to check the
standard-lib and kernel source for the system that you target.
Hmm... Are mutexes a problem if the FD is used exclusively from the
process callback?
All moot anyway. Proper software needs to do
error-handling and doing
that in a rt-callback is out of the question. So you need a non-realtime
thread anyway and if you have that you can directly do i/o there.
Error handling is not a problem here, if those syscalls fail I'll simply
deactivate the JACK client.
anyway, using a ringbuffer to decouple i/o is trivial:
e.g.
https://github.com/jackaudio/example-clients/blob/master/capture_client.c
Thanks for the advice, I'm currently using ringbuffers + worker threads
but being able to do this I/O directly in process() would simplify the
code considerably. I'll consider it, though.
Thank you for your help,
Xavi