I write applications against an API defined as Linux character devices with most of the interesting parts of the API defined as ioctl() calls, where asynchronous events are communicated to the application when the drivers raise POLLPRI through poll(). I would like to try improving the application side of this using libuv.
I have tried out one device where the driver expects read() calls and therefore it raises POLLIN. I used a uv_poll_t with UV_READABLE, and that was fine. However at least two of the devices use POLLPRI exclusively as they change state.
Sketch of code:
int fd = open("/dev/..fancy..", O_RDWR);
if (fd < 0) { ... exit or something ... }
int res = ioctl(fd, FANCY_CONFIGURE, ...);
if (res != 0) { ... exit or something ... }
struct pollfd pfd[1] = { fd, POLLPRI, 0 };
int pr = poll(pfd, 1, -1);
if (pr != 1) { ... }
int events;
res = ioctl(fd, FANCY_GET_EVENTS, &events);
// examine events value to see what happened
I know from the docs and from experiment that if the driver only raises POLLPRI, as it is required by specification to do, then a uv_poll_t won't work.
I'm considering some sort of prep/check/async combination, but other frameworks (bespoke ones) that are built to work with this API use the "big call to poll" pattern and are designed to handle POLLPRI as well. I think if the polls for POLLPRI are properly integrated with whatever else libuv has going on (and I have plenty of use for libuv's sockets too) it would be more efficient and more pleasant.
So if I've missed something, please show me. If I am correct that there is little direct support for POLLPRI, I'd like some elegant proposals from folks who know their way around libuv.
Currently libuv only sets POLLIN for reading. I was curious and made a quick patch which passes our test suite and sets POLLPRI alongside with POLLIN on Linux: https://gist.github.com/saghul/5523f8db12a52a2bc12a
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