WARNING: select() can monitor only file descriptors numbers that are
less than FD_SETSIZE (1024)—an unreasonably low limit for many modern
applications—and this limitation will not change. All modern applica‐
tions should instead use poll(2) or epoll(7), which do not suffer this
limitation.
You don't have to recompile, just do the following (at least on glibc):
#include <sys/types.h> // pull in initial definition of __FD_SETSIZE
#undef __FD_SETSIZE
#define __FD_SETSIZE 32768 // or whatever
#include <sys/select.h> // won't include the internal <bits/types.h> again
This is a rare case when `-Wsystem-headers` is useful to enable (and these days system headers are usually pretty clean) - it will catch if you accidentally define `__FD_SETSIZE` before the system does.
Note that `select` is still the nicest API in a lot of ways - `poll` wastes space gratuitously, `epoll` requires lots of finicky `modify` syscalls, and `io_uring` is frankly not sane.
That said:
* if you're only dealing with a couple FDs, use `poll`.
* it's not that hard to take a day and think about epoll write buffer management. You need to consider every combination of:
epoll state is/isn't checking writability (you want to only change this lazily)
on the previous/current iteration, was there nothing/something in the write buffer?
prior actual write was would-block/actually-incomplete/spuriously-incomplete/complete
current actual write ends up would-block/actually-incomplete/spuriously-incomplete/complete
There are many "correct" answers, but I suspect the optimal answer for epoll is something like: initially, write optimistically (and do this before the wait). If you fail to write anything at all, enable the kernel flag. For FDs that you've previously enabled the flag for, if you don't have anything to write this time, disable the flag; otherwise, don't actually write until after the wait (it is guaranteed to return immediately if the write would be allowed, after all, but you'll also get other events that happen to be ready). If you trust your event handlers to return quickly, you can defer any indicated writes until the next wait, otherwise do them before handling events.
