select RBITS,WBITS,EBITS,TIMEOUT. The meaning of all the *BITS

igoryonya has asked for the wisdom of the Perl Monks concerning the following question:

I am reading documentation on select command and I can't figure out, what these RBITS,WBITS,EBITS mean.

Well, I came to guess, that probably, RBITS - means Read Bits, WBITS - Write Bits, but what about EBITS, then?
What does this construct mean? What is it used for? Manual doesn't explain anything. Just throws these:

select RBITS,WBITS,EBITS,TIMEOUT
my ($nfound, $timeleft) = select(my $rout = $rin, my $wout = $win, my $eout = $ein, $timeout);

without any explanation.

Comment on select RBITS,WBITS,EBITS,TIMEOUT. The meaning of all the *BITS

Replies are listed 'Best First'.
Re: select RBITS,WBITS,EBITS,TIMEOUT. The meaning of all the *BITS by Fletch (Bishop) on Oct 23, 2020 at 02:04 UTC
`EBITS` is for error filehandles. The select builtin is a thin layer over the underlying `select(2)` system call so familiarity with that can help (see your OS' manual pages or a tome like e.g. Stevens, Advanced Programming in the UNIX Environment). Unless you've got a burning desire to program that close to the metal as it were you may be better served using IO::Select (or stepping even further back and using an event loop like Mojo::IOLoop, AnyEvent, or POE). Edit: And I just bothered to pull my copy off the shelf and skim and I'm a bit vague still. That's filehandles which you want to know if/when they have errors or exception conditions present. The cake is a lie. The cake is a lie. The cake is a lie.	[reply] [d/l] [select]
Re^2: select RBITS,WBITS,EBITS,TIMEOUT. The meaning of all the *BITS by Anonymous Monk on Oct 23, 2020 at 02:46 UTC
yeah man select https://perldoc.perl.org/functions/select#select-RBITS,WBITS,EBITS,TIMEOUT even links http://man.he.net/man2/select select links https://en.m.wikipedia.org/wiki/Select_(Unix) links https://pubs.opengroup.org/onlinepubs/9699919799/functions/select.html	[reply]
Re^2: select RBITS,WBITS,EBITS,TIMEOUT. The meaning of all the *BITS by ikegami (Patriarch) on Oct 25, 2020 at 11:16 UTC
EBITS is for error filehandles. No. What does that even mean? It's used to specify the handles to monitor for exceptional conditions. See my answer for details.	[reply]
Re: select RBITS,WBITS,EBITS,TIMEOUT. The meaning of all the *BITS by ikegami (Patriarch) on Oct 25, 2020 at 11:12 UTC
It's used to specify the handles to monitor for exceptional conditions. If you need it, you'd know it. Perl's `select` is a thin wrapper for the system call of the same name. From my `select`(2) man page: The file descriptors in `exceptfds` will be watched for exceptional conditions. (For examples of some exceptional conditions, see the discussion of `POLLPRI` in `poll`(2).) From my `poll`(2) man page: `POLLPRI` There is some exceptional condition on the file descriptor. Possibilities include: There is out-of-band data on a TCP socket (see `tcp`(7)). A pseudoterminal master in packet mode has seen a state change on the slave (see `ioctl_tty`(2)). A `cgroup.events` file has been modified (see `cgroups`(7)). By the way, IO::Select presents a much better interface.	[reply] [d/l] [select]

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