select function question

select function question

[Ameer Armaly]

Hi all.
I have some question about select:
1. one of the parameters is called writefds; what does this mean?
2. what does exceptfds mean?
3. how do I automatically know when an fd has new data?

Re: select function question

[Jan-Benedict Glaw]

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On Fri, 2004-10-01 17:51:08 -0400, Ameer Armaly <ameer@charter.net>
wrote in message <Pine.LNX.4.61.0410011749450.20884@debian>:
> I have some question about select:
> 1. one of the parameters is called writefds; what does this mean?

The first set contains file descriptors you don't want to block upon
read(). The second set (writefds) contains a set of file descriptors you
don't want to block upon write(). Consider pipes or something like that
with a limited local buffer (on the kernel side).

> 2. what does exceptfds mean?

Never used these, but I guess these would make some sense in connection
to timed out network sockets or the like.

> 3. how do I automatically know when an fd has new data?

Actually, you don't:-)  select() will only tell you that you won't block
at the next read() or write() call. It doesn't actually mean that you
really can read any data from it -- it may return an error and set errno
instead!

However, for some file descriptor types (ie. serial ports or network
sockets) there are ways to determine if there's actually data available.

MfG, JBG

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