Very Computer

by **Nancy A. Mill** » Thu, 20 Apr 1995 04:00:00

I'm trying to use the select() call to handle IO for multiple sockets.
I'm a little confused about the FD_ISSET() macro. The man page for
select(3C) says this:

Quote:> FD_ISSET() is nonzero if fd is a member of fdset, zero otherwise.

I had assumed FD_ISSET() was supposed to be used to test whether
fd was ready for IO (after the select() call was made). If this
is not the case, how do you tell which file descriptors are ready
for IO after the select() call returns??

Thanks,
Nancy

by **Jeff Dickso** » Thu, 20 Apr 1995 04:00:00

Quote:
> I'm trying to use the select() call to handle IO for multiple sockets.
> I'm a little confused about the FD_ISSET() macro. The man page for
> select(3C) says this:

> > FD_ISSET() is nonzero if fd is a member of fdset, zero otherwise.

> I had assumed FD_ISSET() was supposed to be used to test whether
> fd was ready for IO (after the select() call was made). If this
> is not the case, how do you tell which file descriptors are ready
> for IO after the select() call returns??

> Thanks,
> Nancy

Have you read the man page? FD_ISSET use is explained therein. It
does pretty much what the name suggests and that is test whether a
particular descripter is set. You see select just returns to you
the number of descriptors that are ready for IO. Finding out which
is what the FD_ISSET macro is for.

Jeff

by **Patrick Horg** » Thu, 20 Apr 1995 04:00:00

After the call to select, the fdset is a subset of the set you put in it
originally! If you passed the fdset for read access, any bits still
pulled up after the select are fds that have data ready for you to read.

Patrick
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by **Matt DiM** » Fri, 21 Apr 1995 04:00:00

>
>> FD_ISSET() is nonzero if fd is a member of fdset, zero otherwise.
>
>I had assumed FD_ISSET() was supposed to be used to test whether
>fd was ready for IO (after the select() call was made). If this

You are correct. The tricky thing going on here is that select(2) actually
modifies the fd_set you pass in, removing the unready fd's from the set.

-Matt

Very Computer

FD_ISSET() - what does it mean?

FD_ISSET() - what does it mean?

FD_ISSET() - what does it mean?

FD_ISSET() - what does it mean?

FD_ISSET() - what does it mean?