Question on select behaviour

exman · 08-29-2006, 06:20 AM

Hi,
I have some specific hardware here and the driver does not seem to support select calls on the filedescriptor as far as I can see (driver does not contain poll()).
Now if I try to use the descriptor in a select call, it returns without changing the fd_sets but also returns no error.
Is that how it is intended to behave?
Any advise would be appreciated.

exman

exman · 08-30-2006, 12:13 PM

Ok I found the answer myself.
http://lwn.net/images/pdf/LDD3/ch03.pdf page 10

Quote:

unsigned int (*poll) (struct file *, struct poll_table_struct *);
The poll method is the back end of three system calls: poll, epoll, and select, all of
which are used to query whether a read or write to one or more file descriptors
would block. The poll method should return a bit mask indicating whether non-
blocking reads or writes are possible, and, possibly, provide the kernel with
information that can be used to put the calling process to sleep until I/O
becomes possible. If a driver leaves its poll method NULL, the device is assumed to
be both readable and writable without blocking.

So it seems that the behaviour of select is correct.
exman