Previously when I needed to herd a bunch of files / devices in real time
from a single process, I have used "select". It's in most of the
textbooks.
Last time I followed things around in the debugger I discovered that
lo and behold, select is implemented in terms of "poll" these days.
So the other day when I needed select again, I thought I'd try "poll"
instead.
Easier, tidier.
So that's the tip'o'the day... if you want to use select.
Don't.
Use "poll"
See "man poll" for more details.
NAME
poll, ppoll - wait for some event on a file descriptor
SYNOPSIS
#include <poll.h>
int poll(struct pollfd *fds, nfds_t nfds, int timeout);
#define _GNU_SOURCE
#include <poll.h>
int ppoll(struct pollfd *fds, nfds_t nfds,
const struct timespec *timeout, const sigset_t *sigmask);
DESCRIPTION
poll() performs a similar task to select(2): it waits for one of a set
of file descriptors to become ready to perform I/O.
The set of file descriptors to be monitored is specified in the fds
argument, which is an array of nfds structures of the following form:
struct pollfd {
int fd; /* file descriptor */
short events; /* requested events */
short revents; /* returned events */
};
The field fd contains a file descriptor for an open file.
The field events is an input parameter, a bit mask specifying the
events the application is interested in.
The field revents is an output parameter, filled by the kernel with the
events that actually occurred. The bits returned in revents can
include any of those specified in events, or one of the values POLLERR,
POLLHUP, or POLLNVAL. (These three bits are meaningless in the events
field, and will be set in the revents field whenever the corresponding
condition is true.)
If none of the events requested (and no error) has occurred for any of
the file descriptors, then poll() blocks until one of the events
occurs.
The timeout argument specifies an upper limit on the time for which
poll() will block, in milliseconds. Specifying a negative value in
timeout means an infinite timeout.
The bits that may be set/returned in events and revents are defined in
<poll.h>:
POLLIN There is data to read.
POLLPRI
There is urgent data to read (e.g., out-of-band data on
TCP socket; pseudo-terminal master in packet mode has
seen state change in slave).
POLLOUT
Writing now will not block.
POLLRDHUP (since Linux 2.6.17)
Stream socket peer closed connection, or shut down writ‐
ing half of connection. The _GNU_SOURCE feature test
macro must be defined in order to obtain this definition.
POLLERR
Error condition (output only).
POLLHUP
Hang up (output only).
POLLNVAL
Invalid request: fd not open (output only).
>snip<
RETURN VALUE
On success, a positive number is returned; this is the number of struc‐
tures which have non-zero revents fields (in other words, those
descriptors with events or errors reported). A value of 0 indicates
that the call timed out and no file descriptors were ready. On error,
-1 is returned, and errno is set appropriately.
John Carter Phone : (64)(3) 358 6639
Tait Electronics Fax : (64)(3) 359 4632
PO Box 1645 Christchurch Email : [email protected]
New Zealand
