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Rework the text of poll(2) for clarity. Bump date.

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dholland 2021-02-09 00:50:47 +00:00
parent 03f821dbf2
commit cf51226d61
1 changed files with 125 additions and 97 deletions
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222
lib/libc/sys/poll.2
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@ -1,4 +1,4 @@
.\" $NetBSD: poll.2,v 1.33 2021/02/07 18:22:51 rillig Exp $
.\" $NetBSD: poll.2,v 1.34 2021/02/09 00:50:47 dholland Exp $
.\"
.\" Copyright (c) 1998, 2005, 2020 The NetBSD Foundation, Inc.
.\" All rights reserved.
@ -27,7 +27,7 @@
.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
.\" POSSIBILITY OF SUCH DAMAGE.
.\"
.Dd May 25, 2020
.Dd February 8, 2021
.Dt POLL 2
.Os
.Sh NAME
@ -56,95 +56,27 @@ and
.Fn ppoll
examine a set of file descriptors to see if some of them are ready for
I/O.
For each object inspected, the caller provides a list of conditions
(called ``events'') to check for, and the kernel returns a list of
conditions that are true.
The intent, as with
.Xr select 2 ,
is to check for whether I/O is possible before performing any, so as
to permit a top-level event loop to process input from many sources
(and output to many destinations)
without blocking on any of them and thus becoming stuck.
.Ss Arguments
The
.Fa fds
argument is a pointer to an array of pollfd structures as defined in
argument is a pointer to an array of pollfd structures, one per file
to inspect, as defined in
.In poll.h
(shown below).
The
.Fa nfds
argument determines the size of the
argument gives the size of the
.Fa fds
array.
.Bd -literal
struct pollfd {
int fd; /* file descriptor */
short events; /* events to look for */
short revents; /* events returned */
};
.Ed
.Pp
The fields of
.Fa struct pollfd
are as follows:
.Bl -tag -width XXXrevents
.It fd
File descriptor to poll.
If the value in
.Em fd
is negative, the file descriptor is ignored and
.Em revents
is set to 0.
.It events
Events to poll for.
(See below.)
.It revents
Events which may occur.
(See below.)
.El
.Pp
The event bitmasks in
.Fa events
and
.Fa revents
have the following bits:
.Bl -tag -width XXXPOLLWRNORM
.It POLLIN
Data other than high priority data may be read without blocking.
.It POLLRDNORM
Normal data may be read without blocking.
.It POLLRDBAND
Data with a non-zero priority may be read without blocking.
.It POLLPRI
High priority data may be read without blocking.
.It POLLOUT
Normal data may be written without blocking.
.It POLLWRNORM
Equivalent to
POLLOUT.
.It POLLWRBAND
Data with a non-zero priority may be written without blocking.
.It POLLERR
An exceptional condition has occurred on the device or socket.
This flag is always checked, even if not present in the
.Fa events
bitmask.
.It POLLHUP
The device or socket has been disconnected.
This flag is always
checked, even if not present in the
.Fa events
bitmask.
Note that
POLLHUP
and
POLLOUT
should never be present in the
.Fa revents
bitmask at the same time.
If the remote end of a socket is closed,
.Fn poll
returns a
POLLIN
event, rather than a
POLLHUP.
.It POLLNVAL
The file descriptor is not open.
This flag is always checked, even
if not present in the
.Fa events
bitmask.
.El
.Pp
If
.Fa timeout
@ -152,14 +84,16 @@ is neither zero nor INFTIM (\-1), it specifies a maximum interval to
wait for any file descriptor to become ready, in milliseconds.
If
.Fa timeout
is INFTIM (\-1), the poll blocks indefinitely.
is INFTIM (\-1), then
.Pn poll
blocks indefinitely.
If
.Fa timeout
is zero, then
.Fn poll
will return without blocking.
.Pp
If
Similarly, if
.Fa ts
is a non-null pointer, it references a timespec structure which specifies a
maximum interval to wait for any file descriptor to become ready.
@ -169,7 +103,7 @@ is a null pointer,
.Fn pollts
and
.Fn ppoll
blocks indefinitely.
block indefinitely.
If
.Fa ts
is a non-null pointer, referencing a zero-valued timespec structure, then
@ -184,11 +118,97 @@ is a non-null pointer, then the
.Fn pollts
and
.Fn ppoll
function shall replace the signal mask of the caller by the set of
functions replace the signal mask of the caller by the set of
signals pointed to by
.Fa sigmask
before examining the descriptors, and shall restore the signal mask
of the caller before returning.
while the call is in progress, and restore the caller's original
signal mask before returning.
.Pp
The
.Vt pollfd
structure:
.Bd -literal
struct pollfd {
int fd; /* file descriptor */
short events; /* events to look for */
short revents; /* events returned */
};
.Ed
.\" without this Pp there is no blank line after the struct which is oogly
.Pp
The fields of
.Fa struct pollfd
are as follows:
.Pp
.Bl -tag -width XXXrevents -compact
.It Fa fd
File descriptor to poll.
(Input)
.It Fa events
Conditions to poll for.
(Input)
.It Fa revents
Conditions that are true.
(Output)
.El
.Pp
.Ss Conditions
There are four conditions that can be placed in
.Fa events
and reported in
.Fa revents :
.Pp
.Bl -tag -width XXXPOLLWRNORM -compact
.It POLLRDNORM
Normal data may be read without blocking.
.It POLLRDBAND
Urgent/out-of-band data may be read without blocking.
.It POLLWRNORM
Normal data may be written without blocking.
.It POLLWRBAND
Urgent/out-of-band data may be written without blocking.
.El
.Pp
There are three more conditions that are always checked for regardless
of
.Fa events
and thus may always be reported in
.Fa revents :
.Pp
.Bl -tag -width XXXPOLLWRNORM -compact
.It POLLERR
An exceptional condition has occurred on the object.
.It POLLHUP
The object has been disconnected.
.It POLLNVAL
The file descriptor is not open.
.El
.Pp
The following additional flags are defined:
.Pp
.Bl -tag -width XXXPOLLWRNORM -compact
.It POLLIN
Synonym for POLLRDNORM.
.It POLLOUT
Synonym for POLLWRNORM.
.It POLLPRI
Synonym for POLLRDBAND.
.El
.Ss Notes
If the value passed in
.Fa fd
is negative, the entry is ignored and
.Fa revents
is set to 0.
(POLLNVAL is
.Em not
set.)
.Pp
No file descriptor will ever produce POLLHUP at the same time as POLLWRNORM.
.\" (XXX but what about POLLWRBAND? POLLRDBAND? POLLRDNORM?)
.Pp
Sockets produce POLLIN rather than POLLHUP when the remote
end is closed.
.Sh RETURN VALUES
.Fn poll
returns the number of descriptors that are ready for I/O, or \-1 if an
@ -204,8 +224,8 @@ the
.Fa fds
array will be unmodified.
.Sh COMPATIBILITY
This implementation differs from the historical one in that a given
file descriptor may not cause
This implementation differs from the historical one in that no individual
file descriptor may cause
.Fn poll
to return with an error.
In cases where this would have happened in the historical implementation
@ -262,9 +282,17 @@ The
function first appeared in
.Nx 10.0 .
.Sh BUGS
The distinction between some of the fields in the
.Fa events
and
.Fa revents
bitmasks is really not useful without STREAMS.
The fields are defined for compatibility with existing software.
As of this writing, in the underlying implementation, POLLIN and
POLLPRI are distinct flags from POLLRDNORM and POLLRDBAND
(respectively) and the behavior is not always exactly identical.
.Pp
The detailed behavior of specific flags is not very portable from one
OS to another.
.\" The old note, which is too vague to be helpful:
.\"
.\" The distinction between some of the fields in the
.\" .Fa events
.\" and
.\" .Fa revents
.\" bitmasks is really not useful without STREAMS.
.\" The fields are defined for compatibility with existing software.