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.Dd February 8, 2021
.Dt POLL 2
.Os
.Sh NAME
.Nm poll, pollts, ppoll
.Nd synchronous I/O multiplexing
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In poll.h
.Ft int
.Fn poll "struct pollfd *fds" "nfds_t nfds" "int timeout"
.In poll.h
.In signal.h
.In time.h
.Ft int
.Fn pollts "struct pollfd * restrict fds" "nfds_t nfds" "const struct timespec * restrict ts" "const sigset_t * restrict sigmask"
.In poll.h
.In signal.h
.In time.h
.Ft int
.Fn ppoll "struct pollfd * restrict fds" "nfds_t nfds" "const struct timespec * restrict ts" "const sigset_t * restrict sigmask"
.Sh DESCRIPTION
.Fn poll ,
.Fn pollts
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, one per file
to inspect, as defined in
.In poll.h
(shown below).
The
.Fa nfds
argument gives the size of the
.Fa fds
array.
.Pp
If
.Fa timeout
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), then
.Fn poll
blocks indefinitely.
If
.Fa timeout
is zero, then
.Fn poll
will return without blocking.
.Pp
Similarly, if
.Fa ts
is not a null pointer, it references a timespec structure which specifies a
maximum interval to wait for any file descriptor to become ready.
If
.Fa ts
is a null pointer,
.Fn pollts
and
.Fn ppoll
block indefinitely.
If
.Fa ts
is not a null pointer, referencing a zero-valued timespec structure, then
.Fn pollts
and
.Fn ppoll
will return without blocking.
.Pp
If
.Fa sigmask
is not a null pointer, then the
.Fn pollts
and
.Fn ppoll
functions replace the signal mask of the caller by the set of
signals pointed to by
.Fa sigmask
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 ugly
.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
.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 returns
\-1 and sets
.Dv errno
if an error occurred.
If the time limit expires,
.Fn poll
returns 0.
If
.Fn poll
returns with an error,
including one due to an interrupted call,
the
.Fa fds
array will be unmodified.
.Sh COMPATIBILITY
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
(e.g. trying to poll a
.Xr revoke 2 Ns d
descriptor), this implementation instead copies the
.Fa events
bitmask to the
.Fa revents
bitmask.
Attempting to perform I/O on this descriptor will then return an error.
This behavior is believed to be more useful.
.Pp
The
.Fn ppoll
function is a wrapper for
.Fn pollts
to provide compatibility with the Linux implementation.
.Sh ERRORS
An error return from
.Fn poll
indicates:
.Bl -tag -width Er
.It Bq Er EFAULT
.Fa fds
points outside the process's allocated address space.
.It Bq Er EINTR
A signal was delivered before the time limit expired and
before any of the selected events occurred.
.It Bq Er EINVAL
The specified time limit is negative or
the number of pollfd structures specified is larger than the current
file descriptor resource limit.
.El
.Sh SEE ALSO
.Xr accept 2 ,
.Xr connect 2 ,
.Xr read 2 ,
.Xr recv 2 ,
.Xr select 2 ,
.Xr send 2 ,
.Xr write 2
.Sh HISTORY
The
.Fn poll
function appeared in
.At V.3 ,
and was added to
.Nx
in
.Nx 1.3 .
The
.Fn pollts
function first appeared in
.Nx 3.0 .
The
.Fn ppoll
function first appeared in
.Nx 10.0 .
.Sh BUGS
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.