path: root/usr/src/man/man2/poll.2

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.TH POLL 2 "Aug 23, 2001"
.SH NAME
poll \- input/output multiplexing
.SH SYNOPSIS
.LP
.nf
#include <poll.h>

\fBint\fR \fBpoll\fR(\fBstruct pollfd\fR \fIfds[]\fR, \fBnfds_t\fR \fInfds\fR, \fBint\fR \fItimeout\fR);
.fi

.SH DESCRIPTION
.sp
.LP
The \fBpoll()\fR function provides applications with a mechanism for
multiplexing input/output over a set of file descriptors.  For each member of
the array pointed to by \fIfds\fR, \fBpoll()\fR examines the given file
descriptor for the event(s) specified in \fIevents\fR. The number of
\fBpollfd\fR structures in the \fIfds\fR array is specified by \fInfds\fR. The
\fBpoll()\fR function identifies those file descriptors on which an application
can read or write data, or on which certain events have occurred.
.sp
.LP
The \fIfds\fR argument specifies the file descriptors to be examined and the
events of interest for each file descriptor.  It is a pointer to an array with
one member for each open file descriptor of interest.  The array's members are
\fBpollfd\fR structures, which contain the following members:
.sp
.in +2
.nf
int     fd;        /* file descriptor */
short   events;    /* requested events */
short   revents;   /* returned events */
.fi
.in -2

.sp
.LP
The \fBfd\fR member specifies an open file descriptor and the \fBevents\fR and
\fBrevents\fR members are bitmasks constructed by a logical \fBOR\fR operation
of any combination of the following event flags:
.sp
.ne 2
.na
\fB\fBPOLLIN\fR\fR
.ad
.RS 14n
Data other than high priority data may be read without blocking. For streams,
this flag is set in \fBrevents\fR even if the message is of zero length.
.RE

.sp
.ne 2
.na
\fB\fBPOLLRDNORM\fR\fR
.ad
.RS 14n
Normal data (priority band equals 0) may be read without blocking. For streams,
this flag is set in \fBrevents\fR even if the message is of zero length.
.RE

.sp
.ne 2
.na
\fB\fBPOLLRDBAND\fR\fR
.ad
.RS 14n
Data from a non-zero priority band may be read without blocking. For streams,
this flag is set in \fBrevents\fR even if the message is of zero length.
.RE

.sp
.ne 2
.na
\fB\fBPOLLPRI\fR\fR
.ad
.RS 14n
High priority data may be received without blocking. For streams, this flag is
set in \fBrevents\fR even if the message is of zero length.
.RE

.sp
.ne 2
.na
\fB\fBPOLLOUT\fR\fR
.ad
.RS 14n
Normal data (priority band equals 0) may be written without blocking.
.RE

.sp
.ne 2
.na
\fB\fBPOLLWRNORM\fR\fR
.ad
.RS 14n
The same as  \fBPOLLOUT\fR.
.RE

.sp
.ne 2
.na
\fB\fBPOLLWRBAND\fR\fR
.ad
.RS 14n
Priority data (priority band > 0) may be written.  This event only examines
bands that have been written to at least once.
.RE

.sp
.ne 2
.na
\fB\fBPOLLERR\fR\fR
.ad
.RS 14n
An error has occurred on the device or stream.  This flag is only valid in the
\fBrevents\fR bitmask; it is not used in the \fBevents\fR member.
.RE

.sp
.ne 2
.na
\fB\fBPOLLHUP\fR\fR
.ad
.RS 14n
A hangup has occurred on the stream. This event and  \fBPOLLOUT\fR are mutually
exclusive; a stream can never be writable if a hangup has occurred. However,
this event and  \fBPOLLIN\fR, \fBPOLLRDNORM\fR, \fBPOLLRDBAND\fR, or
\fBPOLLPRI\fR are not mutually exclusive. This flag is only valid in the
\fBrevents\fR bitmask; it is not used in the \fBevents\fR member.
.RE

.sp
.ne 2
.na
\fB\fBPOLLNVAL\fR\fR
.ad
.RS 14n
The specified \fBfd\fR value does not belong to an open file. This flag is only
valid in the \fBrevents\fR member; it is not used in the \fBevents\fR member.
.RE

.sp
.LP
If the value \fBfd\fR is less than 0, \fBevents\fR is ignored and \fBrevents\fR
is set to 0 in that entry on return from  \fBpoll()\fR.
.sp
.LP
The results of the \fBpoll()\fR query are stored in the \fBrevents\fR member in
the \fBpollfd\fR structure. Bits are set in the \fBrevents\fR bitmask to
indicate which of the requested events are true. If none are true, none of the
specified bits are set in \fBrevents\fR when the \fBpoll()\fR call returns. The
event flags  \fBPOLLHUP\fR, \fBPOLLERR\fR, and  \fBPOLLNVAL\fR are always  set
in \fBrevents\fR if the conditions they indicate are true; this occurs even
though these flags were not present in \fBevents\fR.
.sp
.LP
If none of the defined events have occurred on any selected file descriptor,
\fBpoll()\fR waits at least \fItimeout\fR milliseconds for an event to occur on
any of the selected file descriptors. On a computer where millisecond timing
accuracy is not available, \fItimeout\fR is rounded up to the nearest legal
value available on that system. If the value \fItimeout\fR is 0, \fBpoll()\fR
returns immediately. If the value of \fItimeout\fR is  \(mi1, \fBpoll()\fR
blocks until a requested event occurs or until the call is interrupted.  The
\fBpoll()\fR function is not affected by the \fBO_NDELAY\fR and
\fBO_NONBLOCK\fR flags.
.sp
.LP
The \fBpoll()\fR function supports regular files, terminal and pseudo-terminal
devices, streams-based files, FIFOs and pipes.  The behavior of \fBpoll()\fR on
elements of \fIfds\fR that refer to other types of file is unspecified.
.sp
.LP
The \fBpoll()\fR function supports sockets.
.sp
.LP
A file descriptor for a socket that is listening for connections will indicate
that it is ready for reading, once connections are available.  A file
descriptor for a socket that is connecting asynchronously will indicate that it
is ready for writing, once a connection has been established.
.sp
.LP
Regular files always \fBpoll()\fR \fBTRUE\fR for reading and writing.
.SH RETURN VALUES
.sp
.LP
Upon successful completion, a non-negative value is returned. A positive value
indicates the total number of file descriptors that has been selected (that is,
file descriptors for which the \fBrevents\fR member is non-zero). A value of
\fB0\fR indicates that the call timed out and no file descriptors have been
selected. Upon failure, \fB\(mi1\fR is returned and \fBerrno\fR is set to
indicate the error.
.SH ERRORS
.sp
.LP
The \fBpoll()\fR function will fail if:
.sp
.ne 2
.na
\fB\fBEAGAIN\fR\fR
.ad
.RS 10n
Allocation of internal data structures failed, but the request may be attempted
again.
.RE

.sp
.ne 2
.na
\fB\fBEFAULT\fR\fR
.ad
.RS 10n
Some argument points to an illegal address.
.RE

.sp
.ne 2
.na
\fB\fBEINTR\fR\fR
.ad
.RS 10n
A signal was caught during the \fBpoll()\fR function.
.RE

.sp
.ne 2
.na
\fB\fBEINVAL\fR\fR
.ad
.RS 10n
The argument \fInfds\fR is greater than \fB{OPEN_MAX}\fR, or one of the
\fBfd\fR members refers to a stream or multiplexer that is linked (directly or
indirectly) downstream from a multiplexer.
.RE

.SH ATTRIBUTES
.sp
.LP
See \fBattributes\fR(5) for descriptions of the following attributes:
.sp

.sp
.TS
box;
c | c
l | l .
ATTRIBUTE TYPE	ATTRIBUTE VALUE
_
Interface Stability	Standard
.TE

.SH SEE ALSO
.sp
.LP
\fBIntro\fR(2), \fBgetmsg\fR(2), \fBgetrlimit\fR(2), \fBputmsg\fR(2),
\fBread\fR(2), \fBwrite\fR(2), \fBselect\fR(3C), \fBattributes\fR(5),
\fBstandards\fR(5), \fBchpoll\fR(9E)
.sp
.LP
\fISTREAMS Programming Guide\fR
.SH NOTES
.sp
.LP
Non-STREAMS drivers use  \fBchpoll\fR(9E) to implement  \fBpoll()\fR on these
devices.