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11 .TH POLL 2 "Aug 23, 2001"
12 .SH NAME
13 poll \- input/output multiplexing
14 .SH SYNOPSIS
15 .LP
16 .nf
17 #include <poll.h>
19 \fBint\fR \fBpoll\fR(\fBstruct pollfd\fR \fIfds[]\fR, \fBnfds_t\fR \fInfds\fR, \fBint\fR \fItimeout\fR);
21 \fBint\fR \fBppoll\fR(\fBstruct pollfd *restrict\fR \fIfds\fR, \fBnfds_t\fR \fInfds\fR,
22 \fBconst struct timespec *restrict\fR \fItsp\fR, \fBconst sigset_t *restrict\fR \fIsigmask\fR);
23 .fi
25 .SH DESCRIPTION
26 .LP
27 The \fBpoll()\fR and \fBppoll()\fR functions provides applications with a
28 mechanism for multiplexing input/output over a set of file descriptors. For
29 each member of the array pointed to by \fIfds\fR, \fBpoll()\fR and \fBppoll()\fR
30 examine the given file descriptor for the event(s) specified in \fIevents\fR.
31 The number of \fBpollfd\fR structures in the \fIfds\fR array is specified by
32 \fInfds\fR. The \fBpoll()\fR and \fBppoll()\fR functions identify those file
33 descriptors on which an application can read or write data, or on which certain
34 events have occurred.
35 .sp
36 .LP
37 The \fBppoll()\fR function behaves identically to \fBpoll()\fR, except as follows:
38 .RS +4
39 .TP
40 .ie t \(bu
41 .el o
42 For the \fBppoll\fR function, the timeout period is given in seconds and
43 nanoseconds in an argument of type \fBstruct timespec\fR, where as \fBpoll()\fR
44 takes a timeout in milliseconds in the form of an integer argument.
45 .RE
46 .RS +4
47 .TP
48 .ie t \(bu
49 .el o
50 The \fBppoll()\fR function takes an optional \fIsigmask\fR argument. When a
51 non-\fBNULL\fR pointer is passed, the calling threads signal mask is replaced by
52 the one specified in \fIsigset\fR before examining file descriptors, and
53 restored before returning.
54 .RE
55 .sp
56 .LP
57 The \fIfds\fR argument specifies the file descriptors to be examined and the
58 events of interest for each file descriptor. It is a pointer to an array with
59 one member for each open file descriptor of interest. The array's members are
60 \fBpollfd\fR structures, which contain the following members:
61 .sp
62 .in +2
63 .nf
64 int fd; /* file descriptor */
65 short events; /* requested events */
66 short revents; /* returned events */
67 .fi
68 .in -2
70 .sp
71 .LP
72 The \fBfd\fR member specifies an open file descriptor and the \fBevents\fR and
73 \fBrevents\fR members are bitmasks constructed by a logical \fBOR\fR operation
74 of any combination of the following event flags:
75 .sp
76 .ne 2
77 .na
78 \fB\fBPOLLIN\fR\fR
79 .ad
80 .RS 14n
81 Data other than high priority data may be read without blocking. For streams,
82 this flag is set in \fBrevents\fR even if the message is of zero length.
83 .RE
85 .sp
86 .ne 2
87 .na
88 \fB\fBPOLLRDNORM\fR\fR
89 .ad
90 .RS 14n
91 Normal data (priority band equals 0) may be read without blocking. For streams,
92 this flag is set in \fBrevents\fR even if the message is of zero length.
93 .RE
95 .sp
96 .ne 2
97 .na
98 \fB\fBPOLLRDBAND\fR\fR
99 .ad
100 .RS 14n
101 Data from a non-zero priority band may be read without blocking. For streams,
102 this flag is set in \fBrevents\fR even if the message is of zero length.
103 .RE
105 .sp
106 .ne 2
107 .na
108 \fB\fBPOLLPRI\fR\fR
109 .ad
110 .RS 14n
111 High priority data may be received without blocking. For streams, this flag is
112 set in \fBrevents\fR even if the message is of zero length.
113 .RE
115 .sp
116 .ne 2
117 .na
118 \fB\fBPOLLOUT\fR\fR
119 .ad
120 .RS 14n
121 Normal data (priority band equals 0) may be written without blocking.
122 .RE
124 .sp
125 .ne 2
126 .na
127 \fB\fBPOLLWRNORM\fR\fR
128 .ad
129 .RS 14n
130 The same as \fBPOLLOUT\fR.
131 .RE
133 .sp
134 .ne 2
135 .na
136 \fB\fBPOLLWRBAND\fR\fR
137 .ad
138 .RS 14n
139 Priority data (priority band > 0) may be written. This event only examines
140 bands that have been written to at least once.
141 .RE
143 .sp
144 .ne 2
145 .na
146 \fB\fBPOLLERR\fR\fR
147 .ad
148 .RS 14n
149 An error has occurred on the device or stream. This flag is only valid in the
150 \fBrevents\fR bitmask; it is not used in the \fBevents\fR member.
151 .RE
153 .sp
154 .ne 2
155 .na
156 \fB\fBPOLLHUP\fR\fR
157 .ad
158 .RS 14n
159 A hangup has occurred on the stream. This event and \fBPOLLOUT\fR are mutually
160 exclusive; a stream can never be writable if a hangup has occurred. However,
161 this event and \fBPOLLIN\fR, \fBPOLLRDNORM\fR, \fBPOLLRDBAND\fR, or
162 \fBPOLLPRI\fR are not mutually exclusive. This flag is only valid in the
163 \fBrevents\fR bitmask; it is not used in the \fBevents\fR member.
164 .RE
166 .sp
167 .ne 2
168 .na
169 \fB\fBPOLLNVAL\fR\fR
170 .ad
171 .RS 14n
172 The specified \fBfd\fR value does not belong to an open file. This flag is only
173 valid in the \fBrevents\fR member; it is not used in the \fBevents\fR member.
174 .RE
176 .sp
177 .LP
178 If the value \fBfd\fR is less than 0, \fBevents\fR is ignored and \fBrevents\fR
179 is set to 0 in that entry on return from \fBpoll()\fR and \fBppoll()\fR.
180 .sp
181 .LP
182 The results of the \fBpoll()\fR or \fBppoll()\fR query are stored in the
183 \fBrevents\fR member in the \fBpollfd\fR structure. Bits are set in the
184 \fBrevents\fR bitmask to indicate which of the requested events are true. If
185 none are true, none of the specified bits are set in \fBrevents\fR when either
186 the \fBpoll()\fR or \fBppoll()\fR call returns. The event flags \fBPOLLHUP\fR,
187 \fBPOLLERR\fR, and \fBPOLLNVAL\fR are always set in \fBrevents\fR if the
188 conditions they indicate are true; this occurs even though these flags were not
189 present in \fBevents\fR.
190 .sp
191 .LP
192 If none of the defined events have occurred on any selected file descriptor,
193 \fBpoll()\fR and \fBppoll()\fR wait at least \fItimeout\fR milliseconds for an
194 event to occur on any of the selected file descriptors. On a computer where
195 millisecond timing accuracy is not available, \fItimeout\fR is rounded up to the
196 nearest legal value available on that system. If the value \fItimeout\fR is 0,
197 \fBpoll()\fR returns immediately. If the value of \fItimeout\fR is \(mi1,
198 \fBpoll()\fR blocks until a requested event occurs or until the call is
199 interrupted. If the value of \fBtsp\fR is \fBNULL\fR, then \fBppoll()\fR blocks
200 until a requested event occurs or until the call is interrupted. The
201 \fBpoll()\fR and \fBppoll()\fR functions are not affected by the \fBO_NDELAY\fR
202 and \fBO_NONBLOCK\fR flags.
203 .sp
204 .LP
205 The \fBpoll()\fR and \fBppoll()\fR functions support regular files, terminal and
206 pseudo-terminal devices, streams-based files, FIFOs, pipes, and sockets. The
207 behavior of \fBpoll()\fR and \fBppoll()\fR on elements of \fIfds\fR that refer
208 to other types of file is unspecified.
209 .sp
210 .LP
211 A file descriptor for a socket that is listening for connections will indicate
212 that it is ready for reading, once connections are available. A file
213 descriptor for a socket that is connecting asynchronously will indicate that it
214 is ready for writing, once a connection has been established.
215 .sp
216 .LP
217 Regular files always \fBpoll()\fR and \fBppoll()\fR \fBTRUE\fR for reading and
218 writing.
219 .SH RETURN VALUES
220 .LP
221 Upon successful completion, a non-negative value is returned. A positive value
222 indicates the total number of file descriptors that has been selected (that is,
223 file descriptors for which the \fBrevents\fR member is non-zero). A value of
224 \fB0\fR indicates that the call timed out and no file descriptors have been
225 selected. Upon failure, \fB\(mi1\fR is returned and \fBerrno\fR is set to
226 indicate the error.
227 .SH ERRORS
228 .LP
229 The \fBpoll()\fR and \fBppoll()\fR functions will fail if:
230 .sp
231 .ne 2
232 .na
233 \fB\fBEAGAIN\fR\fR
234 .ad
235 .RS 10n
236 Allocation of internal data structures failed, but the request may be attempted
237 again.
238 .RE
240 .sp
241 .ne 2
242 .na
243 \fB\fBEFAULT\fR\fR
244 .ad
245 .RS 10n
246 Some argument points to an illegal address.
247 .RE
249 .sp
250 .ne 2
251 .na
252 \fB\fBEINTR\fR\fR
253 .ad
254 .RS 10n
255 A signal was caught during the \fBpoll()\fR or \fBppoll()\fR function.
256 .RE
258 .sp
259 .ne 2
260 .na
261 \fB\fBEINVAL\fR\fR
262 .ad
263 .RS 10n
264 The argument \fInfds\fR is greater than \fB{OPEN_MAX}\fR, or one of the
265 \fBfd\fR members refers to a stream or multiplexer that is linked (directly or
266 indirectly) downstream from a multiplexer.
267 .RE
269 .SH ATTRIBUTES
270 .LP
271 See \fBattributes\fR(5) for descriptions of the following attributes:
272 .sp
274 .sp
275 .TS
276 box;
277 c | c
278 l | l .
279 ATTRIBUTE TYPE ATTRIBUTE VALUE
280 _
281 Interface Stability Standard
282 .TE
284 .SH SEE ALSO
285 .LP
286 \fBIntro\fR(2), \fBgetmsg\fR(2), \fBgetrlimit\fR(2), \fBputmsg\fR(2),
287 \fBread\fR(2), \fBwrite\fR(2), \fBselect\fR(3C), \fBattributes\fR(5),
288 \fBstandards\fR(5), \fBchpoll\fR(9E)
289 .sp
290 .LP
291 \fISTREAMS Programming Guide\fR
292 .SH NOTES
293 .LP
294 Non-STREAMS drivers use \fBchpoll\fR(9E) to implement \fBpoll()\fR on these
295 devices.