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Neither the name of the University nor the names of its contributors 17.\" may be used to endorse or promote products derived from this software 18.\" without specific prior written permission. 19.\" 20.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 21.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 22.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 23.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 24.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 25.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 26.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 27.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 28.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 29.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 30.\" SUCH DAMAGE. 31.\" 32.\" @(#)select.2 8.2 (Berkeley) 3/25/94 33.\" $FreeBSD: src/lib/libc/sys/select.2,v 1.14.2.5 2001/12/14 18:34:01 ru Exp $ 34.\" $DragonFly: src/lib/libc/sys/select.2,v 1.8 2007/08/02 00:21:54 swildner Exp $ 35.\" 36.Dd March 25, 1994 37.Dt SELECT 2 38.Os 39.Sh NAME 40.Nm select 41.Nd synchronous I/O multiplexing 42.Sh LIBRARY 43.Lb libc 44.Sh SYNOPSIS 45.In sys/select.h 46.Ft int 47.Fn select "int nfds" "fd_set *readfds" "fd_set *writefds" "fd_set *exceptfds" "struct timeval *timeout" 48.Fn FD_SET fd &fdset 49.Fn FD_CLR fd &fdset 50.Fn FD_ISSET fd &fdset 51.Fn FD_ZERO &fdset 52.Sh DESCRIPTION 53.Fn Select 54examines the I/O descriptor sets whose addresses are passed in 55.Fa readfds , 56.Fa writefds , 57and 58.Fa exceptfds 59to see if some of their descriptors 60are ready for reading, are ready for writing, or have an exceptional 61condition pending, respectively. 62The only exceptional condition detectable is out-of-band 63data received on a socket. 64The first 65.Fa nfds 66descriptors are checked in each set; 67i.e., the descriptors from 0 through 68.Fa nfds Ns No -1 69in the descriptor sets are examined. 70On return, 71.Fn select 72replaces the given descriptor sets 73with subsets consisting of those descriptors that are ready 74for the requested operation. 75.Fn Select 76returns the total number of ready descriptors in all the sets. 77.Pp 78The descriptor sets are stored as bit fields in arrays of integers. 79The following macros are provided for manipulating such descriptor sets: 80.Fn FD_ZERO &fdset 81initializes a descriptor set 82.Fa fdset 83to the null set. 84.Fn FD_SET fd &fdset 85includes a particular descriptor 86.Fa fd 87in 88.Fa fdset . 89.Fn FD_CLR fd &fdset 90removes 91.Fa fd 92from 93.Fa fdset . 94.Fn FD_ISSET fd &fdset 95is non-zero if 96.Fa fd 97is a member of 98.Fa fdset , 99zero otherwise. 100The behavior of these macros is undefined if 101a descriptor value is less than zero or greater than or equal to 102.Dv FD_SETSIZE , 103which is normally at least equal 104to the maximum number of descriptors supported by the system. 105.Pp 106If 107.Fa timeout 108is a non-nil pointer, it specifies the maximum interval to wait for the 109selection to complete. System activity can lengthen the interval by 110an indeterminate amount. 111.Pp 112If 113.Fa timeout 114is a nil pointer, the select blocks indefinitely. 115.Pp 116To effect a poll, the 117.Fa timeout 118argument should be non-nil, pointing to a zero-valued timeval structure. 119.Pp 120Any of 121.Fa readfds , 122.Fa writefds , 123and 124.Fa exceptfds 125may be given as nil pointers if no descriptors are of interest. 126.Pp 127Even if no descriptors are of interest, the timeout works as described, 128effectively putting the process into an interruptible sleep for the 129specified timeout. 130If 131.Fa timeout 132is nil, the process will block until a signal is received. 133.Sh RETURN VALUES 134.Fn Select 135returns the number of ready descriptors that are contained in 136the descriptor sets, 137or -1 if an error occurred. 138If the time limit expires, 139.Fn select 140returns 0. 141If 142.Fn select 143returns with an error, 144including one due to an interrupted call, 145the descriptor sets will be unmodified. 146.Sh ERRORS 147An error return from 148.Fn select 149indicates: 150.Bl -tag -width Er 151.It Bq Er EBADF 152One of the descriptor sets specified an invalid descriptor. 153.It Bq Er EINTR 154A signal was delivered before the time limit expired and 155before any of the selected events occurred. 156.It Bq Er EINVAL 157The specified time limit is invalid. One of its components is 158negative or too large. 159.It Bq Er EINVAL 160.Fa nfds 161was invalid. 162.El 163.Sh SEE ALSO 164.Xr accept 2 , 165.Xr connect 2 , 166.Xr getdtablesize 2 , 167.Xr gettimeofday 2 , 168.Xr read 2 , 169.Xr recv 2 , 170.Xr send 2 , 171.Xr write 2 , 172.Xr clocks 7 173.Sh NOTES 174The default size of 175.Dv FD_SETSIZE 176is currently 1024. 177In order to accommodate programs which might potentially 178use a larger number of open files with 179.Fn select , 180it is possible 181to increase this size by having the program define 182.Dv FD_SETSIZE 183before the inclusion of any header which includes 184.In sys/types.h . 185.Pp 186If 187.Fa nfds 188is greater than the number of open files, 189.Fn select 190is not guaranteed to examine the unused file descriptors. For historical 191reasons, 192.Fn select 193will always examine the first 256 descriptors. 194.Sh HISTORY 195The 196.Fn select 197function call appeared in 198.Bx 4.2 . 199.Sh BUGS 200.St -susv2 201allows systems to modify the original timeout in place. 202Thus, it is unwise to assume that the timeout value will be unmodified 203by the 204.Fn select 205call. 206