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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.\" $FreeBSD$ 34.\" 35.Dd November 17, 2002 36.Dt SELECT 2 37.Os 38.Sh NAME 39.Nm select 40.Nd synchronous I/O multiplexing 41.Sh LIBRARY 42.Lb libc 43.Sh SYNOPSIS 44.In sys/select.h 45.Ft int 46.Fn select "int nfds" "fd_set *readfds" "fd_set *writefds" "fd_set *exceptfds" "struct timeval *timeout" 47.Fn FD_SET fd &fdset 48.Fn FD_CLR fd &fdset 49.Fn FD_ISSET fd &fdset 50.Fn FD_ZERO &fdset 51.Sh DESCRIPTION 52The 53.Fn select 54system call 55examines the I/O descriptor sets whose addresses are passed in 56.Fa readfds , 57.Fa writefds , 58and 59.Fa exceptfds 60to see if some of their descriptors 61are ready for reading, are ready for writing, or have an exceptional 62condition pending, respectively. 63The only exceptional condition detectable is out-of-band 64data received on a socket. 65The first 66.Fa nfds 67descriptors are checked in each set; 68i.e., the descriptors from 0 through 69.Fa nfds Ns No -1 70in the descriptor sets are examined. 71On return, 72.Fn select 73replaces the given descriptor sets 74with subsets consisting of those descriptors that are ready 75for the requested operation. 76The 77.Fn select 78system call 79returns the total number of ready descriptors in all the sets. 80.Pp 81The descriptor sets are stored as bit fields in arrays of integers. 82The following macros are provided for manipulating such descriptor sets: 83.Fn FD_ZERO &fdset 84initializes a descriptor set 85.Fa fdset 86to the null set. 87.Fn FD_SET fd &fdset 88includes a particular descriptor 89.Fa fd 90in 91.Fa fdset . 92.Fn FD_CLR fd &fdset 93removes 94.Fa fd 95from 96.Fa fdset . 97.Fn FD_ISSET fd &fdset 98is non-zero if 99.Fa fd 100is a member of 101.Fa fdset , 102zero otherwise. 103The behavior of these macros is undefined if 104a descriptor value is less than zero or greater than or equal to 105.Dv FD_SETSIZE , 106which is normally at least equal 107to the maximum number of descriptors supported by the system. 108.Pp 109If 110.Fa timeout 111is not a null pointer, it specifies the maximum interval to wait for the 112selection to complete. 113System activity can lengthen the interval by 114an indeterminate amount. 115.Pp 116If 117.Fa timeout 118is a null pointer, the select blocks indefinitely. 119.Pp 120To effect a poll, the 121.Fa timeout 122argument should not be a null pointer, 123but it should point to a zero-valued timeval structure. 124.Pp 125Any of 126.Fa readfds , 127.Fa writefds , 128and 129.Fa exceptfds 130may be given as null pointers if no descriptors are of interest. 131.Sh RETURN VALUES 132The 133.Fn select 134system call 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 system 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 EFAULT 154One of the arguments 155.Fa readfds , writefds , exceptfds , 156or 157.Fa timeout 158points to an invalid address. 159.It Bq Er EINTR 160A signal was delivered before the time limit expired and 161before any of the selected events occurred. 162.It Bq Er EINVAL 163The specified time limit is invalid. 164One of its components is 165negative or too large. 166.It Bq Er EINVAL 167The 168.Fa nfds 169argument 170was invalid. 171.El 172.Sh SEE ALSO 173.Xr accept 2 , 174.Xr connect 2 , 175.Xr getdtablesize 2 , 176.Xr gettimeofday 2 , 177.Xr kqueue 2 , 178.Xr poll 2 , 179.Xr read 2 , 180.Xr recv 2 , 181.Xr send 2 , 182.Xr write 2 , 183.Xr clocks 7 184.Sh NOTES 185The default size of 186.Dv FD_SETSIZE 187is currently 1024. 188In order to accommodate programs which might potentially 189use a larger number of open files with 190.Fn select , 191it is possible 192to increase this size by having the program define 193.Dv FD_SETSIZE 194before the inclusion of any header which includes 195.In sys/types.h . 196.Pp 197If 198.Fa nfds 199is greater than the number of open files, 200.Fn select 201is not guaranteed to examine the unused file descriptors. 202For historical 203reasons, 204.Fn select 205will always examine the first 256 descriptors. 206.Sh STANDARDS 207The 208.Fn select 209system call and 210.Fn FD_CLR , 211.Fn FD_ISSET , 212.Fn FD_SET , 213and 214.Fn FD_ZERO 215macros conform with 216.St -p1003.1-2001 . 217.Sh HISTORY 218The 219.Fn select 220system call appeared in 221.Bx 4.2 . 222.Sh BUGS 223.St -susv2 224allows systems to modify the original timeout in place. 225Thus, it is unwise to assume that the timeout value will be unmodified 226by the 227.Fn select 228system call. 229