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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 112.Dv NULL 113pointer, it specifies the maximum interval to wait for the 114selection to complete. 115System activity can lengthen the interval by 116an indeterminate amount. 117.Pp 118If 119.Fa timeout 120is a 121.Dv NULL 122pointer, the select blocks indefinitely. 123.Pp 124To effect a poll, the 125.Fa timeout 126argument should not be 127.Dv NULL , 128but it should point to a zero-valued timeval structure. 129.Pp 130Any of 131.Fa readfds , 132.Fa writefds , 133and 134.Fa exceptfds 135may be given as 136.Dv NULL 137pointers if no descriptors are of interest. 138.Sh RETURN VALUES 139The 140.Fn select 141system call 142returns the number of ready descriptors that are contained in 143the descriptor sets, 144or -1 if an error occurred. 145If the time limit expires, 146.Fn select 147returns 0. 148If 149.Fn select 150returns with an error, 151including one due to an interrupted system call, 152the descriptor sets will be unmodified. 153.Sh ERRORS 154An error return from 155.Fn select 156indicates: 157.Bl -tag -width Er 158.It Bq Er EBADF 159One of the descriptor sets specified an invalid descriptor. 160.It Bq Er EFAULT 161One of the arguments 162.Fa readfds , writefds , exceptfds , 163or 164.Fa timeout 165points to an invalid address. 166.It Bq Er EINTR 167A signal was delivered before the time limit expired and 168before any of the selected events occurred. 169.It Bq Er EINVAL 170The specified time limit is invalid. 171One of its components is 172negative or too large. 173.It Bq Er EINVAL 174The 175.Fa nfds 176argument 177was invalid. 178.El 179.Sh SEE ALSO 180.Xr accept 2 , 181.Xr connect 2 , 182.Xr getdtablesize 2 , 183.Xr gettimeofday 2 , 184.Xr kqueue 2 , 185.Xr poll 2 , 186.Xr read 2 , 187.Xr recv 2 , 188.Xr send 2 , 189.Xr write 2 , 190.Xr clocks 7 191.Sh NOTES 192The default size of 193.Dv FD_SETSIZE 194is currently 1024. 195In order to accommodate programs which might potentially 196use a larger number of open files with 197.Fn select , 198it is possible 199to increase this size by having the program define 200.Dv FD_SETSIZE 201before the inclusion of any header which includes 202.In sys/types.h . 203.Pp 204If 205.Fa nfds 206is greater than the number of open files, 207.Fn select 208is not guaranteed to examine the unused file descriptors. 209For historical 210reasons, 211.Fn select 212will always examine the first 256 descriptors. 213.Sh BUGS 214.St -susv2 215allows systems to modify the original timeout in place. 216Thus, it is unwise to assume that the timeout value will be unmodified 217by the 218.Fn select 219system call. 220.Sh STANDARDS 221The 222.Fn select 223system call and 224.Fn FD_CLR , 225.Fn FD_ISSET , 226.Fn FD_SET , 227and 228.Fn FD_ZERO 229macros conform with 230.St -p1003.1-2001 . 231.Sh HISTORY 232The 233.Fn select 234system call appeared in 235.Bx 4.2 . 236