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Neither the name of the University nor the names of its contributors.\"    may be used to endorse or promote products derived from this software.\"    without specific prior written permission..\".\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION).\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF.\" SUCH DAMAGE..\".\"     @(#)select.2	8.2 (Berkeley) 3/25/94.\".Dd March 25, 1994.Dt SELECT 2.Os BSD 4.2.Sh NAME.Nm select.Nd synchronous I/O multiplexing.Sh SYNOPSIS.Fd #include <sys/types.h>.Fd #include <sys/time.h>.Fd #include <unistd.h>.Ft int.Fn select "int nfds" "fd_set *readfds" "fd_set *writefds" "fd_set *exceptfds" "struct timeval *timeout".Fn FD_SET fd &fdset.Fn FD_CLR fd &fdset.Fn FD_ISSET fd &fdset.Fn FD_ZERO &fdset.Sh DESCRIPTION.Fn Selectexamines the I/O descriptor sets whose addresses are passed in.Fa readfds ,.Fa writefds ,and.Fa exceptfdsto see if some of their descriptorsare ready for reading, are ready for writing, or have an exceptionalcondition pending, respectively.The first.Fa nfdsdescriptors are checked in each set;i.e., the descriptors from 0 through.Fa nfds Ns No -1in the descriptor sets are examined.On return,.Fn selectreplaces the given descriptor setswith subsets consisting of those descriptors that are readyfor the requested operation..Fn Selectreturns the total number of ready descriptors in all the sets..PpThe descriptor sets are stored as bit fields in arrays of integers.The following macros are provided for manipulating such descriptor sets:.Fn FD_ZERO &fdsetxinitializes a descriptor set.Fa fdsetto the null set..Fn FD_SET fd &fdsetincludes a particular descriptor.Fa fdin.Fa fdset ..Fn FD_CLR fd &fdsetremoves.Fa fdfrom.Fa fdset ..Fn FD_ISSET fd &fdsetis non-zero if.Fa fdis a member of.Fa fdset ,zero otherwise.The behavior of these macros is undefined ifa descriptor value is less than zero or greater than or equal to.Dv FD_SETSIZE ,which is normally at least equalto the maximum number of descriptors supported by the system..PpIf.Fa timeoutis a non-nil pointer, it specifies a maximum interval to wait for theselection to complete.  If .Fa timeoutis a nil pointer, the select blocks indefinitely.  To affect a poll, the.Fa timeoutargument should be non-nil, pointing to a zero-valued timeval structure..PpAny of.Fa readfds ,.Fa writefds ,and.Fa exceptfdsmay be given as nil pointers if no descriptors are of interest..Sh RETURN VALUES.Fn Selectreturns the number of ready descriptors that are contained inthe descriptor sets,or -1 if an error occurred.If the time limit expires,.Fn selectreturns 0.If.Fn selectreturns with an error,including one due to an interrupted call,the descriptor sets will be unmodified..Sh ERRORSAn error return from.Fn selectindicates:.Bl -tag -width Er.It Bq Er EBADFOne of the descriptor sets specified an invalid descriptor..It Bq Er EINTRA signal was delivered before the time limit expired andbefore any of the selected events occurred..It Bq Er EINVALThe specified time limit is invalid.  One of its components isnegative or too large..El.Sh SEE ALSO.Xr accept 2 ,.Xr connect 2 ,.Xr getdtablesize 2 ,.Xr gettimeofday 2 ,.Xr read 2 ,.Xr recv 2 ,.Xr send 2 ,.Xr write 2.Sh BUGSAlthough the provision of.Xr getdtablesize 2was intended to allow user programs to be written independentof the kernel limit on the number of open files, the dimensionof a sufficiently large bit field for select remains a problem.The default size.Dv FD_SETSIZE(currently 256) is somewhat larger thanthe current kernel limit to the number of open files.However, in order to accommodate programs which might potentiallyuse a larger number of open files with select, it is possibleto increase this size within a program by providinga larger definition of.Dv FD_SETSIZEbefore the inclusion of.Aq Pa sys/types.h ..Pp.Fn Selectshould probably return the time remaining from the original timeout,if any, by modifying the time value in place.This may be implemented in future versions of the system.Thus, it is unwise to assume that the timeout value will be unmodifiedby the.Fn selectcall..Sh HISTORYThe.Nmfunction call appeared in.Bx 4.2 .