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     select — synchronous I/O multiplexing


     Standard C Library (libc, -lc)


     #include <sys/select.h>

     select(int nfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds,
         struct timeval *timeout);

     FD_SET(fd, &fdset);

     FD_CLR(fd, &fdset);

     FD_ISSET(fd, &fdset);



     The select() system call examines the I/O descriptor sets whose addresses are passed in
     readfds, writefds, and exceptfds to see if some of their descriptors are ready for reading,
     are ready for writing, or have an exceptional condition pending, respectively.  The only
     exceptional condition detectable is out-of-band data received on a socket.  The first nfds
     descriptors are checked in each set; i.e., the descriptors from 0 through nfds-1 in the
     descriptor sets are examined.  On return, select() replaces the given descriptor sets with
     subsets consisting of those descriptors that are ready for the requested operation.  The
     select() system call returns the total number of ready descriptors in all the sets.

     The descriptor sets are stored as bit fields in arrays of integers.  The following macros
     are provided for manipulating such descriptor sets: FD_ZERO(&fdset) initializes a descriptor
     set fdset to the null set.  FD_SET(fd, &fdset) includes a particular descriptor fd in fdset.
     FD_CLR(fd, &fdset) removes fd from fdset.  FD_ISSET(fd, &fdset) is non-zero if fd is a
     member of fdset, zero otherwise.  The behavior of these macros is undefined if a descriptor
     value is less than zero or greater than or equal to FD_SETSIZE, which is normally at least
     equal to the maximum number of descriptors supported by the system.

     If timeout is not a null pointer, it specifies the maximum interval to wait for the
     selection to complete.  System activity can lengthen the interval by an indeterminate

     If timeout is a null pointer, the select blocks indefinitely.

     To effect a poll, the timeout argument should not be a null pointer, but it should point to
     a zero-valued timeval structure.

     Any of readfds, writefds, and exceptfds may be given as null pointers if no descriptors are
     of interest.


     The select() system call returns the number of ready descriptors that are contained in the
     descriptor sets, or -1 if an error occurred.  If the time limit expires, select() returns 0.
     If select() returns with an error, including one due to an interrupted system call, the
     descriptor sets will be unmodified.


     An error return from select() indicates:

     [EBADF]            One of the descriptor sets specified an invalid descriptor.

     [EFAULT]           One of the arguments readfds, writefds, exceptfds, or timeout points to
                        an invalid address.

     [EINTR]            A signal was delivered before the time limit expired and before any of
                        the selected events occurred.

     [EINVAL]           The specified time limit is invalid.  One of its components is negative
                        or too large.

     [EINVAL]           The nfds argument was invalid.


     accept(2), connect(2), getdtablesize(2), gettimeofday(2), kqueue(2), poll(2), read(2),
     recv(2), send(2), write(2), clocks(7)


     The default size of FD_SETSIZE is currently 1024.  In order to accommodate programs which
     might potentially use a larger number of open files with select(), it is possible to
     increase this size by having the program define FD_SETSIZE before the inclusion of any
     header which includes <sys/types.h>.

     If nfds is greater than the number of open files, select() is not guaranteed to examine the
     unused file descriptors.  For historical reasons, select() will always examine the first 256


     The select() system call and FD_CLR(), FD_ISSET(), FD_SET(), and FD_ZERO() macros conform
     with IEEE Std 1003.1-2001 (“POSIX.1”).


     The select() system call appeared in 4.2BSD.


     Version 2 of the Single UNIX Specification (“SUSv2”) allows systems to modify the original
     timeout in place.  Thus, it is unwise to assume that the timeout value will be unmodified by
     the select() system call.  FreeBSD does not modify the return value, which can cause
     problems for applications ported from other systems.