select(3C)                                                select(3C)


       NAME
             select - synchronous I/O multiplexing

       SYNOPSIS
             #include <sys/time.h>
             #include <sys/types.h>
             #include <sys/select.h>
             select(int nfds, fd_set *readfds, *writefds, *execptfds, struct
                 timeval *timeout);
             FD_SET(int fd, fd_set *fdset);
             FD_CLR(int fd, fd_set *fdset);
             FD_ISSET(int fd, fd_set *fdset);
             FD_ZERO(fd_set *fdset);

       DESCRIPTION
             select examines the I/O descriptor sets whose addresses are
             passed in readfds, writefds, and execptfds to see if any of
             their descriptors are ready for reading, are ready for
             writing, or have an exceptional condition pending,
             respectively.  nfds is the number of bits to be checked in
             each bit mask that represents a file descriptor; the
             descriptors from 0 to 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 return value from the
             call to select() is the number of ready descriptors.

             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 nonzero if fd is a member of fdset, zero
                            otherwise.




                           Copyright 1994 Novell, Inc.               Page 1

      select(3C)                                                select(3C)


            The behavior of these macros is undefined if a descriptor
            value is less than zero or greater than or equal to
            FD_SETSIZE.  FD_SETSIZE is a constant defined in sys/select.h
            and is normally at least equal to the maximum number of
            descriptors supported by the system.

            If timeout is not a NULL pointer, it specifies a maximum
            interval to wait for the selection to complete.  If timeout is
            a NULL pointer, the select blocks indefinitely.  To affect a
            poll, the timeout argument should be a non-NULL pointer,
            pointing to a zero-valued timeval structure.

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

         Return Values
            select returns the number of ready descriptors contained in
            the descriptor sets or -1 if an error occurred.  If the time
            limit expires, then select returns 0.

         Errors
            An error return from select indicates:

            EBADF          One of the I/O descriptor sets specified an
                           invalid I/O descriptor.

            EINTR          A signal was delivered before any of the
                           selected events occurred, or the time limit
                           expired.

            EINVAL         A component of the pointed-to time limit is
                           outside the acceptable range: t_sec must be
                           between 0 and 10^8, inclusive. t_usec must be
                           greater-than or equal to 0, and less than 10^6.

      REFERENCES
            poll(2), read(2), write(2)

      NOTICES
            The default value for FD_SETSIZE (currently 1024) is larger
            than the default limit on the number of open files.  In order
            to accommodate programs that may use a larger number of open
            files with select, it is possible to increase this size within
            a program by providing a larger definition of FD_SETSIZE
            before the inclusion of <sys/types.h>.



                          Copyright 1994 Novell, Inc.               Page 2

       select(3C)                                                select(3C)


             In future versions of the system, select may return the time
             remaining from the original timeout, if any, by modifying the
             time value in place.  It is thus unwise to assume that the
             timeout value will be unmodified by the select call.

             The descriptor sets are always modified on return, even if the
             call returns as the result of a timeout.









































                           Copyright 1994 Novell, Inc.               Page 3