SELECT(2)                                               SELECT(2)



     NAME
          select - synchronous I/O multiplexing - TCP

     SYNOPSIS
          #include <sys/types.h>
          #include <sys/time.h>

          nfound = select(nfds, readfds, writefds, exceptfds, timeout)
          int nfound, nfds;
          fdset *readfds, *writefds, *exceptfds;
          struct timeval *timeout;

          FDSET(fd, &fdset)
          FDCLR(fd, &fdset)
          FDISSET(fd, &fdset)
          FDZERO(&fdset)
          int fd;
          fdset fdset;

     DESCRIPTION
          Select 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 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
          total number of ready descriptors in all the sets is
          returned in nfound.

          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.  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 a non-zero pointer, it specifies a maximum
          interval to wait for the selection to complete.  If timeout
          is a zero pointer, the select blocks indefinitely.  To
          affect a poll, the timeout argument should be non-zero,
          pointing to a zero-valued timeval structure.

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



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     SELECT(2)                                               SELECT(2)



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

     ERRORS
          An error return from select indicates:

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

          [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.

     SEE ALSO
          accept(2), connect(2), read(2), write(2), recv(2), send(2).

     BUGS
          The dimension of a sufficiently large bit field for select
          remains a problem.  The default size FD_SETSIZE (currently
          256) is somewhat larger than the current kernel limit to the
          number of open files.  However, in order to accommodate
          programs which might potentially 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>.

          Select should 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 unmodified by the select call.

          This works only for sockets, pty's, and tty's.

     ORIGIN
          4.3 BSD












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