select(3C)                                                       select(3C)


   NAME
         select - synchronous I/O multiplexing

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

         select(nfds, in0, out0, ex0, tv)
         int nfds;
         fdset *in0, *out0, *ex0;
         struct timeval *tv;

         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
         in0, out0, and ex0 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 -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:  FDZERO(&fdset) initializes a descriptor set fdset to the null
         set.  FDSET(fd, &fdset) includes a particular descriptor fd in
         fdset.  FDCLR(fd, &fdset) removes fd from fdset.  FDISSET(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 FDSETSIZE.  FDSETSIZE 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 in0, out0, and ex0 may be given as NULL pointers if no
         descriptors are of interest.




   7/91                                                                 Page 1

   select(3C)                                                       select(3C)


   RETURN VALUE
         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:  tsec must be between 0 and
                        10^8, inclusive. tusec must be greater-than or equal
                        to 0, and less than 10^6.

   SEE ALSO
         poll(2), read(2), write(2)

   NOTES
         The default value for FDSETSIZE (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 FDSETSIZE before the inclusion of
         <sys/types.h>.

         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.
















   Page 2                                                                 7/91