select(2N)                                             select(2N)



     NAME
          select - synchronous I/O multiplexing

     SYNOPSIS
          #include <sys/time.h>

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

     DESCRIPTION
          select examines the I/O descriptors specified by the bit
          masks readfds, writefds, and execptfds to see if they are
          ready for reading, writing, or have an exceptional condition
          pending, respectively.  File descriptor f is represented by
          the bit 1<<f in the mask.  nfds descriptors are checked,
          that is, the bits from 0 through nfds-1 in the masks are ex-
          amined.  select returns, in place, a mask of those descrip-
          tors which are ready.  The total number of ready descriptors
          is returned.

          If timeout is a nonzero pointer, it specifies a maximum in-
          terval 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 nonzero, pointing to a
          zero valued timeval structure.

          Any of readfds, writefds, and execptfds may be given as 0 if
          no descriptors are of interest.

     RETURN VALUE
          select returns the number of descriptors which are contained
          in the bit masks, 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 bit masks specified an invalid
                         descriptor.

          [EINTR]        A signal was delivered before any of the
                         selected for events occurred or the time lim-
                         it expired.

     SEE ALSO
          accept(2N), connect(2N), recv(2N), readv(2), send(2N), wri-
          tev(2).

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



     April, 1990                                                     1