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
     examined.  select returns, in place, a mask of those
     descriptors which are ready.  The total number of ready
     descriptors is returned.

     If timeout is a nonzero 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 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.

STATUS MESSAGES AND VALUES
     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.

     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 limit expired.

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

SEE ALSO
     accept(2N), connect(2N), recv(2N), readv(2), send(2N),
     writev(2)




January 1992                                                    1