Copyright (C) 1989, Digital Equipment Corporation           
 All rights reserved.                                        
 See the file COPYRIGHT for a full description.              
 Last modified on Wed Jun 22 16:42:22 PDT 1994 by kalsow     
      modified on Wed Mar 16 14:19:17 PST 1994 by wobber     
      modified on Thu Jan 28 10:14:15 PST 1993 by mjordan    
      modified on Mon Nov 18 14:41:43 PST 1991 by muller     

INTERFACE SchedulerPosix;

IMPORT Thread;
FROM Ctypes IMPORT int;

  WaitResult = {Ready, Error, FDError, Timeout};

PROCEDURE WaitProcess(pid: int; VAR status: int): int;
This is a wrapper for POSIX waitpid.

                  timeoutInterval: LONGREAL := -1.0D0): WaitResult;

                  timeoutInterval: LONGREAL := -1.0D0): WaitResult
                  RAISES {Thread.Alerted};
These procedures are functionally equivalent to calling select(2) or poll(2) with a single file descriptor. The major difference is that calls on IOWait and IOAlertWait do not prevent other threads from running. If read is TRUE, then the readfds and exceptfds arguments to select are the singleton set containing fd, and writefds is the empty set. Otherwise, the writefds and exceptfds are non-empty, and readfds is empty.

The return value from IOWait and IOAlertWait indicates that fd was found to be ready for I/O, or that the caller's timeout interval expired, or that an error occured:

-- Ready indicates that fd was found to be ready for I/O (according to the value of the read argument). In other words, a subsequent I/O operation might succeed. If fd is persistently in such a state, then IOWait and IOAlertWait will return Ready.

-- Error indicates that a select call executed on behalf of the caller failed, for instance the supplied fd is not valid.

-- FDError indicates that fd is valid, but it exhibits an exceptional condition.

-- Timeout indicates that the caller's timeout expired. The file descriptor will have been tested at least once before this result is returned.

IOWait and IOAlertWait block until the argument fd is in a reportable state, or until timeoutInterval seconds have passed. If timeoutInterval is negative, an indefinite wait is indicated,

As usual, IOAlertWait is the alertable version, IOWait is the non-alertable version.

A Ready result from IOWait and IOAlertWait does not guarantee that I/O is currently possible on fd. For example, any other thread may preempt during the return sequence and issue a read on fd.

A standard technique for using these procedures is as follows. First make reads non-blocking on the file descriptor fd, and then use a loop of the form:

LOOP status := read (fd, buf, n); IF status = -1 AND errno # EWOULDBLOCK THEN (* error to be handled

ELSIF status = 0 THEN
          (* eof reached *)
        ELSIF status > 0 THEN
          (* status chars available in buf *)

        EVAL SchedulerPosix.IOWait(fd, TRUE);

END SchedulerPosix.