Date: Wed, 16 Sep 1998 16:37:29 +0300 (IDT)
From: Eli Zaretskii <eliz AT is DOT elta DOT co DOT il>
To: DJ Delorie <dj AT delorie DOT com>
cc: djgpp-workers AT delorie DOT com
Subject: Re: `select' feature?
In-Reply-To: <199809131315.PAA11345@tyr.diku.dk>
Message-ID: <Pine.SUN.3.91.980916163518.7343E-100000@is>
MIME-Version: 1.0
Content-Type: TEXT/PLAIN; charset=US-ASCII
Precedence: bulk


On Sun, 13 Sep 1998, Morten Welinder wrote:

> Solaris 2.6 says
> 
>     "On failure, the objects pointed to by the  readfs,  writefs,
>     and  errorfds  arguments  are  not modified.  If the timeout
>     interval expires without the specified condition being  true
>     for  any  of  the  specified  file  descriptors, the objects
>     pointed to by the readfs, writefs,  and  errorfds  arguments
>     have all bits set to 0."

Here's the patch that fixes our `select' in the case of expiring 
timeout.  I also took this opportunity to make the docs a bit more 
explicit.

*** src/libc/compat/time/select.c~0	Tue Jul 23 23:41:26 1996
--- src/libc/compat/time/select.c	Mon Sep 14 10:11:28 1998
*************** select(int nfds,
*** 235,241 ****
--- 235,246 ----
        gettimeofday (&now, 0);
        if (now.tv_sec > then.tv_sec
  	  || (now.tv_sec == then.tv_sec && now.tv_usec >= then.tv_usec))
+       {
+ 	FD_ZERO (readfds);
+ 	FD_ZERO (writefds);
+ 	FD_ZERO (exceptfds);
  	return 0;
+       }
      }
  
      /* We are busy-waiting, so give other processes a chance to run.  */
*** src/libc/compat/time/select.t~0	Sun Jul 23 04:11:10 1995
--- src/libc/compat/time/select.txh	Mon Sep 14 18:39:38 1998
***************
*** 14,23 ****
  
  @subheading Description
  
! This function waits for files to be ready for input or output, or for
! a timeout.  Each fd_set represents a set of bits representing file
! descriptors.  The following macros shall be used to deal with these
! sets:
  
  @table @code
  
--- 14,25 ----
  
  @subheading Description
  
! This function waits for files to be ready for input or output, or to
! have exceptional condition pending, or for a timeout.
! 
! Each @code{fd_set} variable is a bitmap representation of a set of file
! descriptors, one bit for every descriptor.  The following macros shall
! be used to deal with these sets:
  
  @table @code
  
***************
*** 37,49 ****
  
  Return the value of member @var{n} in set @var{p}.
  
  @end table
  
! The @var{timeout} value may be a NULL pointer (no timeout), a pointer
! to a zero-value structure (poll mode), or a pointer to an
! interval-filled structure (timeout).
  
  @subheading Return Value
  
! The number of files ready.  The input sets are replaced with sets that
! describe which files are ready for which operations.
--- 39,167 ----
  
  Return the value of member @var{n} in set @var{p}.
  
+ @item FD_SETSIZE
+ 
+ The maximum number of descriptors supported by the system.
+ 
  @end table
  
! The @var{nfds} parameter is the number of bits to be examined in each of
! the @code{fd_set} sets: the function will only check file descriptors
! @code{0} through @code{@var{nfds} - 1}, even if some bits are set for
! descriptors beyond that.
! 
! On input, some of the bits of each one of the @code{fd_set} sets for
! which the function should wait, should be set using the @code{FD_SET}
! macro.  @code{select} examines only those descriptors whose bits are
! set.
! 
! Any of @code{readfds}, @code{writefds}, and @code{exceptfds} can be a
! @code{NULL} pointer, if the caller is not interested in testing the
! corresponding conditions.
! 
! On output, if @code{select} returns a non-negative value, each
! non- AT code{NULL} argument of the three sets will be replaced with a
! subset in which a bit is set for every descriptor that was found to be,
! respectively, ready for input, ready for output, and pending an
! exceptional condition.  Note that if @code{select} returns -1, meaning a
! failure, the descriptor sets are @emph{unchanged}, so you should always
! test the return value before looking at the bits in the returned sets.
! 
! The @var{timeout} value may be a NULL pointer (no timeout, i.e., wait
! forever), a pointer to a zero-value structure (poll mode, i.e., test
! once and exit immediately), or a pointer to a @code{struct timeval}
! variable (timeout: @code{select} will repeatedly test all the
! descriptors until some of them become ready, or the timeout expires).
  
  @subheading Return Value
  
! On successfull return, @code{select} returns the number of files ready,
! or 0, if the timeout expired.  The input sets are replaced with subsets
! that describe which files are ready for which operations.  If
! @code{select} returns 0 (i.e., the timeout has expired), all the
! non- AT code{NULL} sets have all their bits reset to zero.
! 
! On failure, @code{select} returns -1, sets @code{errno} to a suitable
! value, and leaves the descriptor sets unchanged.
! 
! @subheading Example
! 
! @example
!   struct timeval timeout;
!   fd_set read_fds, write_fds;
!   int i, select_result;
! 
!   timeout.tv_sec = 5;  /* 5-second timeout */
!   timeout.tv_usec = 0;
! 
!   /* Display status of the 5 files open by default.  */
!   for (i = 0; i < 5; i++)
!     @{
! 
!       FD_ZERO (&read_fds);
!       FD_SET (i, &read_fds);
!       select_result = select (i + 1, &read_fds, 0, 0, &timeout);
!       if (select_result == -1)
!         @{
!           fprintf(stderr, "%d: Failure for input", i);
!           perror("");
!         @}
!       else
!         fprintf(stderr,
!                 "%d: %s ready for input\n", i, select_result ? "" : "NOT");
!       FD_ZERO (&write_fds);
!       FD_SET (i, &write_fds);
!       select_result = select (i + 1, 0, &write_fds, 0, &timeout);
!       if (select_result == -1)
!         @{
!           fprintf(stderr, "%d: Failure for output", i);
!           perror("");
!         @}
!       else
!         fprintf(stderr,
!                 "%d: %s ready for output\n", i, select_result ? "" : "NOT");
!     @}
! @end example
! 
! @subheading Implementation Notes
! 
! The following notes describe some details pertinent to the DJGPP
! implementation of @code{select}:
! 
! @itemize @bullet{}
! @item
! While @code{select} waits for the timeout to expire, it repeatedly calls
! the @code{__dpmi_yield} function (@pxref{__dpmi_yield}), so that any
! other programs that run at the same time (e.g., on Windows) get more CPU
! time.
! 
! @item
! A file handle that belongs to a @code{FILE} object created by
! @code{fopen} or @code{fdopen} (@pxref{fopen}) for which @code{feof} or
! @code{ferror} return non-zero, will be reported in the @code{exceptfds}
! set; also, such a handle will be reported not input-ready if there are
! no pending buffered characters in the @code{FILE} object.  This might be
! a feature or a bug, depending on your point of view; in particular, Unix
! implementations usually don't check buffered input.  Portable programs
! should refrain from mixing @code{select} with buffered I/O.
! 
! @item
! DOS doesn't support exceptional conditions, so file handles used for
! unbuffered I/O will @emph{never} be marked in @code{exceptfds}.
! 
! @item
! DOS always returns an output-ready indication for a file descriptor
! connected to a disk file.  So use of @code{writefds} is only meaningful
! for character devices.
! 
! @item
! The usual text-mode input from the keyboard and other character devices
! is line-buffered by DOS.  This means that if you type one character,
! @code{select} will indicate that file handle 0 is ready for input, but a
! call to @code{getc} will still block until the @key{Enter} key is
! pressed.  If you need to make sure that reading a single character won't
! block, you should read either with BIOS functions such as @code{getkey}
! (@pxref{getkey}, or with raw input DOS functions such as @code{getch}
! (@pxref{getch}), or switch the handle to binary mode with a call to
! @code{setmode} (@pxref{setmode}).
! @end itemize
*** src/docs/kb/wc202.t~0	Sun Jul 26 13:34:04 1998
--- src/docs/kb/wc202.txi	Wed Sep 16 15:05:52 1998
***************
*** 447,449 ****
--- 447,453 ----
  interrupted or crashed, and reports the run time with millisecond
  resolution.
  @cindex redir
+ 
+ @code{select} now correctly zeroes out all the bits in the @code{fd_set}
+ arguments when it returns due to expired timeout.
+ @findex select