Re: non-blocking I/O

[Bruno Haible]

Paolo Bonzini wrote:
> Without guessing what your bias is, I also :) prefer to implement 
> {g,s}et_nonblock_flag functions.  It would use either 
> SetNamedPipeHandleState or ioctlsocket (using the socket detection trick 
> in sockets.c to detect sockets, and then GetFileType to detect pipes if 
> it fails).

Here's proposed code to that effect.  Note that the getter function cannot
be implemented: How to determine whether a Woe32 socket is non-blocking?


================================ nonblocking.h ================================
/* Non-blocking I/O for pipe or socket descriptors.
   Copyright (C) 2011 Free Software Foundation, Inc.

   This program is free software: you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation; either version 3 of the License, or
   (at your option) any later version.

   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.

   You should have received a copy of the GNU General Public License
   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */

#ifndef _NONBLOCKING_H
#define _NONBLOCKING_H

#include <stdbool.h>

/* Non-blocking I/O is an I/O mode by which read(), write() calls avoid
   blocking the current thread.  When non-blocking is enabled:
   - A read() call returns -1 with errno set to EAGAIN when no data or EOF
     information is immediately available.
   - A write() call returns -1 with errno set to EAGAIN when it cannot
     transport the requested amount of data (but at most one pipe buffer)
     without blocking.

   There are two modern alternatives to non-blocking I/O:
     - use select() or poll() followed by read() or write() if the descriptor
       is ready,
     - call read() or write() in separate threads.  */


#ifdef __cplusplus
extern "C" {
#endif


#if 0 /* cannot be portably implemented */
/* Return true if I/O to the descriptor DESC is currently non-blocking,
   or false if it is blocking.  */
extern bool get_nonblocking_flag (int desc);
#endif

/* Specify the non-blocking flag for the descriptor DESC.
   Return 0 upon success, or -1 with errno set upon failure.
   The default depends on the presence of the O_NONBLOCK flag for files
   or pipes opened with open() or on the presence of the SOCK_NONBLOCK
   flag for pipes.  */
extern int set_nonblocking_flag (int desc, bool value);


#ifdef __cplusplus
}
#endif

#endif /* _NONBLOCKING_H */
================================ nonblocking.c ================================
/* Non-blocking I/O for pipe or socket descriptors.
   Copyright (C) 2011 Free Software Foundation, Inc.

   This program is free software: you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation; either version 3 of the License, or
   (at your option) any later version.

   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.

   You should have received a copy of the GNU General Public License
   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */

#include <config.h>

/* Specification.  */
#include "nonblocking.h"

#include <errno.h>

#if (defined _WIN32 || defined __WIN32__) && ! defined __CYGWIN__
/* Native Woe32 API.  */

# if 0
bool
get_nonblocking_flag (int desc)
{
  HANDLE h = (HANDLE) _get_osfhandle (desc);
  if (GetFileType (h) == FILE_TYPE_PIPE)
    {
      /* h is a pipe or socket.  */
      DWORD state;
      if (GetNamedPipeHandleState (h, &state, NULL, NULL, NULL, NULL, 0))
        /* h is a pipe.  */
        return (state & PIPE_NOWAIT) != 0;
      else
        /* h is a socket.  */
        ?? How can this be implemented ??
    }
  else
    return false;
}
# endif

int
set_nonblocking_flag (int desc, bool value)
{
  HANDLE h = (HANDLE) _get_osfhandle (desc);
  if (GetFileType (h) == FILE_TYPE_PIPE)
    {
      /* h is a pipe or socket.  */
      DWORD state;
      if (GetNamedPipeHandleState (h, &state, NULL, NULL, NULL, NULL, 0))
        {
          /* h is a pipe.  */
          if ((state & PIPE_NOWAIT) != 0)
            {
              if (value)
                return 0;
              state &= ~PIPE_NOWAIT;
            }
          else
            {
              if (!value)
                return 0;
              state |= PIPE_NOWAIT;
            }
          if (SetNamedPipeHandleState (h, &state, NULL, NULL))
            return 0;
          errno = EINVAL;
          return -1;
        }
      else
        {
          /* h is a socket.  */
          int v = value;
          return ioctl ((SOCKET) h, FIONBIO, &v);
        }
    }
  else
    {
      /* Win32 does not support non-blocking on regular files.  */
      errno = ENOTSUP;
      return -1;
    }
}

#else
/* Unix API.  */

# include <fcntl.h>

/* We don't need the gnulib replacement of fcntl() here.  */
# undef fcntl

# if 0
bool
get_nonblocking_flag (int desc)
{
  int fcntl_flags;

  fcntl_flags = fcntl (desc, F_GETFL, 0);
  if (fcntl_flags < 0)
    return false;
  return (fcntl_flags & O_NONBLOCK) != 0;
}
# endif

int
set_nonblocking_flag (int desc, bool value)
{
  int fcntl_flags;

  fcntl_flags = fcntl (desc, F_GETFL, 0);
  if (fcntl_flags < 0)
    return -1;
  if ((O_NONBLOCK & ~fcntl_flags) == 0)
    return 0;
  return fcntl (desc, F_SETFL, fcntl_flags | O_NONBLOCK);
}

#endif
===============================================================================
--- lib/ioctl.c.orig    Tue Mar 29 17:15:16 2011
+++ lib/ioctl.c Tue Mar 29 16:08:53 2011
@@ -63,6 +63,10 @@
   buf = va_arg (args, void *);
   va_end (args);
 
+  /* We don't support FIONBIO on pipes here.  If you want to make pipe fds
+     non-blocking, use the gnulib 'nonblocking' module, until gnulib implements
+     fcntl F_GETFL / F_SETFL with O_NONBLOCK.  */
+
   sock = FD_TO_SOCKET (fd);
   r = ioctlsocket (sock, req, buf);
   if (r < 0)

-- 
In memoriam Rachel Levy <http://en.wikipedia.org/wiki/Rachel_Levy>