Re: getting EBADF on MSVC

[Paul Eggert]

On 09/18/11 01:36, Bruno Haible wrote:

  This patch protects the dup2() calls in gnulib.

This change has some problems with GNU Emacs, and I suspect there will
be similar problems with printf, close, etc. so let's try to get see
if we can fix these problems with dup2 first.

Emacs has a separate way of building on Windows, one that doesn't
involve gnulib's replacement functions and is not likely to involve
them any time in the near future.  I expect that Emacs developers will
rather omit the files lib/msvc-inval.c, lib/msvc-inval.h, and
m4/msvc-inval.m4 as part of the Emacs distribution, as they're useless
baggage for Emacs.  However, I can't achieve this omission by invoking
gnulib-tool --avoid=msvc-inval, because dup2.c now includes
msvc-inval.h unilaterally.

I'd like a solution where --avoid=msvc-inval works.

Here's one way to do it.  In dup2.c:

#if MSVC_HACK_NEEDED
# include "msvc-inval.h"
#else
# define TRY_MSVC_INVAL if (1)
# define CATCH_MSVC_INVAL else
# define DONE_MSVC_INVAL
#endif

but this leads into the second problem, as discussed below.


  > @@ -64,7 +78,18 @@
  >    if (fd == desired_fd)
  >      return fcntl (fd, F_GETFL) == -1 ? -1 : fd;
  >  # endif
  > -  result = dup2 (fd, desired_fd);
  > +
  > +  TRY_MSVC_INVAL
  > +    {
  > +      result = dup2 (fd, desired_fd);
  > +    }
  > +  CATCH_MSVC_INVAL
  > +    {
  > +      result = -1;
  > +      errno = EBADF;
  > +    }
  > +  DONE_MSVC_INVAL

This style is unsatisfying, because it propagates MSVC workarounds
into the rest of gnulib.  I suspect it won't scale well when used
throughout gnulib for other functions.  I'd rather have a solution
that puts the MSVC workarounds in a separate place, where MSVC-aware
programmers can work on them, a place which people who don't know or
care about MSVC ports can omit and ignore.

I also have some qualms about C macros that expand to series of tokens
with unbalanced braces.

How about something like this instead?  Have dup2.c look
something like this:

     [existing includes]

     #if HAVE_DUP2
     # undef dup2
     # if MSVC_HACK_NEEDED              /* NEW */
     #  include "msvc-hack.h"           /* NEW */
     # endif                            /* NEW */

     int rpl_dup2 (int fd, int desired_fd)
     {
       ...
       result = dup2 (fd, desired_fd);
       ...
     }

and then "msvc-hack.h" can #define dup2 to do whatever it likes.  This
way, the source code to lib/dup2.c needs a relatively small change
(the three lines marked "/* NEW */" above) and can be read by others
largely without worrying about MSVC hacks.

Perhaps you can think of a better approach, but I hope you get
the idea.