Re: Gnulib's freopen replacement and MinGW

[Bruno Haible]

Hi Eli,

> Diffutils 3.2 call xfreopen with its first argument NULL, expecting
> the underlying reopen to handle this.  However, the MS runtime does
> not implement the Posix semantics of such a call, and so, for example,
> MinGW-compiled cmp fails when invoked to compare its stdin with a
> file:
> 
>   D:\gnu\diffutils-3.2\src>cat cmp.c | cmp cmp.c -
>   cmp: failed to reopen `stdin' with mode `rb': No such file or directory
> 
> The following change fixes this:
> 
> 2012-05-05  Eli Zaretskii  <address@hidden>
> 
>       * lib/freopen.c [_WIN32]: Include io.h and fcntl.h.
>       (rpl_freopen) [_WIN32]: If the first argument is NULL, call
>       _setmode to switch STREAM to either binary or text mode, as
>       specified by MODE.

Thanks for the proposed patch, but the problem has already been fixed
differently, by avoiding to use xfreopen() [1].

The question whether to extend gnulib's freopen() to support this use-case
was discussed in the thread [2][3]. The result of that discussion was "no",
because
  - POSIX says that it is "implementation-defined" whether freopen()
    supports this [4],
  - freopen with a NULL filename is not supported on many Unix platforms,
  - there is no point for the caller to use the freopen() API when the
    same effect can be achieved with <binary-io.h>.

Bruno

[1] 
http://git.savannah.gnu.org/gitweb/?p=diffutils.git;a=commitdiff;h=7508234eabe646abcea074baea83612f8115d3b9
[2] http://lists.gnu.org/archive/html/bug-gnulib/2011-08/msg00224.html
[3] http://lists.gnu.org/archive/html/bug-gnulib/2011-09/msg00005.html
[4] http://pubs.opengroup.org/onlinepubs/9699919799/functions/freopen.html