environ: fix link error on 64-bit Cygwin

[Eric Blake]

Forwarding from the gnulib list; is this something we should fix in
cygwin proper?

On 01/31/2018 04:42 AM, Bruno Haible wrote:
> On 64-bit Cygwin, a libunistring build fails like this:
> 
> $ /bin/sh ../libtool  --tag=CC --preserve-dup-deps  --mode=link x86_64-pc-cygwin-gcc  -g -O2  -L/usr/local/cygwin64/lib -Wl,--disable-auto-import -o test-environ.exe test-environ.o libtests.a ../lib/libunistring.la libtests.a
> libtool: link: x86_64-pc-cygwin-gcc -g -O2 -Wl,--disable-auto-import -o .libs/test-environ.exe test-environ.o  -L/usr/local/cygwin64/lib libtests.a ../lib/.libs/libunistring.dll.a -liconv libtests.a -L/usr/local/cygwin64/lib
> test-environ.o:test-environ.c:(.rdata$.refptr.environ[.refptr.environ]+0x0): undefined reference to `environ'
> collect2: error: ld returned 1 exit status
> 
> This fixes it.
> 
> 
> 2018-01-31  Bruno Haible  <bruno@clisp.org>
> 
> 	environ: Fix link error on 64-bit Cygwin.
> 	* lib/unistd.in.h (environ): On Cygwin, redeclare with the
> 	__declspec(dllimport) attribute.
> 	* doc/posix-functions/environ.texi: Mention the Cygwin problem.
> 
> diff --git a/doc/posix-functions/environ.texi b/doc/posix-functions/environ.texi
> index 34ac25a..a6c0095 100644
> --- a/doc/posix-functions/environ.texi
> +++ b/doc/posix-functions/environ.texi
> @@ -24,6 +24,9 @@ shared libraries on Mac OS X 10.5.  Here is a workaround: Instead, one can use
>  #define environ (*_NSGetEnviron())
>  @end smallexample
>  This works at all versions of Mac OS X.
> +@item
> +On Cygwin in 64-bit mode, references to this variable cause a link error when
> +the option @code{-Wl,--disable-auto-import} is in use.
>  @end itemize
>  
>  Portability problems not fixed by Gnulib:
> diff --git a/lib/unistd.in.h b/lib/unistd.in.h
> index 6802e3b..4ef0ffa 100644
> --- a/lib/unistd.in.h
> +++ b/lib/unistd.in.h
> @@ -400,6 +400,13 @@ _GL_WARN_ON_USE (dup3, "dup3 is unportable - "
>  
>  
>  #if @GNULIB_ENVIRON@
> +# if defined __CYGWIN__
> +/* The 'environ' variable is defined in a DLL. Therefore its declaration needs
> +   the '__declspec(dllimport)' attribute, but the system's <unistd.h> lacks it.
> +   This leads to a link error on 64-bit Cygwin when the option
> +   -Wl,--disable-auto-import is in use.  */
> +_GL_EXTERN_C __declspec(dllimport) char **environ;
> +# endif
>  # if !@HAVE_DECL_ENVIRON@
>  /* Set of environment variables and values.  An array of strings of the form
>     "VARIABLE=VALUE", terminated with a NULL.  */
> 
> 
> 

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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