swab wrapper fails mingw.org's MinGW compilation of GDB

swab wrapper fails mingw.org's MinGW compilation of GDB

[Eli Zaretskii]

The following snippet from Gnulib's unistd.h causes a compilation
error when building the current development version of GDB 11:

  #if @GNULIB_MDA_SWAB@
  /* On native Windows, map 'swab' to '_swab', so that -loldnames is not
     required.  In C++ with GNULIB_NAMESPACE, avoid differences between
     platforms by defining GNULIB_NAMESPACE::creat always.  */
  # if defined _WIN32 && !defined __CYGWIN__
  #  if !(defined __cplusplus && defined GNULIB_NAMESPACE)
  #   undef swab
  #   define swab _swab
  #  endif
  _GL_CXXALIAS_MDA (swab, void, (char *from, char *to, int n));
  # else
  _GL_CXXALIAS_SYS (swab, void, (const void *from, void *to, ssize_t n));
  # endif
  _GL_CXXALIASWARN (swab);
  #endif

The problem is that mingw.org's MinGW uses a slightly different
prototype of _swab:

  _CRTIMP __cdecl __MINGW_NOTHROW  void _swab (const char *, char *, size_t);

So the difference between the prototypes causes this compilation error
in C++ programs:

       CXX    unittests/string_view-selftests.o
     In file included from ./../gdbsupport/common-defs.h:86,
		      from ./defs.h:28,
		      from unittests/string_view-selftests.c:26:
     ./../gnulib/import/unistd.h: In member function 'gnulib::_gl_swab_wrapper::operator gnulib::_gl_swab_wrapper::type() const':
     ./../gnulib/import/unistd.h:2543:1: error: invalid conversion from 'void (__attribute__((cdecl)) *)(const char*, char*, size_t)' {aka 'void (__attribute__((cdecl)) *)(const char*, char*, unsigned int)'} to 'gnulib::_gl_swab_wrapper::type' {aka 'void (*)(char*, char*, int)'} [-fpermissive]
      2543 | _GL_CXXALIAS_MDA (swab, void, (char *from, char *to, int n));
	   | ^~~~~~~~~~~~~~~~
	   | |
	   | void (__attribute__((cdecl)) *)(const char*, char*, size_t) {aka void (__attribute__((cdecl)) *)(const char*, char*, unsigned int)}

The suggested fix is as follows (__MINGW32_VERSION is defined by
mingw.org's MinGW, but not by MinGW64):

--- unistd.h~	2021-03-29 09:15:08.782625000 +0300
+++ unistd.h	2021-03-29 10:19:46.485750000 +0300
@@ -2540,7 +2540,11 @@ _GL_WARN_ON_USE (sleep, "sleep is unport
 #   undef swab
 #   define swab _swab
 #  endif
+#  if defined __MINGW32_VERSION
+_GL_CXXALIAS_MDA (swab, void, (const char *from, char *to, size_t n));
+#  else
 _GL_CXXALIAS_MDA (swab, void, (char *from, char *to, int n));
+#  endif
 # else
 _GL_CXXALIAS_SYS (swab, void, (const void *from, void *to, ssize_t n));
 # endif

Re: swab wrapper fails mingw.org's MinGW compilation of GDB

[Bruno Haible]

Eli Zaretskii wrote in
<https://lists.gnu.org/archive/html/bug-gnulib/2021-03/msg00135.html>:
> The following snippet from Gnulib's unistd.h causes a compilation
> error when building the current development version of GDB 11:
> 
>   #if @GNULIB_MDA_SWAB@
>   /* On native Windows, map 'swab' to '_swab', so that -loldnames is not
>      required.  In C++ with GNULIB_NAMESPACE, avoid differences between
>      platforms by defining GNULIB_NAMESPACE::creat always.  */
>   # if defined _WIN32 && !defined __CYGWIN__
>   #  if !(defined __cplusplus && defined GNULIB_NAMESPACE)
>   #   undef swab
>   #   define swab _swab
>   #  endif
>   _GL_CXXALIAS_MDA (swab, void, (char *from, char *to, int n));
>   # else
>   _GL_CXXALIAS_SYS (swab, void, (const void *from, void *to, ssize_t n));
>   # endif
>   _GL_CXXALIASWARN (swab);
>   #endif
> 
> The problem is that mingw.org's MinGW uses a slightly different
> prototype of _swab:
> 
>   _CRTIMP __cdecl __MINGW_NOTHROW  void _swab (const char *, char *, size_t);
> 
> So the difference between the prototypes causes this compilation error
> in C++ programs:
> 
>        CXX    unittests/string_view-selftests.o
>      In file included from ./../gdbsupport/common-defs.h:86,
> 		      from ./defs.h:28,
> 		      from unittests/string_view-selftests.c:26:
>      ./../gnulib/import/unistd.h: In member function 'gnulib::_gl_swab_wrapper::operator gnulib::_gl_swab_wrapper::type() const':
>      ./../gnulib/import/unistd.h:2543:1: error: invalid conversion from 'void (__attribute__((cdecl)) *)(const char*, char*, size_t)' {aka 'void (__attribute__((cdecl)) *)(const char*, char*, unsigned int)'} to 'gnulib::_gl_swab_wrapper::type' {aka 'void (*)(char*, char*, int)'} [-fpermissive]
>       2543 | _GL_CXXALIAS_MDA (swab, void, (char *from, char *to, int n));
> 	   | ^~~~~~~~~~~~~~~~
> 	   | |
> 	   | void (__attribute__((cdecl)) *)(const char*, char*, size_t) {aka void (__attribute__((cdecl)) *)(const char*, char*, unsigned int)}

Thanks for the report. A similar problem also exists on Solaris and HP-UX.
The patch below should fix it. Tested on Solaris with CC and g++.

> The suggested fix is as follows (__MINGW32_VERSION is defined by
> mingw.org's MinGW, but not by MinGW64):

It is good to know how to distinguish older mingw from mingw64; thanks
for this info. But here we don't need to make the distinction, since we
have a _GL_CXXALIAS_MDA_CAST macro for these situations.


2021-06-20  Bruno Haible  <bruno@clisp.org>

	unistd: Avoid compilation error in C++ mode on Solaris, HP-UX, mingw.
	Reported by Eli Zaretskii <eliz@gnu.org> in
	<https://lists.gnu.org/archive/html/bug-gnulib/2021-03/msg00135.html>.
	* lib/unistd.in.h (swab): Consider different declarations on Solaris,
	HP-UX, and old mingw.

diff --git a/lib/unistd.in.h b/lib/unistd.in.h
index d4d4ba7..73c882f 100644
--- a/lib/unistd.in.h
+++ b/lib/unistd.in.h
@@ -2034,9 +2034,17 @@ _GL_WARN_ON_USE (sleep, "sleep is unportable - "
 #   undef swab
 #   define swab _swab
 #  endif
-_GL_CXXALIAS_MDA (swab, void, (char *from, char *to, int n));
-# else
+/* Need to cast, because in old mingw the arguments are
+                             (const char *from, char *to, size_t n).  */
+_GL_CXXALIAS_MDA_CAST (swab, void, (char *from, char *to, int n));
+# else
+#  if defined __hpux /* HP-UX */
+_GL_CXXALIAS_SYS (swab, void, (const char *from, char *to, int n));
+#  elif defined __sun && !defined _XPG4 /* Solaris */
+_GL_CXXALIAS_SYS (swab, void, (const char *from, char *to, ssize_t n));
+#  else
 _GL_CXXALIAS_SYS (swab, void, (const void *from, void *to, ssize_t n));
+#  endif
 # endif
 _GL_CXXALIASWARN (swab);
 #endif