Re: status of new 'c++defs' module?

[Bruno Haible]

John W. Eaton wrote:
> I received a report about the following error on an OS X system:
> 
>   In file included from /usr/include/sys/time.h:197,
>                    from ../libgnu/sys/time.h:40,
>                    from ../libgnu/sys/select.h:51,
>                    from /usr/include/unistd.h:519,
>                    from ../libgnu/unistd.h:29,
>                    from 
> /usr/include/c++/4.2.1/i686-apple-darwin10/bits/os_defines.h:61,
>                    from 
> /usr/include/c++/4.2.1/i686-apple-darwin10/bits/c++config.h:41,
>                    from /usr/include/c++/4.2.1/cstddef:50,
>                    from Array.h:30,
>                    from MArray.h:28,
>                    from dMatrix.h:27,
>                    from ODESFunc.h:26,
>                    from ODES.h:26,
>                    from ODES.cc:27:
>   ../libgnu/stdlib.h:737: error: previous declaration of ‘int 
> _gl_warn_on_use’ with ‘C++’ linkage
>   ../libgnu/time.h:350: error: conflicts with new declaration with ‘C’ linkage
> 
> so it seems that _gl_warn_on_use should be declared extern "C" to
> avoid this kind of problem.

Yes, I agree. We cannot control whether all gnulib include files are included
in C++ mode. Here apparently <time.h> is included in C mode. This may be a
bug in the system include files; it may also lead to unintended consequences.

But anyway, it's safe to make the change you suggested. I've committed this:


2010-03-20  Bruno Haible  <address@hidden>

        Make _GL_WARN_ON_USE usable in C++ and C mode in the same compilation.
        * build-aux/warn-on-use.h (_GL_WARN_EXTERN_C): New macro.
        (_GL_WARN_ON_USE, _GL_WARN_ON_USE_CXX): Likewise.
        Reported by John W. Eaton <address@hidden>.

--- build-aux/warn-on-use.h.orig        Sun Mar 21 00:48:23 2010
+++ build-aux/warn-on-use.h     Sun Mar 21 00:47:33 2010
@@ -70,7 +70,7 @@
 
 # else /* Unsupported.  */
 #  define _GL_WARN_ON_USE(function, message) \
-extern int _gl_warn_on_use
+_GL_WARN_EXTERN_C int _gl_warn_on_use
 # endif
 #endif
 
@@ -87,6 +87,16 @@
      __attribute__ ((__warning__ (msg)))
 # else /* Unsupported.  */
 #  define _GL_WARN_ON_USE_CXX(function,rettype,parameters_and_attributes,msg) \
-extern int _gl_warn_on_use
+_GL_WARN_EXTERN_C int _gl_warn_on_use
+# endif
+#endif
+
+/* _GL_WARN_EXTERN_C declaration;
+   performs the declaration with C linkage.  */
+#ifndef _GL_WARN_EXTERN_C
+# if defined __cplusplus
+#  define _GL_WARN_EXTERN_C extern "C"
+# else
+#  define _GL_WARN_EXTERN_C extern
 # endif
 #endif


Additionally, it raises my attention to the fact that .h files must not have
#include statements inside   extern "C" { ... }  blocks. This fixes the two
violations of this rule that I can see in gnulib.


2010-03-20  Bruno Haible  <address@hidden>

        Ensure no #include statements inside extern "C" { ... }.
        * lib/obstack.h: Shrink extern "C" { ... } region so that it does not
        contain #include statements.
        * lib/time.in.h: Likewise.

--- lib/obstack.h.orig  Sun Mar 21 01:12:15 2010
+++ lib/obstack.h       Sun Mar 21 01:07:00 2010
@@ -103,10 +103,6 @@
 
 #ifndef _OBSTACK_H
 #define _OBSTACK_H 1
-
-#ifdef __cplusplus
-extern "C" {
-#endif
 
 /* We need the type of a pointer subtraction.  If __PTRDIFF_TYPE__ is
    defined, as with GNU C, use that; that way we don't pollute the
@@ -139,6 +135,10 @@
 
 #include <string.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 struct _obstack_chunk           /* Lives at front of each chunk. */
 {
   char  *limit;                 /* 1 past end of this chunk */
--- lib/time.in.h.orig  Sun Mar 21 01:12:15 2010
+++ lib/time.in.h       Sun Mar 21 01:08:00 2010
@@ -46,10 +46,6 @@
 
 /* The definition of _GL_WARN_ON_USE is copied here.  */
 
-# ifdef __cplusplus
-extern "C" {
-# endif
-
 /* Some systems don't define struct timespec (e.g., AIX 4.1, Ultrix 4.3).
    Or they define it with the wrong member names or define it in <sys/time.h>
    (e.g., FreeBSD circa 1997).  */
@@ -57,6 +53,11 @@
 #  if @SYS_TIME_H_DEFINES_STRUCT_TIMESPEC@
 #   include <sys/time.h>
 #  else
+
+#   ifdef __cplusplus
+extern "C" {
+#   endif
+
 #   undef timespec
 #   define timespec rpl_timespec
 struct timespec
@@ -64,11 +65,12 @@
   time_t tv_sec;
   long int tv_nsec;
 };
-#  endif
-# endif
 
-# ifdef __cplusplus
+#   ifdef __cplusplus
 }
+#   endif
+
+#  endif
 # endif
 
 /* Sleep for at least RQTP seconds unless interrupted,  If interrupted,