Re: Gnulib's alloca.h and MinGW

[Bruno Haible <bruno@clisp.org>]

Hi Eli,

> A recent MinGW runtime changed its definition of alloca from  an
> inline function to a macro.  As result, there's a warning when
> compiling Gnulib's malloca.c:
> 
>        CC       malloca.o
>      In file included from malloca.h:21,
> 		      from malloca.c:22:
>      ./alloca.h:46: warning: "alloca" redefined
> 	46 | #  define alloca __builtin_alloca
> 	   |
>      In file included from ./alloca.h:44,
> 		      from malloca.h:21,
> 		      from malloca.c:22:
>      d:\usr\include\alloca.h:81: note: this is the location of the previous definition
> 	81 | #define alloca( __request )  __builtin_alloca( __request )
> 	   |
> 
> Proposed patch is below.  Thanks.

Thanks for the report.

I like your patch. It is better than the '#undef alloca' solution that I
had considered before.


2020-06-29  Bruno Haible  <bruno@clisp.org>

	alloca-opt: Fix warning on mingw.
	Reported and solution by Eli Zaretskii <eliz@gnu.org> in
	<https://lists.gnu.org/archive/html/bug-gnulib/2020-06/msg00069.html>.
	* lib/alloca.in.h: On mingw, include <alloca.h> and then test again
	whether alloca is defined.

diff --git a/lib/alloca.in.h b/lib/alloca.in.h
index 73516ca..beb022c 100644
--- a/lib/alloca.in.h
+++ b/lib/alloca.in.h
@@ -35,13 +35,16 @@
  */
 
 #ifndef alloca
+  /* Some version of mingw have an <alloca.h> that causes trouble when
+     included after 'alloca' gets defined as a macro.  As a workaround,
+     include this <alloca.h> first and define 'alloca' as a macro afterwards
+     if needed.  */
+# if defined __GNUC__ && (defined _WIN32 && ! defined __CYGWIN__) && @HAVE_ALLOCA_H@
+#  include_next <alloca.h>
+# endif
+#endif
+#ifndef alloca
 # ifdef __GNUC__
-   /* Some version of mingw have an <alloca.h> that causes trouble when
-      included after 'alloca' gets defined as a macro.  As a workaround, include
-      this <alloca.h> first and define 'alloca' as a macro afterwards.  */
-#  if (defined _WIN32 && ! defined __CYGWIN__) && @HAVE_ALLOCA_H@
-#   include_next <alloca.h>
-#  endif
 #  define alloca __builtin_alloca
 # elif defined _AIX
 #  define alloca __alloca