stdlib: support non-GCC __attribute__

[gnulib.git] / ChangeLog

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+2011-02-12  Paul Eggert  <eggert@cs.ucla.edu>
+
+       stdlib: support non-GCC __attribute__
+
+       Fix a serious and tricky problem encountered when attempting to
+       add the getloadavg module to Emacs.  Emacs worked fine on RHEL
+       5.5, but it crashed due to memory corruption on Solaris 10 with
+       Sun C 5.11.  Emacs normally ORs 3-bit tags into their low-order
+       bits that are otherwise zero.  This tagging is optional inside
+       Emacs but is preferred and is used when __attribute__ ((__aligned
+       (8))) works, as it does with both recent-enough GCC and with Sun C
+       5.11.  However, Sun C 5.11 is not GCC and does not #define
+       __GNUC__ and __GNUC_MINOR__.
+
+       When I added the getloadavg module to Emacs, it brought in
+       stdlib.in.h, which contained this fragment:
+
+          #ifndef __attribute__
+          # if __GNUC__ < 2 || (__GNUC__ == 2 && __GNUC_MINOR__ < 8)
+          #  define __attribute__(Spec)   /* empty */
+          # endif
+          #endif
+
+       When files that include <stdlib.h> were compiled with Sun C 5.11,
+       the above code disabled __attribute__ ((__aligned (8))), which
+       caused variables to not be properly aligned, which eventually led
+       to the pointer corruption mentioned above.  (This was a bit hard
+       to diagnose, unfortunately.)
+
+       Several "#define __attribute__(X) /* empty */" code snippets need
+       to be eradicated from Gnulib to work with non-GCC compilers that
+       support __attribute__.  The Autoconf way to do this is to test for
+       each kind of attribute that we want support for, and selectively
+       enable that in source code.
+
+       Fix this problem just for stdlib.h, by adding a test for the
+       __noreturn__ attribute, and change stdlib.in.h to use that test
+       when needed.  This technique can be easily generalized to the
+       other *.in.h files and attributes, and a similar technique can be
+       used for *.h and *.c files.  This patch is enough to solve the
+       problem for Emacs + getloadavg, and I thought I'd publish it for
+       feedback before undertaking further, similar fixes in other
+       modules.
+
+       This patch does not arrange to #define HAVE_ATTRIBUTE_NORETURN
+       because it's not needed for stdlib.h.  It merely substitutes the
+       value directly into stdlib.h.  We may well need to #define it, or
+       similar symbols, for other modules, but it's nice to also have an
+       option to not #define it for applications like Emacs that do not
+       need it.
+
+       * lib/stdlib.in.h (__attribute__): Do not #define.
+       (_GL_ATTRIBUTE_NORETURN): New macro, which in stdlib.h needs to
+       be defined only if the _Exit module is also used.
+       * m4/_Exit.m4 (gl_FUNC__EXIT): Require gl_ATTRIBUTE_NORETURN.
+       * m4/stdlib_h.m4 (gl_STDLIB_H_DEFAULTS): Subst
+       HAVE_ATTRIBUTE_NORETURN and default it to 1, its value on GNU
+       platforms.
+       * modules/_Exit (Files): Add m4/attribute.m4.
+       * modules/stdlib (Makefile.am): Substitute HAVE_ATTRIBUTE_NORETURN.
+       * m4/attribute.m4: New file.
+
 2011-02-12  Bruno Haible  <bruno@clisp.org>
 
        wcsrtombs: Work around bug on native Windows.