Re: AC_CHECK_DECLS vs. Clang

[Noah Misch]

On Mon, Apr 13, 2015 at 09:12:52PM -0400, Noah Misch wrote:
> On Sun, Mar 08, 2015 at 12:13:44AM -0500, Noah Misch wrote:
> > The Clang compiler furnishes implicit declarations for built-in functions,
> > even when the identifier in question is not used as a function designator.
> > This thwarts AC_CHECK_DECLS:
> > 
> > configure:11565: checking whether strlcpy is declared
> > configure:11565: clang -c -Wall -Wmissing-prototypes -Wpointer-arith 
> > -Wdeclaration-after-statement -Wendif-labels -Wmissing-format-attribute 
> > -Wformat-security -fno-strict-aliasing -fwrapv -g -O2  -D_GNU_SOURCE 
> > -I/usr/include/libxml2  conftest.c >&5
> > conftest.c:166:10: warning: implicitly declaring library function 'strlcpy' 
> > with type 'unsigned long (char *, const char *, unsigned long)'
> >   (void) strlcpy;
> >          ^
> > conftest.c:166:10: note: please include the header <string.h> or explicitly 
> > provide a declaration for 'strlcpy'
> > 1 warning generated.
> > configure:11565: $? = 0
> > configure:11565: result: yes
> > 
> > 
> > Full config.log:
> > http://buildfarm.postgresql.org/cgi-bin/show_stage_log.pl?nm=treepie&dt=2015-03-07%2006%3A22%3A38&stg=config
> > 
> > Clang has always behaved this way, but strlcpy() and strlcat() were 
> > unaffected
> > until Clang 3.0 added them as builtins.  To fix this, I plan to introduce
> > ac_c_decl_warn_flag by analogy to ac_c_preproc_warn_flag.  With no explicit
> > declarations, a typical compiler (ac_c_decl_warn_flag='') reports an error 
> > for
> > "(void) strchr;".  Clang (ac_c_decl_warn_flag=yes) reports a warning, but
> > adding a declaration silences the warning.  A compiler that emits some 
> > warning
> > with or without a declaration is unusable.
> 
> Here is the patch.  There's a risk that incidental warnings will make
> AC_CHECK_DECL wrongly report a declaration missing.  If, for example, an
> AC_CHECK_DECL test includes a header that elicits a deprecation warning, that
> test will report any declaration missing.  The Autoconf test suite does pass
> under "clang -Weverything -Wno-unused-macros -Wno-main" or "clang -Wno-main".
> "-Wno-unused-macros" suppresses warnings about unused confdefs.h macros like
> PACKAGE_URL, warnings which would otherwise snag on one of the new sanity
> checks.  (Unrelated to AC_CHECK_DECL, the "C unit tests" group needs
> "-Wno-main".)  I considered -fno-builtin instead of checking for warnings, but
> the loss of realism outweighs the benefit of mitigating this risk.

In the absence of objections, I will commit this next week.