https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97998
Bug ID: 97998
Summary: GCC should not complain about double definitions of
macros whose names begin with __STDC_
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: preprocessor
Assignee: unassigned at gcc dot gnu.org
Reporter: zackw at panix dot com
Target Milestone: ---
Normally, GCC does not warn about a second definition of a preprocessor macro
with the exact same expansion:
#define some_ordinary_macro 1
#define some_ordinary_macro 1 /* no diagnostic */
However, GCC 9 and 10 *do* complain about this kind of redefinition if the name
of the macro begins with __STDC_:
#define __STDC_anything 1
#define __STDC_anything 1 /* warning: "__STDC_anything" redefined */
In particular, it will complain about double definitions of the various
__STDC_WANT_*__ macros. The upcoming autoconf 2.70 will start defining many of
those macros when the script uses AC_USE_SYSTEM_EXTENSIONS, and this broke code
in APR's configure script that included the internal 'confdefs.h' (which
accumulates the various #defines that will be emitted into config.h) a second
time (autoconf test programs always include confdefs.h before doing anything
else)
I think GCC should treat __STDC_ macros the same as ordinary macros - no
warning for a second definition with the same expansion. N.B. clang already
behaves this way.
(If the second definition has a *different* expansion, GCC complains regardless
of the macro's name, and that behavior should not be changed.)
