There doesn't seem to be an easy way to suppress this warning in a macro like #define MILL(in) ((in) > 1000000) where the macro may be passed different sizes of ints including shorts. I would have expected the warning to be suppressed when a cast is present like: #define MILL(in) ((int)(in) > 1000000) but it isn't.
Environment: System: CYGWIN_NT-5.1 DHX98431 1.5.14s(0.124/4/2) 20050319 16:46:09 i686 unknown unknown Cygwin host: i686-pc-cygwin build: i686-pc-cygwin target: i686-pc-cygwin configured with: /gcc/3.4/gcc-3.4.1-1/configure --verbose --prefix=/usr --exec-prefix=/usr --sysconfdir=/etc --libdir=/usr/lib --libexecdir=/usr/lib --mandir=/usr/share/man --infodir=/usr/share/info --enable-languages=c,ada,c++,f77,java,objc --enable-nls --without-included-gettext --enable-libgcj --with-system-zlib --enable-interpreter --enable-threads=posix --enable-java-gc=boehm --enable-sjlj-exceptions --disable-version-specific-runtime-libs --disable-win32-registry How-To-Repeat: Given warnme.c: #define MILL(in) ((in) > 1000000) int foo(short s) { return MILL(s); } int bar(short s) { return MILL((int)s); } int baz(short s) { int i=s; return MILL(i); } with preprocessor output warnme.i: # 1 "warnme.c" # 1 "<built-in>" # 1 "<command line>" # 1 "warnme.c" int foo(short s) { return ((s) > 1000000); } int bar(short s) { return (((int)s) > 1000000); } int baz(short s) { int i=s; return ((i) > 1000000); } $ gcc -Wall -save-temps -c warnme.c warnme.c: In function `foo': warnme.c:2: warning: comparison is always false due to limited range of data type warnme.c: In function `bar': warnme.c:3: warning: comparison is always false due to limited range of data type The warning in foo is expected; the warning in bar is not. baz is an ugly workaround. ------- Additional Comments From sthoenna at efn dot org 2005-03-21 00:16 ------- Fix: An ugly workaround is to assign to a temporary variable of large enough data type where the macro is called, and pass the temporary variable instead. -- Summary: unable to use cast to suppress "warning: comparison is always false due to limited range of data type" Product: gcc Version: 3.4.1 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c AssignedTo: unassigned at gcc dot gnu dot org ReportedBy: sthoenna at efn dot org CC: gcc-bugs at gcc dot gnu dot org GCC build triplet: i686-pc-cygwin GCC host triplet: i686-pc-cygwin GCC target triplet: i686-pc-cygwin http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20573