I'm not really sure whether this is gcc bug or what but it used to work with gcc 4.4.x and older.
Problem description: -------------------- I include <ucontext.h> which resides in /usr/include/sys/ucontext.h on my system this header contains (among others): #ifdef __USE_GNU enum { ... }; #endif So to use this I need to #define __USE_GNU But for some reason the preprocessor removes this part from included file like the __USE_GNU was never defined. It happens no matter whether I define __USE_GNU in source or on command line. OS: --- Arch Linux 64 bit GCC version: ------------ Using built-in specs. COLLECT_GCC=gcc COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-unknown-linux-gnu/4.5.0/lto-wrapper Target: x86_64-unknown-linux-gnu Configured with: ../configure --prefix=/usr --enable-languages=c,c++,fortran,objc,obj-c++,ada --enable-shared --enable-threads=posix --enable-__cxa_atexit --enable-clocale=gnu --enable-gnu-unique-object --enable-lto --enable-plugin --disable-multilib --disable-libstdcxx-pch --with-system-zlib --with-ppl --with-cloog --libdir=/usr/lib --libexecdir=/usr/lib --mandir=/usr/share/man --infodir=/usr/share/info Thread model: posix gcc version 4.5.0 20100610 (prerelease) (GCC) but I was able to reproduce it with the stable 4.5.0 -- Summary: [4.5 regression?] strange #define behavior Product: gcc Version: 4.5.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: preprocessor AssignedTo: unassigned at gcc dot gnu dot org ReportedBy: l dot jirkovsky at gmail dot com http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44837