The default C++ mode, when no -std= is specified, for g++ 9.3.0 is gnu++14 (it's the same for g++ 10.1.0, too). > Looks like it is set by default, even when specifying the C++ standard version for g++ Why this happens is explained in libstd++'s faq: https://gcc.gnu.org/onlinedocs/libstdc++/faq.html#faq.predefined On Wed, Jun 17, 2020 at 12:14 PM Jonny Grant <jg@xxxxxxxx> wrote: > > > > On 17/06/2020 11:22, Jakub Wilk wrote: > > * Jonny Grant <jg@xxxxxxxx>, 2020-06-16, 12:36: > >> SYNOPSIS top > >> #define _GNU_SOURCE > >> #include <unistd.h> > >> #include <sys/types.h> > >> > >> pid_t gettid(void); > >> > >> I can compile on Ubuntu without #define _GNU_SOURCE and call gettid() > >> > >> Maybe that line can be removed? > > > > I don't think so. Here's what I get (with GCC 9): > > > > $ gcc test.c > > test.c: In function ‘main’: > > test.c:6:9: warning: implicit declaration of function ‘gettid’; did you mean ‘getgid’? [-Wimplicit-function-declaration] > > 6 | return gettid(); > > | ^~~~~~ > > | getgid > > > > Prepending "#define _GNU_SOURCE" fixes the above. > > Hi Jakub, > > Apologies, I should have shared my test case, I was compiling as C++ > // g++ -Wall -o tid tid.cpp > > #include <unistd.h> > #include <sys/types.h> > > int main() > { > return gettid(); > } > > > > $ g++ -Wall -o tid tid.cpp > tid.cpp:3: warning: "_GNU_SOURCE" redefined > 3 | #define _GNU_SOURCE > | > <command-line>: note: this is the location of the previous definition > > > Looks like it is set by default, even when specifying the C++ standard version for g++ > g++ -std=c++11 -Wall -o tid tid.cpp > > > Checked, yes, if compiling as C, see the same result as you. > > Not sure how it gets enabled by default in my c++ > > $ g++ --version > g++ (Ubuntu 9.3.0-10ubuntu2) 9.3.0 > > Cheers, Jonny > > -- Regards, Ponnuvel P
