On Wed, 2020-05-13 at 13:21 +0300, Pavel Fedin via Cygwin wrote: > While compiling various software packages for Cygwin i notice that very > often i have to add something like #define _GNU_SOURCE to > them in order to compile correctly. Meanwhile on Linux they compile with no > problems at all. I've narrowed it down to -std=??? > option using a simple test case: [snip] > $ g++ test.cpp -o test - compiles OK > $ g++ test.cpp -o test -std=c++14 - error: 'strdup' was not declared in this > scope; did you mean 'strcmp'? > > By printing out predefined macros (-dM -E) i found out that -std=something > option adds " #define __STRICT_ANSI__ 1" to builtin > macros, but removes all *_SOURCE definitions, so _DEFAULT_SOURCE is not > triggered any more.
That is what -std=c* is supposed to mean, that you are declaring strict ISO-standard language conformance. > I've compared the behavior with Linux system. On Linux -std=c++14 also > defines __STRICT_ANSI__, but various *_SOURCE macros are not > omitted. That's because _GNU_SOURCE is defined unconditionally by g++ on Linux, which overrides the __STRICT_ANSI__ defined by -std=c*. IIUC this is done only to handle the use of non-ISO functions in libstdc++, particularly in the implementation of newer C++ standards. The bottom line is, if you are using POSIX C extensions, then you shouldn't be declaring -std=c* without the appropriate _*_SOURCE. > Isn't this a Cygwin bug? Not really, just that our g++ is somewhat more strict. If anything, allowing use of functions outside the declared standards (the behaviour on Linux) is the bug, and it's been on my to-do list for a long time to fix. Fixing this isn't as simple as removing the unconditional define; the C library functions used by newer C++ need to be appropriately guarded, and then those specific guards used in libstdc++. It was a major project to get this working on Cygwin. > By the way, clang does not suffer from this problem. Clang also defines _GNU_SOURCE unconditionally when compiling C++, even on Cygwin. Perhaps *that* should be considered the bug. -- Yaakov -- Problem reports: https://cygwin.com/problems.html FAQ: https://cygwin.com/faq/ Documentation: https://cygwin.com/docs.html Unsubscribe info: https://cygwin.com/ml/#unsubscribe-simple
