When is it necessary to use the flag -stdlib=libstdc++?

Question

When is it necessary to use use the flag -stdlib=libstdc++ for the compiler and linker when compiling with gcc?

Does the compiler automatically use libstdc++?

I am using gcc4.8.2 on Ubuntu 13.10 and I would like to use the c++11 standard. I already pass -std=c++11 to the compiler.

Solution 1

On Linux: In general, all commonly available linux distributions will use libstdc++ by default, and all modern versions of GCC come with a libstdc++ that supports C++11. If you want to compile c++11 code here, use one of:

• g++ -std=c++11 input.cxx -o a.out (usually GNU compiler)
• g++ -std=gnu++11 input.cxx -o a.out

On OS X before Mavericks: g++ was actually an alias for clang++ and Apple's old version of libstdc++ was the default. You could use libc++ (which included c++11 library support) by passing -stdlib=libc++. If you want to compile c++11 code here, use one of:

• g++ -std=c++11 -stdlib=libc++ input.cxx -o a.out (clang, not GNU compiler!)
• g++ -std=gnu++11 -stdlib=libc++ input.cxx -o a.out (clang, not GNU compiler!)
• clang++ -std=c++11 -stdlib=libc++ input.cxx -o a.out
• clang++ -std=gnu++11 -stdlib=libc++ input.cxx -o a.out

On OS X since Mavericks: libc++ is the default and you should not pass any -stdlib=<...> flag. Since Xcode 10, building against libstdc++ is not supported at all anymore. Existing code built against libstdc++ will keep working because libstdc++.6.dylib is still provided, but compiling new code against libstdc++ is not supported.

• clang++ -std=c++11 input.cxx -o a.out
• clang++ -std=gnu++11 input.cxx -o a.out

Solution 2

When is it necessary to use use the flag -stdlib=libstdc++ for the compiler and linker when compiling with gcc?

Short answer: never

Longer answer: -stdlib is a Clang flag and will not work with any version of GCC ever released. On Mac OS X sometimes the gcc and g++ commands are actually aliases for Clang not GCC, and the version of libstdc++ that Apple ships is ancient (circa 2008) so of course it doesn't support C++11. This means that on OS X when using Clang-pretending-to-be-GCC, you can use -stdlib=libc++ to select Clang's new C++11-compatible library, or you can use -stdlib=libstdc++ to select the pre-C++11 antique version of libstdc++ that belongs in a museum. But on GNU/Linux gcc and g++ really are GCC not Clang, and so the -stdlib option won't work at all.

Does the compiler automatically use libstdc++?

Yes, GCC always uses libstdc++ unless you tell it to use no standard library at all with the -nostdlib option (in which case you either need to avoid using any standard library features, or use -I and -L and -l flags to point it to an alternative set of header and library files).

I am using gcc4.8.2 on Ubuntu 13.10 and I would like to use the c++11 standard. I already pass -std=c++11 to the compiler.

You don't need to do anything else. GCC comes with its own implementation of the C++ standard library (libstdc++) which is developed and tested alongside GCC itself so the version of GCC and the version of libstdc++ are 100% compatible. If you compile with -std=c++11 then that enables the C++11 features in g++ compiler and also the C++11 features in the libstdc++ headers.

Solution 3

The compiler uses the libstdc++ automatically, if you use the g++ frontend, not the gcc frontend.

Comments

[Aaron McDaid]

Indeed. Just to clarify: The OP asked "when compiling with gcc?". If one runs gcc on a bunch of .o files, then I think it assume they're just C programs and it doesn't link in any C++ stuff. But if you use g++ (or if you have any cpp files on the gcc command line), then I think it'll realise that stdc++ should be included. But I'm not too certain about this. Is this what you're saying?

[Raymond Valdes]

If I understand this correctly, then libstdc++ supports c++11 for linux but not OSX10.9. -> c++11 code on osx10.9 must be compiled/linked with -stdlib=libc++.

[Raymond Valdes]

To clarify my question, when I said gcc I meant the gnu compiler collection (as a whole). Since I'm talking about c++ code, then I would be using the g++ frontend.

[Bill Lynch]

@raymondvaldes: That's correct. Apple refuses to distribute newer versions of libstdc++ that would contain C++11 support.

[hithwen]

Can I use libstdc++ with clang (not clang++)?

[Bill Lynch]

@hithwen: libstdc++ is the C++ standard library. It doesn't make much sense to use it with clang instead of clang++.

[Bill Lynch]

@hithwen: I don't think your problem is related to this answer. I'd recommend creating a new question to ask this.

[nn0p]

What's the difference between libc++ and libstdc++? Are they both c++ standard runtime library?

[Bill Lynch]

@nn0p yes. They are two different implementations of the c++ standard library. One by the gcc folks, one by the llvm folks.

[garyF]

Wow. This one was not obvious.

[Jonathan Wakely]

@RaymondValdes when referring to the whole it's conventional to say GCC, to distinguish it from the gcc driver program.

[Jonathan Wakely]

@Torsten, what you're referring to is automatically linking to libstdc++ if you use g++ rather than gcc. If you compile a C++ file (one with an extension like .cc or .C or .cpp) with gcc then it compiles the code using the C++ front-end and automatically makes the libstdc++ headers available via #include, exactly the same as when you compile with g++. Only the linking step handles libstdc++ differently depending whether you use gcc or g++.

[user2864740]

To avoid the silly "g++" and "gcc" nonsense on OS X, it's probably time-saving to explicitly use g++-mp-9 (or whatever flavor of GCC install has been performed) during compilation, if that's the goal - in which case the real GNU libstdc++ library will be used by the real GNU G++ compiler, not the old OS X "libstdc++" (or newer libc++) library on faked compatibility-shim "g++".

[tjysdsg]

I used clang in my project, but for CI the compiler is set to gcc by default. So this is exactly the problem that occurred. gcc doesn't support -stdlib so the CI always failed

[Louis Dionne]

This answer is good, but it should be updated to say that as of recently, -stdlib=libstdc++ should simply not be used on macOS, since libstdc++ is not shipped in the Apple SDKs anymore.

[Bill Lynch]

@LouisDionne: I wrote the original answer a decade ago. Feel free to edit it as appropriate.