Can Different Gcc Dialects Be Linked Together

Can different GCC dialects be linked together?

A priori, no. The safest solution is to assume that all of the compiler options are identical, except when the compiler specifically documents that the option doesn't affect binary compatibility. (Documentation which is sorely lacking in most compilers.) In practice, in the lack of documentation, it seems a safe bet that options which control warnings (-W... in g++) won't affect binary compatibility, and that options which affect code generation (language level, etc.) might: g++ generally maintains compatibility across different levels of optimization, where as VC++ doesn't.

Another real problem is defining preprocessor symbols in the command line. Again, the safest bet is that all of the defines be identical, but also again, some common sense is in order: one can hardly expect the standard library to have been compiled with preprocessor symbols which are used in your project (things like MYPROG_CONFIG_FILE_LOCATION, say). On the other hand, be aware that preprocessor definitions of _GLIBCXX_DEBUG and _GLIBCXX_DEBUG_PEDANTIC will affect binary compatibility (although g++ ensures that you will get a library version which works with them if you use them consistently).

With regards to your question: I would not expect to much impact on binary compatibility due to standard version, but it would hardly surprise me if the choice affects some pre-defined preprocessor symbols, in a way that would break binary compatibility in the library, much as if you'd compiled some of the modules with _GLIBCXX_DEBUG, and some without. It might work, but I wouldn't count on it.

Can you mix c++ compiled with different versions of the same compiler

Different generations of the same compiler sometimes can be compatible with each other, but not always. For example, GCC 4.7.0 changed its C/C++ ABI, meaning libraries compiled with 4.7.0+ and 4.7.0- are not likely to be compatible with each other (so in your example, the library compiled with 4.6 will not be compatible with the library compiled with 4.9). There can also be ABI bugs within a given compiler release, as happened in GCC 4.7.0/4.7.1:

GCC versions 4.7.0 and 4.7.1 had changes to the C++ standard library which affected the ABI in C++11 mode: a data member was added to std::list changing its size and altering the definitions of some member functions, and std::pair's move constructor was non-trivial which altered the calling convention for functions with std::pair arguments or return types. The ABI incompatibilities have been fixed for GCC version 4.7.2 but as a result C++11 code compiled with GCC 4.7.0 or 4.7.1 may be incompatible with C++11 code compiled with different GCC versions and with C++98/C++03 code compiled with any version.

The GCC ABI Policy and Guidelines page indicates they try to maintain forward compatibility, but not backward compatibility:

Versioning gives subsequent releases of library binaries the ability to add new symbols and add functionality, all the while retaining compatibility with the previous releases in the series. Thus, program binaries linked with the initial release of a library binary will still run correctly if the library binary is replaced by carefully-managed subsequent library binaries. This is called forward compatibility.

The reverse (backwards compatibility) is not true. It is not possible to take program binaries linked with the latest version of a library binary in a release series (with additional symbols added), substitute in the initial release of the library binary, and remain link compatible.

That page also has some fairly lengthy explanations on the versioning system GCC uses to mark different versions of given components, as well as an explanation for the versioning behind GCC itself:

Allowed Changes

• The following will cause the library minor version number to increase, say from "libstdc++.so.3.0.4" to "libstdc++.so.3.0.5".

• Adding an exported global or static data member

• Adding an exported function, static or non-virtual member function

• Adding an exported symbol or symbols by additional instantiations

• Other allowed changes are possible.

Prohibited Changes

The following non-exhaustive list will cause the library major version number to increase, say from "libstdc++.so.3.0.4" to "libstdc++.so.4.0.0".

• Changes in the gcc/g++ compiler ABI

• Changing size of an exported symbol

• Changing alignment of an exported symbol

• Changing the layout of an exported symbol

• Changing mangling on an exported symbol

• Deleting an exported symbol

• Changing the inheritance properties of a type by adding or removing base classes

• Changing the size, alignment, or layout of types specified in the C++ standard. These may not necessarily be instantiated or otherwise exported in the library binary, and include all the required locale facets, as well as things like std::basic_streambuf, et al.

• Adding an explicit copy constructor or destructor to a class that would otherwise have implicit versions. This will change the way the compiler deals with this class in by-value return statements or parameters: instead of passing instances of this class in registers, the compiler will be forced to use memory. See the section on Function Calling Conventions and APIs of the C++ ABI documentation for further details.

Note the bolded bit. In a perfect world, GCC versions with the same major release number would be binary-compatible. This isn't a perfect world, so test very very carefully before you go mixing compiler versions like this, but in general you'll probably be okay.

Is it safe to link C++17, C++14, and C++11 objects

Which combinations of these objects is it and isn't it safe to link into a single binary? Why?

For GCC it is safe to link together any combination of objects A, B, and C. If they are all built with the same version then they are ABI compatible, the standard version (i.e. the -std option) doesn't make any difference.

Why? Because that's an important property of our implementation which we work hard to ensure.

Where you have problems is if you link together objects compiled with different versions of GCC and you have used unstable features from a new C++ standard before GCC's support for that standard is complete. For example, if you compile an object using GCC 4.9 and -std=c++11 and another object with GCC 5 and -std=c++11 you will have problems. The C++11 support was experimental in GCC 4.x, and so there were incompatible changes between the GCC 4.9 and 5 versions of C++11 features. Similarly, if you compile one object with GCC 7 and -std=c++17 and another object with GCC 8 and -std=c++17 you will have problems, because C++17 support in GCC 7 and 8 is still experimental and evolving.

On the other hand, any combination of the following objects will work (although see note below about libstdc++.so version):

• object D compiled with GCC 4.9 and -std=c++03
• object E compiled with GCC 5 and -std=c++11
• object F compiled with GCC 7 and -std=c++17

This is because C++03 support is stable in all three compiler versions used, and so the C++03 components are compatible between all the objects. C++11 support is stable since GCC 5, but object D doesn't use any C++11 features, and objects E and F both use versions where C++11 support is stable. C++17 support is not stable in any of the used compiler versions, but only object F uses C++17 features and so there is no compatibility issue with the other two objects (the only features they share come from C++03 or C++11, and the versions used make those parts OK). If you later wanted to compile a fourth object, G, using GCC 8 and -std=c++17 then you would need to recompile F with the same version (or not link to F) because the C++17 symbols in F and G are incompatible.

The only caveat for the compatibility described above between D, E and F is that your program must use the libstdc++.so shared library from GCC 7 (or later). Because object F was compiled with GCC 7, you need to use the shared library from that release, because compiling any part of the program with GCC 7 might introduce dependencies on symbols that are not present in the libstdc++.so from GCC 4.9 or GCC 5. Similarly, if you linked to object G, built with GCC 8, you would need to use the libstdc++.so from GCC 8 to ensure all symbols needed by G are found. The simple rule is to ensure the shared library the program uses at run-time is at least as new as the version used to compile any of the objects.

Another caveat when using GCC, already mentioned in the comments on your question, is that since GCC 5 there are two implementations of std::string available in libstdc++. The two implementations are not link-compatible (they have different mangled names, so can't be linked together) but can co-exist in the same binary (they have different mangled names, so don't conflict if one object uses std::string and the other uses std::__cxx11::string). If your objects use std::string then usually they should all be compiled with the same string implementation. Compile with -D_GLIBCXX_USE_CXX11_ABI=0 to select the original gcc4-compatible implementation, or -D_GLIBCXX_USE_CXX11_ABI=1 to select the new cxx11 implementation (don't be fooled by the name, it can be used in C++03 too, it's called cxx11 because it conforms to the C++11 requirements). Which implementation is the default depends on how GCC was configured, but the default can always be overridden at compile-time with the macro.

Linking object files built using different versions of GCC

The safest would be to give them a .so and its corresponding header with the stable binary API. To be binary stable that API should not accept or return any std:: types like std::string or std::vector<> because the binary layout of std:: types may change from version to version.

And it should be linked statically with libstdc++ and libgcc_s, so that your clients don't have to link against a particular version of libstdc++.

You can also pack all your .o files into one .a for convenience, so that when you add a new .o file your clients don't have to update their makefiles to link against the new .o.

Compiling and linking with a different versions of gcc on linux

Linux has a C++ Application Binary Interface (ABI), which has been around for a while. This means that the calling conventions and name mangling across compilers on Linux is fixed. Therefore, as long as the libraries are compatible, you should be able to compiler with different compilers (or different versions of the same compiler) and have code which correctly and reliably links together.

Not entirely the ELF requirements per se...

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