I am currently writing a library and am considering moving from GCC 4.1.2 to 4.5.2 (latest release) of GCC. If I compile my code into a static library can I assume compiler compatibility (on the same OS obviously) should be a non-issue for clients?
EDIT
To further clarify: if I provide a client a statically linked library compiled with gcc 4.5.2, what restrictions does this place on users of this library in terms of the compiler and version they must use?
Just came across this which I believe answers my question from http://gcc.gnu.org/bugs/#nonbugs:
ABI changes The C++ application binary
interface (ABI) consists of two
components: the first defines how the
elements of classes are laid out, how
functions are called, how function
names are mangled, etc; the second
part deals with the internals of the
objects in libstdc++. Although we
strive for a non-changing ABI, so far
we have had to modify it with each
major release. If you change your
compiler to a different major release
you must recompile all libraries that
contain C++ code. If you fail to do so
you risk getting linker errors or
malfunctioning programs. Some of our
Java support libraries also contain
C++ code, so you might want to
recompile all libraries to be safe. It
should not be necessary to recompile
if you have changed to a bug-fix
release of the same version of the
compiler; bug-fix releases are careful
to avoid ABI changes. See also the
compatibility section of the GCC
manual.
Remark: A major release is designated
by a change to the first or second
component of the two- or three-part
version number. A minor (bug-fix)
release is designated by a change to
the third component only. Thus GCC 3.2
and 3.3 are major releases, while
3.3.1 and 3.3.2 are bug-fix releases for GCC 3.3. With the 3.4 series we
are introducing a new naming scheme;
the first release of this series is
3.4.0 instead of just 3.4.
From this as I understand it I'll need to ensure clients are linking my library in with a major-release compatable version of gcc.
It doesn't really matter if you are providing a static library or dynamic library, the users will still need to use a compatable compiler/linker to link against it. Usually when GCC does a ABI change they offer a switch that can be set to use the old ABI. I know that they did that when they went from 3.x to 4.x and even a couple of the releases within the 4.x series.
Related
This may be a dumb question, but I am a novice in the compilation of a C or C++ project through the command line. I am currently trying to cross-compile some frameworks to run in an aarch64 device, which has a built-in gcc5.4.0. However, some of the libraries require at least gcc7.5.0. So my first question is:
Can a framework that is compiled with gcc7.5.0 version can be used in an environment where gcc5.4.0 is present?
Moreover, some processes the I try to run on the device also requires gcc7.5.0. Therefore, my second question is that:
Can an object file (.o, e.g. the output of the compiled .c / .cpp file) that is compiled with gcc7.5.0 run on a system with gcc5.4.0?
Lastly, some processes require the libraries compiled in gcc5.4.0 and gcc7.5.0 to be used together. Therefore, I have to link the .so files that are generated by both gcc5.4.0 and gcc7.5.0. Hence, my last question is:
Can one create an executable by using libraries together that are compiled with different gcc versions?
Thanks.
For your first two questions: the presence or absence of a compiler in the system does not matter. I've run software on machines where no compiler was installed (it was compiled elsewhere). I've run software on machines where a bunch of compilers are installed, included multiple version of g++.
For your last question: the thing which matter when linking (either statically, at link time, or dynamically at execution time) libraries is called ABI. There are two aspects of the ABI: for the language itself, and for the standard libraries. For both, GCC maintains forward compatibility: using the latest compiler and standard library, you can link with object files compiled with older compiler (and for older standard library) since something like 3.4. In some more restricted cases (less well documented), you can even use the older compiler and standard library.
If you want to execute a program dynamically linked (which is usually the default with GCC) with a newer version of the standard library than the one on your system, you need to ensure it is found. That's a whole other subject, but here are two key words to help you find information for Unix (I know nothing about how Windows handle this, I presume but could be wrong that MacOS is a unix for this purpose): LD_LIBRARY_PATH and rpath.
Obviously meeting bugs is always a possibility and some care is needed in some cases. So here are some relevant links with the details.
GCC Compiler option related to ABI.
Standard library documentation related to ABI.
On the use on the C++11 library ABI. C++11 put additional requirements on some types (notably std::string) which prevented GCC to keep its old ABI for those types. Thus a new ABI was introduced in GCC 5 or so and if you want to be compatible with the previous one you have to pay attention. (Note that distributers sometimes forced the old ABI even with GCC version 5 and later)
Older related SO question
I've written a fairly substantial C++11 library, and I'm planning to allow download of pre-compiled versions from my website.
So I've set up an automated build which compiles the library with clang and make it available for download, but this has exposed a problem: if I try to use the clang-compiled library with GCC I get undefined references (mainly related to std::string). I think this is related to the GCC dual-ABI changes in GCC 5.1 but I'm not sure how to fix it.
My question is, what flags should I set, or practices should I follow to make a C++ library compatible with both clang and GCC?
Or should I give up and compile two separate libraries?
As already mentioned in several places (eg. here) libc++ is not fully binary compatible with libstdc++.
There are several options, but some of them are somewhat not-so-straightforward.
Compile two separate libraries - always working solution.
Remove incompatible containers from your interface (eg. std::string) - but this might be lot of work and sometimes not a good idea.
Instruct your library GCC users that you link with libc++ and that they need to do the same, basic steps here. But I guess most GCC users do not want to do this.
Use clang with libstdc++ using -stdlib=libstdc++ flag to be compatible with libstdc++ (as suggested in other answer). This solution might be harder to setup on some platforms though.
I would suggest as already mentioned in comments to go with option 1.
There are several options:
Don't distribute it in binary form. Instead, make it easy to build everywhere (e.g. by using CMake, or autotools or ...)
Make it header only. This is by far the simplest solution but might not be what you want. It only really makes sense for templated code, and incurs a heavy impact on compile-time performance of your library.
Tell people to link with libstdc++ when using Clang and your library. Suboptimal solution (I for one like to check my code against libc++ as well as libstdc++), but (virtually) every Linux user has libstdc++ installed anyway. Make sure to pick a slightly older version (the one shipped in the latest Debian Stable distro is a good choice), because newer versions might introduce new symbols olders versions are missing. New versions should be ABI compatible anyway.
Note the situation for Visual Studio users is even worse, where every single compiler release mandates a new binary because they guarantee absolutely nothing with respect to the C++ libraries' or compiler's ABI.
Another option is for your shared library to not expose any C++ standard library types in its interface. And have a header file supplied with your shared library that converts std::string to types consumed by your library, such as struct my_string_span { char const *begin, *end; }; and other standard containers as necessary.
Having gone through a number of similar questions (see below), I don't think any of them cover this case.
Problem
Is it possible to dlopen and use a C++11 shared library compiled using gcc 4.9 from a C++03 application compiled with gcc 4.1? Both the library and application use libstdc++, including on the API (but not std::list, which is apparently a problem). I can't change the way the application is compiled (sadly), and must be certain that existing code doesn't break (e.g. if it ends up dynamically linking to a new version of libstdc++.so).
As far as I understand it (which is not very), on Linux the dynamic linker uses a flat namespace, meaning that is not possible to have the same symbol defined in two libraries. Does statically linking the library against the newer libstdc++ help at all here, or is there perhaps some other way?
Some similar questions that don't seem to answer this
C++11 backwards compatibility
C++03 library with C++11 source code
C++11 compatibility with existing libraries/frameworks
Can a compiled C++11 library (lib,dll,etc.) be linked in older C++ compilers? [softwareengineering.stackexchange.com]
If you do that, you definitely need to use the newer libstdc++.so.6, which should be compatible with the system libstdc++.so.6 based on GCC 4.1 (in the sense that GCC upstream intends to preserve ABI compatibility for the library). It would be a very good idea to use the libstdc++.so.6 library that came with the GCC 4.9 compiler.
Once you do that, it is supposed to work, unless you hit the few of the compatibility gotchas you already listed, and as long as the C++ part of the library interface actually sticks to the C++98 subset and does not use any constructs which are not expressible in the language subset.
All this assumes that the library was actually compiled on the the system which uses GCC 4.1, which probably has something like glibc 2.5. If the library was compiled on a completely different, newer system, then you will likely run into library compatibility issues beyond libstdc++.so.6, and these libraries tend to be even harder to upgrade.
(There is also the possibility that the library was explicitly compiled for use with the 4.1-based system libstdc++.so.6 and everything just works, just as if by magic, but then you wouldn't be asking here, I suppose.)
If you still have problems there is the option to use a statically linked C-API library as border between the two, application and your library, see this post.
Also you may be able to explicitely demand an old version of a symbol, see this post
GCC is great with ABI-compatibility as long as you use the same C++ standard [1].
But it strikes me that if a shared library compiled by GCC 4.3 in C++03 mode exposes, say, a std::string, this is going to be a different std::string than that understood by an executable compiled by GCC 4.8 in C++11 mode.
The reason I ask is that I am planning to deploy a program compiled by GCC 4.8 in C++11 mode on CentOS 6, whose maximum packaged GCC is 4.3... and some of the shared libraries (be they third-party C++ libraries or more system-level stuff) will presumably therefore all be C++03. But if that were the case, we'd never be able to deploy any C++11 programs on older Linux distributions, which seems unlikely.
Am I naive to think there might be a problem here? And, if there is, how can I resolve it?
There is a wonderful page on this matter: https://gcc.gnu.org/wiki/Cxx11AbiCompatibility
In short, C++11 in gcc is mostly ABI compatible with c++98, but there are a couple of mismatches. Page above lists all of those.
To alleviate the issue I can suggest following approach:
Clearly identify all your dependencies which are C++ libraries. You usually do not have too many of them - boost comes to mind first, do you have anything else?
Than you check if the symbols your app needs are in the list of broken ABI (see above). If they are not, you are in the clear.
If they are, you recompile the lib, and either distribute it as shared lib together with your app (playing with Rpath flags to make sure your app loads your version) or link statically against it.
Just in case, you might as well link statically against libstdc++.
Actually, you can distribute a program compiled with a newer g++ compiler on a vanilla CentOS 6 platform. There are several ways to do this: The easiest is to use the DevToolset 3, which will give you g++ 4.9.2 (the dev toolset 2 will give you gcc 4.8.2). Then, just compile your application with this g++. When distributing your software, you need to make sure to also ship the libstdc++.so that is being shipped with g++ 4.9. Either set the LD_LIBRARY_PATH so it gets picked up on startup, or set the RPATH to tell your executable where to look first for libraries.
Essentially, you can do this also with newer compilers, but then you first need to compile the compiler itself. If you don't want to compile a compiler first, go with a respective dev toolset and you should be fine.
Yes, you can also try to statically link libstdc++.a. Search for the option -static-libstdc++:
When the g++ program is used to link a C++ program, it normally automatically links against libstdc++. If libstdc++ is available as a shared library, and the -static option is not used, then this links against the shared version of libstdc++. That is normally fine. However, it is sometimes useful to freeze the version of libstdc++ used by the program without going all the way to a fully static link. The -static-libstdc++ option directs the g++ driver to link libstdc++ statically, without necessarily linking other libraries statically.
But if you statically link, you will not get any security updates etc. Granted, you will not get the updates, if you ship libstdc++.so on your own as well, but incremental updates maybe easier.
And with respect to running your application: The rule of thumb is: Compile on the oldest platform you need to support, then your binaries (with self-shipped libstdc++ and other required libs) will likely work also on newer versions. That is, if you compile on CentoOS 6, and it works, then you can expect it to also work on CentOS 7. On a related subject, this is exactly the reason why for instance AppImage and related solutions recommend to build on an old system.
In my company we use gcc 5.1.0, compiled and used on CentOS 5.5 (with old gcc on-board).
When we deploy our application we also redistribute libstdc++.so and libgcc_s.so, compiled from gcc 5.1.0 sources.
For example:
/opt/ourapp/lib/libstdc++.so
/opt/ourapp/lib/libgcc_s.so
/opt/ourapp/bin/myapp
And for starting the binary correctly we execute:
LD_LIBRARY_PATH=/opt/ourapp/lib/ myapp.
Hope it helps.
Drawbacks:
At least you can't use native gdb on such an environment because DWARF format incompatibilities.
If you build your C++11 program with the define _GLIBCXX_USE_CXX11_ABI=0 (see this) and the option --abi-version=2 (see this) you should be compatible with any library build with GCC 4.3, including libstdc++.
The default ABI version was 2 through 4.9, it seems like a safe assumption that CentOS uses the default ABI.
The _GLIBCXX_USE_CXX11_ABI macro will affect the standard library's types, to use the same layout as the pre C++11 version. This will introduce some C++11 conformance issues (the reason why they were changed in the first place), things like the complexity of std::list<>::size().
The --abi-version= command line option affects the compiler's ABI, calling conventions, name mangling etc. The default ABI was 2 from 3.4 through 4.9.
I have an application with STL objects used as part of the C++ interface for plug-in writers.
I know the best option for compatibility would have been to use a C interface instead, but that's not currently feasible.
I know that everything from GCC 3.4 to 4.8 in libstdc++ has been highly compatible in terms of ABI.
So for example, if I compile with GCC 4.1, and a plug-in vendor writes code compiled with GCC 4.7, then barring corner cases all will be well on a platform with a libstdc++ version corresponding to GCC 4.7 or later, provided STL useage is internal to the .so only, and that the external .so interface is using pure C, which sadly is not the case for me.
So, I'm curious about what the case will be with respect to the STL classes used as part of the plug-in interface. Can I safely pass STL objects between shared objects that weren't compiled with the same compiler version (e.g. 4.1 and 4.8)? And is there anything I need to be mindful of with regards to how templates are compiled and resolved if people use different compiler options?
I suspect it'll be problematic. However, there's a chance the symbol versioning magic done by the GCC folks might somehow make this work.
For this question, I am only interested in pre-C++11 compilation and linking. I am also only interested in Linux and Mac OS X, using GCC.
I already answered this on the mailing list but as Marc said, it will just work.
It doesn't make any difference whether you use the library internally to your DSO or in the interface, the library doesn't care and is backwards compatible back to GCC 3.4 either way.