GCC compatibility: inline namespaces and ABI tags

Keeping libraries binary-compatible with old versions is hard. Recently, GCC was in the unenviable situation of having to switch its std::string implementation.1 GCC used inline namespaces and ABI tags to minimize the extent of breakage and to ensure that old and new versions could only be combined in a safe way. Here, we'll have a look at those mitigation techniques: what they are and how they work.

First, some background: GCC used to have a copy-on-write implementation for std::string. However, C++11 does not allow this anymore because of new iterator and reference invalidation rules. So, GCC 5.1 introduced a new implementation of std::string. The new version was not binary-compatible with the old one: exchanging strings between old (pre-5.1) and new code would crash. We say the application binary interface (ABI) changed.

To understand the consequences of this, let's look at several scenarios:

  • A program uses only code compiled with GCC < 5.1: only old strings, works
  • A program uses only code compiled with GCC >= 5.1: only new strings, works
  • A program mixes code compiled with different GCC versions: both types of strings exist in the program, it could crash.

Let's dig into the last bullet point. When would it crash? Whenever "new" code accesses an "old" string or vice versa. Here are some examples where f() is compiled with an older GCC version and called from "new" code:

void f(int i);                 // (a) safe, no std::string involved
void f(const std::string& s);  // (b) will crash when f accesses s
std::string f();               // (c) will crash when the returned string is used

Given how common std::string is, such crashes would happen frequently if "old" and "new" code were combined. Unfortunately, it's surprisingly easy to end up with a program with some pre-5.1 parts and some newer ones. It's sufficient to link a "new" executable to an "old" library. Given the bad consequences, the GCC developers needed to solve this.

The solution is to change the symbol names of the GCC 5.1 std::string and all functions using it. Because the linker uses symbol names to resolve function calls into libraries, this would cause link-time errors for cases (b) and (c) while (a) would still work. Exactly the intended behavior.

How could this be done? The mechanism that converts C++ names into symbol names is called name mangling. The generated symbol names contain information about namespaces, function names, argument types, etc. Putting the new std::string into a separate namespace would change the symbol names of all functions accepting std::string as argument. But that's crazy—std::string needs to be in namespace std, right?

The solution for this is inline namespaces. All classes, functions, and templates declared in an inline namespace are automatically imported into the parent namespace. Their mangled name still references the original location, though.

Here's what GCC 5.1 does:

namespace std {
    inline namespace __cxx11 {
        template<typename _CharT, ...>  basic_string;
        typedef basic_string<char>      string;
    }
}

Looking at f(const std::string& s), what would the symbol names be when compiled with older and newer GCC versions?

Older GCCGCC 5.1
Symbol name_Z1fRKSs_Z1fRKNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE
Decoded symbol name2f(std::basic_string<char, ...> const&)f(std::__cxx11::basic_string<char, ...> const&)

Indeed, the symbol names are different and the linker would give an error if GCC 5.1 code called f(const std::string &s) from a library compiled with an older GCC version. This solves the compatibility problem for all functions taking std::string as argument.

One problem remains: case (c) from above, std::string f(). The return value type of a function is not part of its mangled name.3 Thus, the symbol name wouldn't change for functions that return std::string, which could still lead to runtime crashes.

GCC 5.1 solves this with the use of ABI tags. From the documentation:

The abi_tag attribute can be applied to a function, variable, or class declaration. It modifies the mangled name of the entity to incorporate the tag name, in order to distinguish the function or class from an earlier version with a different ABI

[...]

When a type involving an ABI tag is used as the type of a variable or return type of a function where that tag is not already present in the signature of the function, the tag is automatically applied to the variable or function.

GCC applies the attribute __abi_tag__ ("cxx11")) to the std::__cxx11 namespace. This affects all classes therein, including the new version of std::string. The ABI tag propagates to all functions that return a string and changes their symbol name.

Let's look at the symbol names for std::string f() for different compiler versions:

Older GCCGCC 5.1
Symbol name_Z1fv_Z1fB5cxx11v
Decoded symbol name2f()f[abi:cxx11]()

Again, the symbol name is different, and the "cxx11" ABI tag is applied to std::string f() with GCC 5.1. This completes the second part of GCC's solution for the std::string ABI change.

These two methods to induce symbol name changes for anything using std::string make the migration path to GCC 5.1 much easier. If the program links correctly it should work, and be safe from difficult-to-debug runtime errors. There would be more to discuss about the GCC 5.1 changes, such as how libstdc++.so still exports the old std::string implementation for compatibility. But this post has gone on too long already, so let's leave it at that :-)


1 GCC 5.1 also introduced a new version of std::list. It is handled analogously to std::string.
2 Decoded using c++filt from the binutils package.
3 The return value type doesn't need to be part of the symbol name because it doesn't get used for overload resolution.

published October 29, 2016
tags c++

Building External Libraries with Boost Build

I recently ran into the problem of having to use an external library not available for our distribution. Of course, it would be possible to build Debian or RPM packages, deploy them, and then link against those. However, building sanely-behaving library packages is tricky.

The easier way is to include the source distribution in a sub-folder of the project and integrate it into our build system, Boost Build. It seemed prohibitive to create Boost Build rules to build the library from scratch (from *.cpp to *.a/*.so) because the library had fairly complex build requirements. The way to go was to leverage the library's own build system.

Easier said than done. I only arrived at a fully integrated solution after seeking help on the Boost Build Mailing List (thanks Steven, Vladimir). This is the core of it:

path-constant LIBSQUARE_DIR : . ;

make libsquare.a
    : [ glob-tree *.c *.cpp *.h : generated-file.c gen-*.cpp ]
    : @build-libsquare
    ;
actions build-libsquare
{
    ( cd $(LIBSQUARE_DIR) && make )
    cp $(LIBSQUARE_DIR)/libsquare.a $(<)
}

alias libsquare
    : libsquare.a                                        # sources
    :                                                    # requirements
    :                                                    # default-build
    : <include>$(LIBSQUARE_DIR) <dependency>libsquare.a  # usage-requirements
    ;

The first line defines a filesystem constant for the library directory, so the build process becomes independent of the working directory.

The next few lines tell Boost Build about a library called libsquare.a, which is built by calling make. The cp command copies the library to the target location expected by Boost Build ($(<)). glob-tree defines source files that should trigger library rebuilds when changed, taking care to exclude files generated by the build process itself.

The alias command defines a Boost Build target to link against the library. The real magic is the <dependency>libsquare.a directive: it is required because the library build may produce header files used by client code. Thus, build-libsquare must run before compiling any dependent C++ files. Adding libsquare.a to an executable's dependency list won't quite enforce this: it only builds libsquare.a in time for the linking step (*.oexecutable), but not for the compilation (*.cpp*.o). In contrast, the <dependency>libsquare.a directive propagates to all dependent build steps, including the compilation, and induces the required dependencies.

I created an example project on GitHub that demonstrates this. It builds a simple executable relying on a library built via make. The process is fully integrated in the Boost Build framework and triggered by a single call to bjam (or bb2).

If you need something along these lines give this solution a try!

Remarks on enable_shared_from_this

std::enable_shared_from_this is a template base class that allows derived classes to get a std::shared_ptr to themselves. This can be handy, and it's not something that C++ classes can normally do. Calling std::shared_ptr<T>(this) is not an option as it creates a new shared pointer independent of the existing one, which leads to double destruction.

The caveat is that before calling the shared_from_this() member function, a shared_ptr to the object must already exist, otherwise undefined behavior results. In other words, the object must already be managed by a shared pointer.

This presents an interesting issue. When using this technique, there are member functions (those that rely on shared_from_this()) that can only be called if the object is managed via a shared_ptr. This is a rather subtle requirement: the compiler won't enforce it. If violated, the object may even work at runtime until a problematic code path is executed, which may happen rarely – a nice little trap. At the very least, this should be prominently mentioned in the class documentation. But frankly, relying on the documentation to communicate such a subtle issue sounds wrong.

The correct solution is to let the compiler enforce it. Make the constructors private and provide a static factory method that returns a shared_ptr to a new instance. Take care to delete the copy constructor and the assignment operator to prevent anyone from obtaining non-shared-pointer-managed instances this way.

Another point worth mentioning about enable_shared_from_this is that the member functions it provides, shared_from_this() and weak_from_this(), are public. Not only the object itself can retrieve it's owning shared_ptr, everyone else can too. Whether this is desirable is essentially an API design question and depends on the context. To restrict access to these functions, use private inheritance.

Overall, enable_shared_from_this is an interesting tool, if a bit situational. However, it requires care to use safely, in a way that prevents derived classes from being used incorrectly.

published May 21, 2016
tags c++

Boost Range Highlights

Last week, I presented Boost Range for Humans: documentation for the Boost Range library with concrete code examples. This week we'll talk about some of the cool features in Boost Range.

irange

boost::irange() is the C++ equivalent to Python's range() function. It returns a range object containing an arithmetic series of numbers:

boost::irange(4, 10)    -> {4, 5, 6, 7, 8, 9}
boost::irange(4, 10, 2) -> {4, 6, 8}

Together with indexed() (see below), it serves as a range-based alternative to the classic C for loop.

combine

boost::combine() takes two or more input ranges and creates a zipped range – a range of tuples where each tuple contains corresponding elements from each input range.

The input ranges must have equal size.

std::string str = "abcde";
std::vector<int> vec = {1, 2, 3, 4, 5};
for (const auto & zipped : boost::combine(str, vec)) {
    char c; int i;
    boost::tie(c, i) = zipped;

    // Iterates over the pairs ('a', 1), ('b', 2), ...
}

Algorithms

Most if not all algorithms from the C++ standard library that apply to containers (via begin/end iterator pairs) have been wrapped in Boost Range. Examples include copy(), remove(), sort(), count(), find_if().

Adaptors

Adaptors are among the most interesting concepts Boost Range has to offer.

There are generally two ways to use adaptors, either via function syntax or via a pipe syntax. While the former is handy for simple cases, while the latter allows chaining data transformations into an easy-to-read pipeline.

bool is_even(int n) { return n % 2 == 0; }
const std::vector<int> vec = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };

// Function-style call:
for (int i : boost::adaptors::filter(vec, is_even)) { ... }

// Pipe-style call:
for (int i : vec | boost::adaptors::filtered(is_even)) { ... }

To see the power of the latter syntax, consider a transformation pipeline:

int square(int n) { return n*n; }
std::map<int, int> input_map = { ... };

using boost::adaptors;
auto result = input_map | map_values
                        | filtered(is_even)
                        | transformed(square);

indexed

The boost::adaptors::indexed() adapter warrants special mention: it is analogous to Python's enumerate(). Given a Range, it gives access to the elements as well as their index within the range. Boost 1.56 or higher is required for this to work properly.

accumulate

boost::accumulate() by default sums all the items in an input range, but other reduction functions can be supplied as well. Together with range adapters, this makes map-reduce pipelines easy to write.

std::vector<int> vec = {1, 2, 3, 4, 5};
int product = boost::accumulate(vec, 1, std::multiplies<int>());

as_literal

boost::as_literal() may be less a highlight and more of a crutch, but it bears mentioning still. Boost Range functions accept a wide variety of types, among them strings. C++ style strings (std::string) always work as expected, but with character arrays (char[]), there is an ambiguity as to whether the argument should be interpreted as array (including the terminal '\0' character) or as string (excluding the terminator).

By default, Boost Range treats them as arrays, which they are, after all. In practice, this is often a pitfall for newcomers. If any string-related range operations don't work as expected, this is a common reason.

To force the library to treat character arrays as strings, they can be wrapped in an as_literal() call. Alternatively, the C strings can be cast to std::string as well.

Summary

There are several interesting aspects about Boost Range. It plays very well with C++11's range-based for loops and makes code operating on containers much easier to write and (most importantly) read. In addition, it makes it possible to lay out data processing pipelines a lot more clearly.

Container iteration and modification becomes as easy as it is in modern scripting languages, which is a huge, huge step for the C++ language.

Let's hope that C++17 brings similar capabilities in the standard library. Until then, Boost Range is the way to go, so check out the docs and try it yourself.