A C++ gotcha on Snow Leopard

On Snow Leopard, Apple decided to build g++ and the standard C++ library with “fully dynamic strings” enabled. What this means for you relates to the empty string.

When fully dynamic strings are off (as was true in Leopard), there exists a single global variable representing the empty string. This variable lives in the data segment of libstdc++, and so it does not exist on the heap. Whenever a string is deconstructed, the standard library would check whether that string’s address matches matches the empty string’s: if so, it does nothing; if not, it calls free.

With fully dynamic strings on, there is no global empty string. All strings are on the heap, and once their reference count goes to zero, they get deallocated. Where this creates a problem is if you mix and match code. If a library that does have fully dynamic strings enabled (aka the standard library) receives an empty string from code which does not have it enabled (aka, the app you just built), it will try to free it and your application will crash.

Here’s a reproducible case for this issue using Boost:

#include <string>
#include <sstream>
#include <boost/variant.hpp>

int main()
{
  std::ostringstream buf;
  boost::variant<bool, std::string> data;
  data = buf.str();
  data = false;
  return 0;
}

In this case – which really happened to me – I created an empty string by calling ostringstream::str(). Since I don’t have fully dynamic string on, its address is in data space, not on the heap. I pass this string to boost::variant, which makes a copy of that address. Later, when the variant is reassigned false, it calls ~basic_string to deconstruct the string. Since my standard library is compiled with fully dynamic strings, the destructor for basic_string doesn’t recognize that its the “special” empty string, so it tries to free it.

The solution to this problem is three-fold:

1. You must be using the g++ that comes with Xcode, or if you build your own (say, via MacPorts), you must configure it using --enable-fully-dynamic-string. I’ve already submitted a patch to this effect to the MacPorts crew.

2. All libraries must be compiled with -D_GLIBCXX_FULLY_DYNAMIC_STRING.

3. Your own code must be compiled with -D_GLIBCXX_FULLY_DYNAMIC_STRING.

You’ll know if this issue is biting you by looking at a stack trace in gdb. You’ll see a crash somewhere inside basic_string’s _M_destroy (which calls free). Move up the trace a bit and check whether the string it’s trying to free is 0 bytes long.

To recap: what’s happened is that an empty string constructed by code without fully dynamic strings got deallocated by code that was. That is, most likely you, or a library you built, handed an empty std::string to the system library.