Help understanding segfault with std::map/boost::unordered_map

Question

Hello,

I have some code to handle resource (images, fonts, mesh data, etc.) management using a template'd static class, allowing client code to do something like:

ResourceManager<Texture>::init("data/textures");
ResourceManager<Font>::init("data/fonts");
// later ...
boost::shared_ptr<const Texture> tex = ResourceManager<Texture>::getResource("wall.png");
boost::shared_ptr<const Font> font = ResourceManager<Font>::getResource("Arial.ttf");
// later ...
ResourceManager<Texture>::release();

The "resource type" must have a constructor taking a const std::string&.

getResource is implemented as follows:

static boost::shared_ptr<const ResourceType> getResource(const std::string& fileName)
{
    boost::shared_ptr<ResourceType> resource;

    typename table_t::const_iterator itr = _resources.find(fileName);
    if (itr == _resources.end()) {
        resource.reset(new ResourceType(_dataDirectory + fileName));
        _resources[fileName] = resource;
    } else {
        resource = itr->second;
    }

    return resource;
}

table_t is defined as typedef typename boost::unordered_map< std::string, boost::shared_ptr<ResourceType> > table_t;

_resources is of type table_t.

The problem is with boost::unordered_map I get a segfault on the call to find (originating from find_iterator). However, with std::map instead, I either get a segfault on the insert operation (originating from _Rb_tree_decrement), or on the call to find (originating from string::compare).

The problem only occurs the 2nd time a resource is requested (fileName is valid when the failure occurs).

As this is happening with both map and unordered_map I'm assuming I must be doing something bizarre somewhere to cause this, any ideas?

Thanks.

EDIT: Still having the problem, I was wrong about it only happening the 2nd time a resource is requested. However, the first 2 calls to get a resource are successful, it's the 3rd call that is causing the segfault (each call is for a different resource).

Here is a stack trace:

Program received signal SIGSEGV, Segmentation fault.
0x00000000004b4978 in boost::unordered_detail::hash_table<boost::unordered_detail::map<std::string, boost::hash<std::string>, std::equal_to<std::string>, std::allocator<std::pair<std::string const, boost::shared_ptr<Texture> > > > >::find_iterator (this=0x7aed80, bucket=0x38, k=...)
    at /usr/local/include/boost/unordered/detail/table.hpp:55
55          node_ptr it = bucket->next_;
(gdb) bt
#0  0x00000000004b4978 in boost::unordered_detail::hash_table<boost::unordered_detail::map<std::string, boost::hash<std::string>, std::equal_to<std::string>, std::allocator<std::pair<std::string const, boost::shared_ptr<Texture> > > > >::find_iterator (this=0x7aed80, bucket=0x38, k=...)
    at /usr/local/include/boost/unordered/detail/table.hpp:55
#1  0x00000000004b294c in boost::unordered_detail::hash_table<boost::unordered_detail::map<std::string, boost::hash<std::string>, std::equal_to<std::string>, std::allocator<std::pair<std::string const, boost::shared_ptr<Texture> > > > >::find (this=0x7aed80, k=...)
    at /usr/local/include/boost/unordered/detail/table.hpp:583
#2  0x00000000004b07c1 in boost::unordered_map<std::string, boost::shared_ptr<Texture>, boost::hash<std::string>, std::equal_to<std::string>, std::allocator<std::pair<std::string const, boost::shared_ptr<Texture> > > >::find (this=0x7aed80, k=...)
    at /usr/local/include/boost/unordered/unordered_map.hpp:423
#3  0x00000000004ae7c6 in ResourceManager<Texture>::getResource (fileName=...) at /home/tim/Projects/gameproj/app/ResourceManager.hpp:52
#4  0x00000000004ce7fc in Map::loadCellTextures (this=0x7fffffffdfc0, in=...) at /home/tim/Projects/gameproj/app/Map.cpp:57
#5  0x00000000004ce632 in Map (this=0x7fffffffdfc0, fileName=...) at /home/tim/Projects/gameproj/app/Map.cpp:30
#6  0x0000000000495702 in Game::init (xResolution=1024, yResolution=768) at /home/tim/Projects/gameproj/app/Game.cpp:116
#7  0x0000000000494fa0 in Game::run (xResolution=1024, yResolution=768) at /home/tim/Projects/gameproj/app/Game.cpp:38
#8  0x0000000000487f1d in Main::run (xResolution=1024, yResolution=768) at /home/tim/Projects/gameproj/app/Main.cpp:28
#9  0x0000000000487db5 in main (argc=1, argv=0x7fffffffe398) at /home/tim/Projects/gameproj/app/main.cpp:10

Answer 1

The problem only occurs the 2nd time a resource is requested

This suggests to me that your code might be freeing the resource - the first time it's all fine, then you free it, then the next time the container tries to access that memory, it segfaults.

Answer 2

I can't spot any obvious errors, have you tried Valgrind (assuming you run some kind of *nix system)? It's an invaluable tool for finding memory errors, and this looks like it might be one of those.