unordered_map with string in managed_shared_memory fails - boost

Question

This is my code:
int main (int argc, char *argv[])
{
typedef int KeyType;
typedef string MappedType;
typedef std::pair<KeyType, MappedType> ValueType;
typedef boost::interprocess::allocator<ValueType, boost::interprocess::managed_shared_memory::segment_manager> ShmAlloc;
typedef boost::unordered_map<KeyType, MappedType, boost::hash<KeyType>, std::equal_to<KeyType>, ShmAlloc> ShmHashMap;
boost::interprocess::managed_shared_memory segment(boost::interprocess::open_or_create, "ContainerSharedMemory", 65536);
if(argc == 2 && string(argv[1]) == "clear")
{
boost::interprocess::shared_memory_object::remove("ContainerSharedMemory");
return 0;
}
ShmHashMap *hash_map = segment.find_or_construct<ShmHashMap>(boost::interprocess::unique_instance)(segment.get_segment_manager());
if(hash_map == NULL)
{
cout << "find_or_construct error" << endl;
return 0;
}
for(int i = 0; i < 5; ++i) {
ShmHashMap::iterator iter = hash_map->find(i);
if (iter == hash_map->end()) {
hash_map->insert(ValueType(i, "test"));
}
}
cout << "all..." << endl;
for(ShmHashMap::iterator iter = hash_map->begin(); iter != hash_map->end(); ++iter)
{
cout << iter->first << "|" << iter->second << endl;
}
cout << "end..." << endl;
return 0;
}
Everything is ok when MappedType is int, but a segment fault whit this code like this:
Rerun this program to access hash map in shared memory will coredump
----------------------------edit again----------------------------------
the problem about string is solved by sehe, thank you
and if i design a template class want to hide that detail, how could i do? if there is some perfect way
template<typename MappedType>
struct ComplexMappedType
{
ComplexMappedType(): t_access(0), t_expire(0) {}
ComplexMappedType(const MappedType& v, uint32_t a, uint32_t e): value(v), t_access(a), t_expire(e) {}
MappedType value;
uint32_t t_access;
uint32_t t_expire;
};
template <typename KeyType, typename MappedType>
class MMSHashMap
{
private:
typedef ComplexMappedType<MappedType> DataType;
typedef std::pair<KeyType, DataType> ValueType;
typedef boost::interprocess::allocator<ValueType, boost::interprocess::managed_shared_memory::segment_manager> ShmAlloc;
typedef boost::unordered_map<KeyType, DataType, boost::hash<KeyType>, std::equal_to<KeyType>, ShmAlloc> ShmHashMap;
public:
MMSHashMap(const std::string& name, size_t size, float e_thr, float e_scale);
~MMSHashMap() {delete pMemorySegment;}
size_t getMEMSize() { return pMemorySegment->get_size(); }
size_t getMEMFreeSize() { return pMemorySegment->get_free_memory(); }
bool get(const KeyType& key, MappedType& value, uint32_t& expire);
bool set(const KeyType& key, const MappedType& value, uint32_t expire);
bool del(const KeyType& key);
private:
void doCapacityElimination();
std::string _name;
boost::interprocess::managed_shared_memory* pMemorySegment;
boost::shared_mutex mutex, mutex_eliminate;
float fEliminateThreshold, fEliminateScale;
};

Answer 1

Of course. std::string allocates from the heap.
The heap is in your process address space, so any other process reading the same shared memory is going to get a wrong raw pointer there and invoke UB.
You need to use a shared-memory allocator with the strings too.
Live On Coliru (using mapped file for Coliru)
With shared memory:
#include <boost/interprocess/managed_shared_memory.hpp>
#include <boost/interprocess/allocators/allocator.hpp>
#include <boost/container/scoped_allocator.hpp>
#include <boost/container/string.hpp>
#include <boost/unordered_map.hpp>
#include <iostream>
namespace bip = boost::interprocess;
int main (int argc, char *argv[])
{
typedef int KeyType;
typedef boost::container::basic_string<char, std::char_traits<char>, bip::allocator<char, bip::managed_shared_memory::segment_manager> > MappedType;
typedef std::pair<KeyType, MappedType> ValueType;
typedef boost::interprocess::allocator<ValueType, boost::interprocess::managed_shared_memory::segment_manager> ShmAlloc;
typedef boost::unordered_map<KeyType, MappedType, boost::hash<KeyType>, std::equal_to<KeyType>, boost::container::scoped_allocator_adaptor<ShmAlloc> > ShmHashMap;
boost::interprocess::managed_shared_memory segment(boost::interprocess::open_or_create, "ContainerSharedMemory", 65536);
if(argc == 2 && std::string(argv[1]) == "clear")
{
boost::interprocess::shared_memory_object::remove("ContainerSharedMemory");
return 0;
}
ShmHashMap *hash_map = segment.find_or_construct<ShmHashMap>(boost::interprocess::unique_instance)(segment.get_segment_manager());
if(hash_map == NULL)
{
std::cout << "find_or_construct error" << std::endl;
return 0;
}
for(int i = 0; i < 5; ++i) {
ShmHashMap::iterator iter = hash_map->find(i);
if (iter == hash_map->end()) {
hash_map->insert(ValueType(i, MappedType { "hello", segment.get_segment_manager() }));
}
}
std::cout << "all..." << std::endl;
for(ShmHashMap::iterator iter = hash_map->begin(); iter != hash_map->end(); ++iter)
{
std::cout << iter->first << "|" << iter->second << std::endl;
}
std::cout << "end..." << std::endl;
}
Prints
all...
4|hello
3|hello
2|hello
1|hello
0|hello
end...