Why is Valgrind stating that my implementation of std::map<T, T> produces a memory leak?

https://stackoverflow.com/questions/766080

c++
valgrind

12-09-2019
|

Question

Valgrind is outputting the following:

==14446== 2,976 (176 direct, 2,800 indirect) bytes in 2 blocks are definitely lost in loss record 23 of 33
==14446==    at 0x4C2506C: operator new(unsigned long) (in /usr/lib64/valgrind/amd64-linux/vgpreload_memcheck.so)
==14446==    by 0x41C487: __gnu_cxx::new_allocator<std::_Rb_tree_node<std::pair<unsigned const, vimrid::imaging::ImageMatrixColumn> > >::allocate(unsigned long, void const*) (new_allocator.h:92)
==14446==    by 0x41C4AB: std::_Rb_tree<unsigned, std::pair<unsigned const, vimrid::imaging::ImageMatrixColumn>, std::_Select1st<std::pair<unsigned const, vimrid::imaging::ImageMatrixColumn> >, std::less<unsigned>, std::allocator<std::pair<unsigned const, vimrid::imaging::ImageMatrixColumn> > >::_M_get_node() (stl_tree.h:357)
==14446==    by 0x41C915: std::_Rb_tree<unsigned, std::pair<unsigned const, vimrid::imaging::ImageMatrixColumn>, std::_Select1st<std::pair<unsigned const, vimrid::imaging::ImageMatrixColumn> >, std::less<unsigned>, std::allocator<std::pair<unsigned const, vimrid::imaging::ImageMatrixColumn> > >::_M_create_node(std::pair<unsigned const, vimrid::imaging::ImageMatrixColumn> const&) (stl_tree.h:366)
==14446==    by 0x5036E9A: std::_Rb_tree<unsigned, std::pair<unsigned const, vimrid::imaging::ImageMatrixColumn>, std::_Select1st<std::pair<unsigned const, vimrid::imaging::ImageMatrixColumn> >, std::less<unsigned>, std::allocator<std::pair<unsigned const, vimrid::imaging::ImageMatrixColumn> > >::_M_insert_(std::_Rb_tree_node_base const*, std::_Rb_tree_node_base const*, std::pair<unsigned const, vimrid::imaging::ImageMatrixColumn> const&) (stl_tree.h:852)
==14446==    by 0x5037027: std::_Rb_tree<unsigned, std::pair<unsigned const, vimrid::imaging::ImageMatrixColumn>, std::_Select1st<std::pair<unsigned const, vimrid::imaging::ImageMatrixColumn> >, std::less<unsigned>, std::allocator<std::pair<unsigned const, vimrid::imaging::ImageMatrixColumn> > >::_M_insert_unique(std::pair<unsigned const, vimrid::imaging::ImageMatrixColumn> const&) (stl_tree.h:1148)
==14446==    by 0x5037227: std::_Rb_tree<unsigned, std::pair<unsigned const, vimrid::imaging::ImageMatrixColumn>, std::_Select1st<std::pair<unsigned const, vimrid::imaging::ImageMatrixColumn> >, std::less<unsigned>, std::allocator<std::pair<unsigned const, vimrid::imaging::ImageMatrixColumn> > >::_M_insert_unique_(std::_Rb_tree_const_iterator<std::pair<unsigned const, vimrid::imaging::ImageMatrixColumn> >, std::pair<unsigned const, vimrid::imaging::ImageMatrixColumn> const&) (stl_tree.h:1188)
==14446==    by 0x50375CD: std::map<unsigned, vimrid::imaging::ImageMatrixColumn, std::less<unsigned>, std::allocator<std::pair<unsigned const, vimrid::imaging::ImageMatrixColumn> > >::insert(std::_Rb_tree_iterator<std::pair<unsigned const, vimrid::imaging::ImageMatrixColumn> >, std::pair<unsigned const, vimrid::imaging::ImageMatrixColumn> const&) (stl_map.h:496)
==14446==    by 0x50376DE: std::map<unsigned, vimrid::imaging::ImageMatrixColumn, std::less<unsigned>, std::allocator<std::pair<unsigned const, vimrid::imaging::ImageMatrixColumn> > >::operator[](unsigned const&) (stl_map.h:419)
==14446==    by 0x5036A43: vimrid::imaging::ImageMatrixRow::operator[](unsigned) (ImageMatrixRow.cpp:10)
==14446==    by 0x5034BBB: vimrid::imaging::ImageMatrix::_getRotatedCopy(double, vimrid::imaging::ImageMatrix&) (ImageMatrix.cpp:151)
==14446==    by 0x503350A: vimrid::imaging::processing::ImageFilter& vimrid::imaging::ImageMatrix::GetRotatedCopy<vimrid::imaging::processing::ImageFilter>(double) (ImageMatrix.h:48)

What could this possibly mean?

//ImageMatrixRow.cpp:8-11
ImageMatrixColumn &ImageMatrixRow::operator[](VUInt32 columnIndex)
{
    return columns[columnIndex];
}

//ImageMatrix.cpp:151
target[x][y][0] = source[roundX][roundY][0];

//ImageMatrix.h:48
return *(T*)&_getRotatedCopy(degrees, CopyDimensions());

Solution

It's probably because of a pool allocator. From Valgrind FAQ:

My program uses the C++ STL and string classes. Valgrind reports 'still reachable' memory leaks involving these classes at the exit of the program, but there should be none.

First of all: relax, it's probably not a bug, but a feature. Many implementations of the C++ standard libraries use their own memory pool allocators. Memory for quite a number of destructed objects is not immediately freed and given back to the OS, but kept in the pool(s) for later re-use. The fact that the pools are not freed at the exit() of the program cause Valgrind to report this memory as still reachable. The behaviour not to free pools at the exit() could be called a bug of the library though.