segfault in dynamic_bitset::reference assign operator

[ben.collins@…]

I will of course take a look at this later (i.e., I've go to stop here and do the work I get paid to do), but for now, this will serve as a heads up to any one who's interested. Perhaps I'm not initializing bset correctly, but I couldn't easily tell that from the headers or the online documentation.

test code:

$ cat bitset.cpp
#include <iostream>
#include <string>
#include <boost/dynamic_bitset.hpp>

int main()
{
  boost::dynamic_bitset<> bset;

  bset[0] = true;
  bset[0] = false;

  return 0;
}

compile + gdb session:

$ g++ -I ~/local/include/boost-1_35/ -o bitset bitset.cpp
$ ./bitset
Segmentation fault
$ g++ -g -I ~/local/include/boost-1_35/ -o bitset bitset.cpp
$ gdb bitset
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(gdb) r
Starting program: /users/bcollins/test/C++/bitset

Program received signal SIGSEGV, Segmentation fault.
0x08048877 in boost::dynamic_bitset<unsigned long, std::allocator<unsigned long> >::reference::do_set (this=0xbf856240)
    at /users/bcollins/local/include/boost-1_35/boost/dynamic_bitset/dynamic_bitset.hpp:115
115             void do_set() { m_block |= m_mask; }
(gdb) bt
#0  0x08048877 in boost::dynamic_bitset<unsigned long, std::allocator<unsigned long> >::reference::do_set (this=0xbf856240)
    at /users/bcollins/local/include/boost-1_35/boost/dynamic_bitset/dynamic_bitset.hpp:115
#1  0x080488a3 in boost::dynamic_bitset<unsigned long, std::allocator<unsigned long> >::reference::do_assign (this=0xbf856240, x=true)
    at /users/bcollins/local/include/boost-1_35/boost/dynamic_bitset/dynamic_bitset.hpp:118
#2  0x080488d1 in boost::dynamic_bitset<unsigned long, std::allocator<unsigned long> >::reference::operator= (this=0xbf856240, x=true)
    at /users/bcollins/local/include/boost-1_35/boost/dynamic_bitset/dynamic_bitset.hpp:103
#3  0x080486c9 in main () at bitset.cpp:9
(gdb) f 0
#0  0x08048877 in boost::dynamic_bitset<unsigned long, std::allocator<unsigned long> >::reference::do_set (this=0xbf856240)
    at /users/bcollins/local/include/boost-1_35/boost/dynamic_bitset/dynamic_bitset.hpp:115
115             void do_set() { m_block |= m_mask; }
(gdb) p m_block
$1 = (long unsigned int &) @0x0: Cannot access memory at address 0x0
(gdb)

[smckay@…]

A new dynamic_bitset with no size is empty. You're trying to assign to an empty vector.

I think it would work if you initialized bset with a size of 1.

[Gennaro Prota]

This isn't a bug: as noted in the previous comment, the size of bset is zero, and evaluating bset[0] violates the precondition n < this->size(), which holds for both the operator[]() members, and thus invokes undefined behavior.