How to see boost::multi_index hashed index's data using gdb

I'd like to see the data contained by boost::multi_index(version 1.67.0) using gdb. First I tried https://github.com/ruediger/Boost-Pretty-Printer. It seems that hashed indexes such as hashed_unique are not supported.

I noticed that if the first index is a supported type such as sequenced, Boost-Pretty-Printer works fine. However, I cannot edit the code now. I need to debug a core file and a binary executable.

I tried to understand the internal structure of multi_index with hashed index.

I worte the following test code:

#include <boost/multi_index_container.hpp>
#include <boost/multi_index/hashed_index.hpp>
#include <boost/multi_index/identity.hpp>

namespace mi = boost::multi_index;

struct t_hash{};

using elems = mi::multi_index_container<
    int,
    mi::indexed_by<
        mi::hashed_unique<
            mi::tag<t_hash>,
            mi::identity<int>
        >
    >
>;

int main() {
    elems es { 0x12, 0x34 };
    return 0; // set break point here and (gdb) p es
}

https://wandbox.org/permlink/UtMfVRI4rT5AXUOZ

When I print es, (gdb) p es I got the following output:

$1 = {
  <boost::base_from_member<std::allocator<boost::multi_index::detail::hashed_index_node<boost::multi_index::detail::index_node_base<int, std::allocator<int> >, boost::multi_index::detail::hashed_unique_tag> >, 0>> = {
    member = {
      <__gnu_cxx::new_allocator<boost::multi_index::detail::hashed_index_node<boost::multi_index::detail::index_node_base<int, std::allocator<int> >, boost::multi_index::detail::hashed_unique_tag> >> = {<No data fields>}, <No data fields>}
  },
  <boost::multi_index::detail::header_holder<boost::multi_index::detail::hashed_index_node<boost::multi_index::detail::index_node_base<int, std::allocator<int> >, boost::multi_index::detail::hashed_unique_tag>*, boost::multi_index::multi_index_container<int, boost::multi_index::indexed_by<boost::multi_index::hashed_unique<boost::multi_index::tag<t_hash, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na>, boost::multi_index::identity<int>, mpl_::na, mpl_::na>, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na>, std::allocator<int> > >> = {
    <boost::noncopyable_::noncopyable> = {<No data fields>},
    members of boost::multi_index::detail::header_holder<boost::multi_index::detail::hashed_index_node<boost::multi_index::detail::index_node_base<int, std::allocator<int> >, boost::multi_index::detail::hashed_unique_tag>*, boost::multi_index::multi_index_container<int, boost::multi_index::indexed_by<boost::multi_index::hashed_unique<boost::multi_index::tag<t_hash, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na>, boost::multi_index::identity<int>, mpl_::na, mpl_::na>, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na>, std::allocator<int> > >:
    member = 0x55555576ee70
  },
  <boost::multi_index::detail::hashed_index<boost::multi_index::identity<int>, boost::hash<int>, std::equal_to<int>, boost::multi_index::detail::nth_layer<1, int, boost::multi_index::indexed_by<boost::multi_index::hashed_unique<boost::multi_index::tag<t_hash, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na>, boost::multi_index::identity<int>, mpl_::na, mpl_::na>, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na>, std::allocator<int> >, boost::mpl::v_item<t_hash, boost::mpl::vector0<mpl_::na>, 0>, boost::multi_index::detail::hashed_unique_tag>> = {
    <boost::multi_index::detail::index_base<int, boost::multi_index::indexed_by<boost::multi_index::hashed_unique<boost::multi_index::tag<t_hash, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na>, boost::multi_index::identity<int>, mpl_::na, mpl_::na>, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na>, std::allocator<int> >> = {<No data fields>},
    members of boost::multi_index::detail::hashed_index<boost::multi_index::identity<int>, boost::hash<int>, std::equal_to<int>, boost::multi_index::detail::nth_layer<1, int, boost::multi_index::indexed_by<boost::multi_index::hashed_unique<boost::multi_index::tag<t_hash, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na>, boost::multi_index::identity<int>, mpl_::na, mpl_::na>, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na>, std::allocator<int> >, boost::mpl::v_item<t_hash, boost::mpl::vector0<mpl_::na>, 0>, boost::multi_index::detail::hashed_unique_tag>:
    key = {
      <boost::multi_index::detail::non_const_identity_base<int>> = {<No data fields>}, <No data fields>},
    hash_ = {
      <boost::hash_detail::hash_base<int>> = {
        <std::unary_function<int, unsigned long>> = {<No data fields>}, <No data fields>}, <No data fields>},
    eq_ = {
      <std::binary_function<int, int, bool>> = {<No data fields>}, <No data fields>},
    buckets = {
      <boost::multi_index::detail::bucket_array_base<true>> = {
        <boost::noncopyable_::noncopyable> = {<No data fields>},
      },
      members of boost::multi_index::detail::bucket_array<std::allocator<int> >:
      size_index_ = 0,
      spc = {
        <boost::noncopyable_::noncopyable> = {<No data fields>},
        members of boost::multi_index::detail::auto_space<boost::multi_index::detail::hashed_index_base_node_impl<std::allocator<char> >, std::allocator<int> >:
        al_ = {
          <__gnu_cxx::new_allocator<boost::multi_index::detail::hashed_index_base_node_impl<std::allocator<char> > >> = {<No data fields>}, <No data fields>},
        n_ = 54,
        data_ = 0x55555576ee90
      }
    },
    mlf = 1,
    max_load = 53
  },
  members of boost::multi_index::multi_index_container<int, boost::multi_index::indexed_by<boost::multi_index::hashed_unique<boost::multi_index::tag<t_hash, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na>, boost::multi_index::identity<int>, mpl_::na, mpl_::na>, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na>, std::allocator<int> >:
  node_count = 2
}

I found node_count = 2. It seems that the number of elements. I tried digging the member variables. e.g.) member, buckets, and so on. But I cannot found the data 0x12 and 0x34, so far.

How do I get them?

Edit: 2018/07/28 11:51 JST

Thanks for the comments, I found the solution. I summarize two approaches based on the comments.

Runtime shared library loading based on @sehe 's approach,

Write a debug print function (debug_print()) as a shared library (dp.so).
run gdb. gdb target_executable_file
Do set environment LD_PRELOAD ./dp.so.
Set breakpoints.
Do r
When the breakpoint hits, do call debug_print(data). data is the target you want to see.

This approach does't need to re-compile the target. However, when I load core file, debug_print() is no longer on memory. So this approach doesn't work with core file. I'm looking for forcibly load dp.so after core-file loaded, but I couldn't find the way, so far.

Trace multi_index's internal structure using gdb

This approach is work with core-file. This approach is based on @Joaquín M López Muñoz 's comment.

Run gdb with core file. gdb target_executable_file core_file.
Do the following command to access the data. T is multi_index's element type. data is multi_index container variable. N is number of indexes.

1st data

p *(T*)((char*)(*data.member).prior_ - sizeof(T) - 0x10 * (N - 1))

2nd data

p *(T*)((char*)(*(*data.member).prior_).prior_ - sizeof(T) - 0x10 * (N - 1))

...follow the same pattern.

Thank you very much @sele and @Joaquín M López Muñoz !!

Edit: 2018/07/28 15:22 JST

I implemented hashed indexes support for Boost-Pretty-Printer. It is based on the above approach.

I sent the pull request for that:

https://github.com/ruediger/Boost-Pretty-Printer/pull/36

Edit: 2018/07/28 15:42 JST

I figured out what 0x10 mean. It is pointer size multiplies 2. So in 64bit environment, 64bit = 8byte, 8 * 2 = 16 = 0x10. In 32bit environment 32bit = 4byte, 4 * 2 = 8 = 0x08.

I also updated the pull request.

Edit: 2018/08/02 09:30 JST

Finally, I sent two pull requests and both are merged. Now, we can simple use Boost-Pretty-Printer and print the multi_index container that has hashed_index as the first index.

https://github.com/ruediger/Boost-Pretty-Printer/pull/36

https://github.com/ruediger/Boost-Pretty-Printer/pull/37

Here is internal structure and iterating algorithm:

https://speakerdeck.com/redboltz/boost-multi-index-version-equals-1-dot-56-dot-0-internal-structure-and-iteration-algorithm-for-gdb-boost-prerry-printer

Solution

Solution for new Boost version

If you are using Boost 1.56.0 or later, you can print the container using Boost-Pretty-Printer.

Install Boost-Pretty-Printer

The github site is https://github.com/ruediger/Boost-Pretty-Printer

Installation manual is https://github.com/ruediger/Boost-Pretty-Printer#installation

Do print command on gdb.

Then, you can get the result.

If you have already installed Boost-Pretty-Printer and couldn't getpretty print result, you need to update it.

hashed_index support is introduced since https://github.com/ruediger/Boost-Pretty-Printer/commit/d8557f664e0dd3d11bb0464d8f670e99946e88b9

Solution for old Boost version

If you are using Boost version less than 1.56.0, you have some choices.

1. Updating boost.

Just update your boost library and re-compile your program.

Pros

If you can do, it's the easiest way.

Cons

It requires re-compile the target.
It might introduce comatibility problem to your application.
It doesn't work with existing binary executable and core file.

2. Dynamic loading shared library

Writing debug print function. Here is the code example:

dp.cpp

#include <boost/multi_index_container.hpp>
#include <boost/multi_index/hashed_index.hpp>
#include <boost/multi_index/identity.hpp>

namespace mi = boost::multi_index;

struct t_hash{};

// define the same structure of the debug target
using elems = mi::multi_index_container<
    int,
    mi::indexed_by<
        mi::hashed_unique<
            mi::tag<t_hash>,
            mi::identity<int>
        >
    >
>;

#include <iostream>

// debug print function
void dp(elems const& es) {
    for (auto const& e : es) {
        std::cout << e << std::endl;
    }
}

Compile it using the same compiler option as the target. And add -fPIC for shared library.

clang++ -g -c dp.cpp -fPIC

Then you get dp.o

Link it as a shared library

clang++ -shared -o dp.so dp.o

Then you get dp.so

Run gdb. gdb target_executable_file

Do set environment LD_PRELOAD ./dp.so.

Set breakpoints.

Do r

When the breakpoint hits, do call dp(data). data is the target you want to see

Pros

Re-compile the target is not required.

Cons

Doesn't work with core file.

3. Develop Boost-Pretty-Print support for older version

Unfortunately, current Boost-Pretty-Print hashed_indexes supports only Boost 1.56.0 or later. But you can implement older support by yourself.

Here is the data structure description that is focused on iteration and iteration algorithm for >= 1.56 version. I think that it is a good hint to implement older version iteration algorithm.

https://speakerdeck.com/redboltz/boost-multi-index-version-equals-1-dot-56-dot-0-internal-structure-and-iteration-algorithm-for-gdb-boost-prerry-printer

And my pull requests (merged). https://github.com/ruediger/Boost-Pretty-Printer/pull/36

https://github.com/ruediger/Boost-Pretty-Printer/pull/37

For < 1.56 version, I wrote the program to check the internal structure.

#include <boost/multi_index_container.hpp>
#include <boost/multi_index/hashed_index.hpp>
#include <boost/multi_index/identity.hpp>
#include <boost/multi_index/member.hpp>
#include <boost/lexical_cast.hpp>

#include <iostream>
#include <string>

namespace mi = boost::multi_index;

struct t_hash{};

using elems = mi::multi_index_container<
    int,
    mi::indexed_by<
        mi::hashed_non_unique<
            mi::tag<t_hash>,
            mi::identity<int>
        >
    >
>;

int main(int argc, char** argv) {
    auto size = boost::lexical_cast<int>(argv[1]);
    elems es;
    for (int i = 0; i != size; ++i) {
        es.insert(i);
    }
    return 0; // break here
}

And I got the following gdb output:

b 30
r 5

p es.member
0x555555770e70

p (*es.member).next_
0x555555771038

p (*(*es.member).next_).next_
0x555555770e78

p (*(*(*es.member).next_).next_).next_
0x555555771038

...


x /200xb 0x555555771030

0x555555771030: 0x30    0x10    0x77    0x55    0x55    0x55    0x00    0x00
0x555555771038: 0x78    0x0e    0x77    0x55    0x55    0x55    0x00    0x00
0x555555771040: 0x00    0x00    0x00    0x00    0x00    0x00    0x00    0x00
0x555555771048: 0x21    0x00    0x00    0x00    0x00    0x00    0x00    0x00


0x555555771050: 0x00    0x00    0x00    0x00    0x00    0x00    0x00    0x00
                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
0x555555771058: 0x90    0x0e    0x77    0x55    0x55    0x55    0x00    0x00
0x555555771060: 0x00    0x00    0x00    0x00    0x00    0x00    0x00    0x00
0x555555771068: 0x21    0x00    0x00    0x00    0x00    0x00    0x00    0x00


0x555555771070: 0x01    0x00    0x00    0x00    0x00    0x00    0x00    0x00
                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
0x555555771078: 0x98    0x0e    0x77    0x55    0x55    0x55    0x00    0x00
0x555555771080: 0x00    0x00    0x00    0x00    0x00    0x00    0x00    0x00
0x555555771088: 0x21    0x00    0x00    0x00    0x00    0x00    0x00    0x00


0x555555771090: 0x02    0x00    0x00    0x00    0x00    0x00    0x00    0x00
                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
0x555555771098: 0xa0    0x0e    0x77    0x55    0x55    0x55    0x00    0x00
0x5555557710a0: 0x00    0x00    0x00    0x00    0x00    0x00    0x00    0x00
0x5555557710a8: 0x21    0x00    0x00    0x00    0x00    0x00    0x00    0x00


0x5555557710b0: 0x03    0x00    0x00    0x00    0x00    0x00    0x00    0x00
                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
0x5555557710b8: 0xa8    0x0e    0x77    0x55    0x55    0x55    0x00    0x00
0x5555557710c0: 0x00    0x00    0x00    0x00    0x00    0x00    0x00    0x00
0x5555557710c8: 0x21    0x00    0x00    0x00    0x00    0x00    0x00    0x00


0x5555557710d0: 0x04    0x00    0x00    0x00    0x00    0x00    0x00    0x00
                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
0x5555557710d8: 0xb0    0x0e    0x77    0x55    0x55    0x55    0x00    0x00
0x5555557710e0: 0x00    0x00    0x00    0x00    0x00    0x00    0x00    0x00
0x5555557710e8: 0x21    0xef    0x00    0x00    0x00    0x00    0x00    0x00

0x5555557710f0: 0x00    0x00    0x00    0x00    0x00    0x00    0x00    0x00
...

This information and @Joaquín M López Muñoz's comment also could be a good hint to implement the < 1.56 Boost-Pretty-Printer.

Pros

Re-compile the target is not required.
Work well with core file.

Cons

You need to implement Boost-Pretty-Printer support.