Mongocxx segmentation fault issue

Hello everyone,

I work on a Linux application based on C++ & Qt and I try to properly build mongocxx and link to it. Everything works smoothly, but the app crashes on launch. Here is the stack trace:

Program received signal SIGSEGV, Segmentation fault.
0x0000555555d6d070 in bson_free ()
(gdb) where
#0  0x0000555555d6d070 in bson_free ()
#1  0x00007ffff3f21b63 in bson_string_free (string=0x555555e4d320, free_segment=free_segment@entry=false)
    at /home/lukasz/qliqsoft/mongo-c-driver/src/libbson/src/bson/bson-string.c:101
#2  0x00007ffff3f9086b in _set_platform_string (handshake=0x7ffff3ff7780 <gMongocHandshake>)
    at /home/lukasz/qliqsoft/mongo-c-driver/src/libmongoc/src/mongoc/mongoc-handshake.c:378
#3  _mongoc_handshake_init () at /home/lukasz/qliqsoft/mongo-c-driver/src/libmongoc/src/mongoc/mongoc-handshake.c:442
#4  0x00007ffff3f8ae33 in _mongoc_do_init () at /home/lukasz/qliqsoft/mongo-c-driver/src/libmongoc/src/mongoc/mongoc-init.c:140
#5  0x00007ffff456547f in __pthread_once_slow (once_control=0x7ffff3ff7734 <once>, init_routine=0x7ffff3f8adf0 <_mongoc_do_init>)
    at pthread_once.c:116
#6  0x00007ffff6c6cb7b in std::unique_ptr<mongocxx::v_noabi::instance::impl, std::default_delete<mongocxx::v_noabi::instance::impl> > core::v1::make_unique<mongocxx::v_noabi::instance::impl, void, std::unique_ptr<mongocxx::v_noabi::logger, std::default_delete<mongocxx::v_noabi::logger> > >(std::unique_ptr<mongocxx::v_noabi::logger, std::default_delete<mongocxx::v_noabi::logger> >&&) () from /lib/x86_64-linux-gnu/
#7  0x00007ffff6c6c8fd in mongocxx::v_noabi::instance::instance(std::unique_ptr<mongocxx::v_noabi::logger, std::default_delete<mongocxx::v_noabi::logger> >) () from /lib/x86_64-linux-gnu/
#8  0x00007ffff6c6c9d9 in mongocxx::v_noabi::instance::instance() () from /lib/x86_64-linux-gnu/
#9  0x000055555578b47f in main (argc=1, argv=0x7fffffffde88) at ../qliqdesktop/src/qliqstor/service/main.cpp:37
(gdb) quit
A debugging session is active.

So creating an mongocxx::instance variable is the very first thing that application does, but I get segmentation fault. I think that it is something related to mongocxx dependencies, so how I actually build the library?

I needed mongoc, so I cloned the official repository, built it, and installed using official guideline. I used version from r1.17 branch. I could install from Ubuntu repository, but that version is 1.16 and the newest mongocxx requires at least 1.17.

So that worked and I cloned mongocxx repository from releases/stable branch, built it and installed. Everything using commands from official guideline.

App builds without problems, ldd sees mongocxx and bsoncxx, but I get that segmentation fault. Do you have any ideas how to solve this issue?

Hi @Lukasz_Kosinski,

Thank you for including the stack trace. From that, I suspect the mongocxx::instance is going out of scope and being destroyed at some point during the lifetime of your application.

The mongocxx::instance should be created exactly once for the lifetime of the application (

If that is not the issue, can you include the relevant snippet from your application code where the mongocxx::instance is being created?

Thanks @Kevin_Albertson. I think that it is not the case as I’m pretty sure that I create only one instance.

The effect is the same if I run the following code:

#include <mongocxx/instance.hpp>

int main(int argc, char *argv[])
    mongocxx::instance mongoInstace;

    return 1;

Just wanted to ensure me :slight_smile:

@Lukasz_Kosinski the example code you provided to does not result in a SIGSEV on my system. Can you provide the CMake commands you used for configuring/installing the C and C++ drivers? The CMake configuration commands along with their output would be most helpful, along with the output of ldd ./a.out from the compiled example?

Thanks, @Roberto_Sanchez. I had a fresh look today and it looks like I had another, old driver in my include path. After removing that older driver it worked like charm.
Thanks for your proposals.

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