C++ function not exported by .so seemingly because of pybind11 parameters

Question

I am currently trying to define a shared library that I aim to use from a Python C++ extension as well as from vanilla C++ applications.

I managed to build the shared library, and tried to link a simple C++ application against it to test its functionalities, but one of the functions of the shared library is treated as an undefined reference by the linker. After checking with nm --demangle --dynamic --defined-only --extern-only libmylib.so, I realized the function is not being exported by the shared library, but I have no idea why.

The function's signature is as follows:

void bootstrap_mylib(std::vector<std::string> python_path,
        std::vector<std::string> python_scripts,
        std::string interface_module,
        std::function<void (pybind11::module_, pybind11::object)> interface_module_initializer);

Everything goes well if I comment out the last parameter, so the problem seems to be coming from the way I declare the dependencies with pybind11 somehow.

Here are the relevant parts of my CMakeLists.txt:

set(CMAKE_CXX_COMPILER /usr/bin/g++)

project(monilog LANGUAGES CXX VERSION 0.0.1)

set(PYBIND11_PYTHON_VERSION 3.8)
find_package(pybind11 REQUIRED)
include_directories(${PYTHON_INCLUDE_DIRS})

add_library(mylib SHARED MyLib.cc MyLib.h)
set_property(TARGET mylib PROPERTY CXX_STANDARD 17)

target_link_libraries(mylib ${PYTHON_LIBRARIES})

set_target_properties(mylib PROPERTIES VERSION ${PROJECT_VERSION})
set_target_properties(mylib PROPERTIES SOVERSION 1)
set_target_properties(mylib PROPERTIES PUBLIC_HEADER MyLib.h)

Any idea of what I might be doing wrong?

Edit: minimal working example

Here is a minimal example of my problem, consisting of the following files:

example.h

#include <pybind11/stl.h>

namespace Example
{
    void simple_func(std::string some_string);

    void pybind11_func(pybind11::function some_func);
}

example.cc

#include "example.h"

namespace Example
{
    
    void simple_func(std::string some_string)
    {
        std::cout << "Simple function" << '\n';
    }

    void pybind11_func(pybind11::function some_func)
    {
        std::cout << "Pybind11 function" << '\n';
    }
}

CMakeLists.txt

cmake_minimum_required(VERSION 3.10)
cmake_policy(SET CMP0074 NEW)

# SET VARIABLES
set(CMAKE_CXX_COMPILER /usr/bin/g++)
set(CMAKE_CXX_STANDARD 17)

project(example CXX)

set(PYBIND11_PYTHON_VERSION 3.8)
find_package(pybind11 REQUIRED)
# include_directories(${PYTHON_INCLUDE_DIRS})
include_directories(${pybind11_INCLUDE_DIRS})

add_library(example SHARED example.cc example.h)

target_link_libraries(example ${PYTHON_LIBRARIES})

When I build the project, if I then search for func in the exposed symbols, I get the following result:

> nm -D libexample.so | grep "func"
00000000000039d9 T _ZN7Example11simple_funcENSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE

pybind11_func is thus not exported, while simple_func is correctly exported.

Answer 1

Namespace pybind11 has hidden visibility.

/usr/include/pybind11/detail/common.h:#    
   define PYBIND11_NAMESPACE pybind11 __attribute__((visibility("hidden")))

so all functions that have anything to do with that namespace (like having an argument type from it) are also hidden.

You can override this by explicitly setting visibility to default:

__attribute__((visibility("default"))) 
void bootstrap_mylib( ... )