why not shared registry between main and shared library???

[hbwang15]

Because spdlog is header-only, building shared libraries and using it in a main program will not share the registry between them. This means that calls to functions like spdlog::set_level(spdlog::level::level_enum::info) will not change loggers in DLLs.

why not shared registry between main and shared library???

[tt4g]

This can be described as a C/C++ language specification and security limitation.

Since a shared library may be linked and used by multiple applications, the OS reserves memory storage dedicated to the library when the library is loaded into memory.
The objects generated by the library code are placed in the memory storage reserved for the library (this allows objects generated by a single library to be shared by multiple applications).

The library's memory storage cannot be accessed directly by different applications (this is a memory access violation).

See also:

• https://stackoverflow.com/questions/1178127/shared-libraries-memory-space
• https://unix.stackexchange.com/questions/116327/loading-of-shared-libraries-and-ram-usage

[tt4g]

The same logger has been registered twice with spdlog::register_logger().

• https://github.com/hbwang15/spdlogInSharedLib/blob/65bdec50d0d3a926dbe6e6f5f6443519543a3922/lib.cpp#L23
• https://github.com/hbwang15/spdlogInSharedLib/blob/65bdec50d0d3a926dbe6e6f5f6443519543a3922/main.cpp#L20

Loggers in the registry are identified by spdlog::logger::name(), so loggers with the same name cannot be registered.

https://github.com/gabime/spdlog/blob/v1.9.1/include/spdlog/details/registry-inl.h#L301

You need to remove either spdlog::register_logger().

[hbwang15]

but the example is from https://github.com/gabime/spdlog/wiki/How-to-use-spdlog-in-DLLs

[hbwang15]

i am so confused by your explain, if the dll is not shared the register with the main, why exception happens

[tt4g]

I'm sorry. It seems that my explanation was insufficient.

I think your first question was written after looking at the Wiki: https://github.com/gabime/spdlog/wiki/How-to-use-spdlog-in-DLLs
The content here contains notes on using spdlog inside DLLs on Windows OS.
In other words, it is not relevant for general UNIX Like OS.
Please consider that my answer so far is also mainly about memory space in Windows OS.

I assume that your code is built and running on a UNIX Like OS.
As far as I know, in that environment, the shared library (.so) and the application share the static variable.

For more information, please browse this Stack Overflow post: https://stackoverflow.com/questions/19373061/what-happens-to-global-and-static-variables-in-a-shared-library-when-it-is-dynam

[hbwang15]

Thank you very much for your answers