How to initialize a shared library on Linux

Question

I am developing a shared library using C++ under Linux, and I would like this library to use log4cxx for logging purposes. However, I'm not sure how to set this up. For log4cxx to work, I need to create a logger object. How can I make sure this object is created when my library is loaded?

I suspect that it will be easiest to create the logger object as a global variable and then use it from any of the source files of my library, declaring it as extern in the headers. But how can I have the logger created automatically once an application connects to the library?

I know that in DLLs for Windows, there's a thing as REASON_FOR_CALL == PROCESS_ATTACH; is there a similar thing under Linux?

Answer 1

In C++ under Linux, global variables will get constructed automatically as soon as the library is loaded. So that's probably the easiest way to go.

If you need an arbitrary function to be called when the library is loaded, use the constructor attribute for GCC:

__attribute__((constructor)) void foo(void) {
    printf("library loaded!\n");
}

Constructor functions get called by the dynamic linker when a library is loaded. This is actually how C++ global initialization is implemented.

Answer 2

If you want your code to be portable you should probably try something like this:

namespace {
  struct initializer {
    initializer() {
      std::cout << "Loading the library" << std::endl;
    }

    ~initializer() {
      std::cout << "Unloading the library" << std::endl;
    }
  };
  static initializer i;
}

Answer 3

Using a global (or a local-static wrapped up in a function) is nice... but then you enter the land of static initialization fiasco (and the actual destruction is not pretty either).

I would recommend to have a look at the Singleton implementation of Loki.

There are various lifetime policies, one of which is Phoenix and will help you avoid this fiasco.

When you are at it, read Modern C++ Design which explains the problems encountered by the Singleton in depth as well as the uses for the various policies.