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.