How exactly does __attribute__((constructor)) work?

Question

It seems pretty clear that it is supposed to set things up.

1. When exactly does it run?
2. Why are there two brackets?
3. Is __attribute__ a function? A macro? Syntax?
4. Does this work in C? C++?
5. Does the function it works with need to be static?
6. When does __attribute__((destructor)) run?

Example in Objective C:

__attribute__((constructor))
static void initialize_navigationBarImages() {
  navigationBarImages = [[NSMutableDictionary alloc] init];
}

__attribute__((destructor))
static void destroy_navigationBarImages() {
  [navigationBarImages release];
}

Answer 1

1. It's run when a shared library is loaded, typically during program startup.
2. That's how all GCC attributes are; presumably to distinguish them from function calls.
3. GCC-specific syntax.
4. Yes.
5. No.
6. The destructor is run when the shared library is unloaded, typically at program exit.

So, the way the constructors and destructors work is that the shared object file contains special sections (.ctors and .dtors on ELF) which contain references to the functions marked with the constructor and destructor attributes, respectively. When the library is loaded/unloaded the dynamic loader program (ld.so or somesuch) checks whether such sections exist, and if so, calls the functions referenced therein.

Come to think of it, there is probably some similar magic in the normal static linker, so that the same code is run on startup/shutdown regardless if the user chooses static or dynamic linking.

Answer 2

Excellent answer by @janneb, very useful. Just for completeness, here is a link to the related GCC docs.

I'm glad I stumbled onto this question, it just allowed me to eliminate two dozen lines of boilerplate code and redundant function calls for doing some library-wide initialization in my Objective-C framework.

NOTE: Although this syntax is specific to GCC, LLVM and Clang have been written to support it as well, and code compiled using Clang-LLVM seems to work just like GCC-compiled code.