How Does `__attribute__((constructor))` Initialize and Clean Up Shared Libraries?

Demystifying "__attribute__((constructor))": A Comprehensive Analysis

"__attribute__((constructor))" is a versatile mechanism used to initialize and clean up code within shared libraries. Let's delve into the intricacies of how it operates:

When is "__attribute__((constructor))" Triggered?

This attribute initializes code when the shared library is loaded, typically during program startup. It plays a crucial role in setting up resources and performing necessary initializations before the shared library becomes available for use.

Parentheses in "__(constructor)__"

The parentheses in "__attribute__((constructor))__" are a defining characteristic of GCC attributes. They distinguish these attributes from function calls, providing a clear syntax for specifying function initialization.

Nature of "__attribute__"

"__attribute__" is specific syntax introduced by GCC. It is neither a function nor a macro but rather a special directive that modifies function declarations.

Compatibility with C and C

"__attribute__((constructor))__" is supported in both C and C . It provides a convenient means of initializing shared libraries regardless of the programming language used.

Requirement of Static Functions

The functions marked with "__attribute__((constructor))__" do not need to be static. They can be regular global functions that the constructor attribute initializes during library loading.

Counterpart for Cleanup: "__attribute__((destructor))__"

Just as "__attribute__((constructor))__" initializes code, "__attribute__((destructor))__" runs its specified code when the shared library is unloaded, typically at program exit. It is responsible for cleanup operations and releasing allocated resources.

Execution Mechanism

Shared object files contain ".ctors" sections that hold references to functions marked with "__attribute__((constructor))__." During library loading, the dynamic loader invokes these functions. A similar mechanism exists in the static linker for initialization in statically linked programs.

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