Why Are There Two Constructor Symbols in C Libraries?

Dual Definition of Constructor Symbols in C Libraries

In C , constructor definitions appear twice in libraries when examined with nm. This is due to the Itanium C ABI, which requires multiple constructor entries for polymorphism support.

The mangled names for the constructors indicate their purpose:

• C1: Complete object constructor
• C2: Base object constructor

Even though a class may not have virtual base classes, GCC emits both constructors as a result of the ABI's support for polymorphism. This behavior is considered a known issue in GCC, where the compiler could potentially optimize this behavior away when polymorphism is not involved.

The different types of constructors have the following meanings:

• Complete object constructor: Constructs the entire object, including virtual base classes.
• Base object constructor: Creates the object itself and initializes data members and non-virtual base classes.
• Allocating object constructor: Not commonly seen, performs the actions of the complete object constructor and additionally allocates memory using operator new.

While this dual emission of constructor symbols does not typically cause problems, it can lead to confusion when examining library symbols with tools like nm.

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