Do Distinct C Functions Always Have Distinct Function Pointer Addresses?

Function Pointer Equality and Distinct Addresses

Question:

In C , are function pointers for distinct functions guaranteed to have distinct addresses? Specifically, for the following code:

void foo() {}
void bar() {}

template<class T> void foo() { }

Are &foo != &bar and &foo != &foo guaranteed to be true?

Answer:

The standard does not explicitly require function pointers for distinct functions to have distinct addresses. However, it does allow implementations to optimize functions with identical definitions, and this optimization can result in identical function addresses.

In fact, Microsoft Visual C (MSVC) aggressively folds functions with identical implementations, assigning them the same address. This behavior is considered non-conforming.

On the other hand, the Gold linker offers a safer setting that maintains distinct addresses for functions, even if they have identical definitions.

Detailed Explanation:

The C standard defines equality for function pointers as follows:

• Two function pointers compare equal if they point to the same function.
• Alternatively, if both function pointers represent the same address, they are also considered equal.

The latter condition gives latitude for implementations to alias different functions and does not explicitly require pointers to different functions to be unequal.

However, taking the address of a function is observable behavior, and changing the address can violate the "as-if" rule. This rule requires that the implementation's behavior be indistinguishable from the behavior specified in the standard.

Therefore, while the standard does not explicitly prohibit function address aliasing, it can be argued that it violates the "as-if" rule, leading to non-conforming behavior.

Observations:

• Changing function addresses can disrupt programs that rely on unique function addresses, such as those using function pointers as unique values.
• Some linkers, including Gold and LLVM's lld, perform optimizations that avoid problematic address aliasing only when necessary.

Conclusion:

While the standard allows for function address aliasing, it is generally recommended to avoid this optimization as it can create portability and reliability issues.

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