Can I Get the Address of a Standard Library Function in C ?

Can I Take the Address of a Function Defined in Standard Library?

Overview

Can a C program take the address of a function defined in the standard library, and if so, under what conditions?

Short Answer

No, not in general.

Explanation

According to [namespace.std], forming a pointer to a standard library function is unspecified behavior unless the function is designated as addressable. This applies to member functions as well.

The First Call to std::invoke

std::invoke(std::boolalpha, std::cout);

In this example, std::boolalpha is an addressable function designated by [fmtflags.manip]. Therefore, this line is well-formed and equivalent to:

std::cout.setf(std::ios_base::boolalpha);

The Second Call to std::invoke

std::invoke(static_cast<ctype_func>(std::tolower), 'A');

However, for functions defined in , [cctype.syn] states that the specific functions are defined according to the C standard library header . Nowhere in is tolower designated as addressable, so the behavior of this program is unspecified (possibly ill-formed) due to the attempt to form a pointer to tolower.

Conclusion

The unexpected output is not guaranteed, and the code may not even compile. This also applies to member functions declared in the C standard library.

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