What does this External Linkage directive mean?

Question

Can someone explain this note in C++ primer 5th edition to me:

Note

The functions that C++ inherits from the C library are permitted to be defined as C functions but are not required to be C functions—it’s up to each C++ implementation to decide whether to implement the C library functions in C or C++.

Answer 1

I will give you an example of one function, that could be "treated" as C or C++. std::toupper() can also be written as toupper(). I believe the first one uses some safety checks of C++, while the other one is strictly C. But what it boils down to is:

#include <cctype>
#include <iostream>

int main (void) {
    char c = 'b';

    std::cout << toupper(c);                                                                     

    return 0;
}

compiles using C way, while:

#include <cctype>
#include <iostream>

int main (void) {
    char c = 'b';

    std::cout << std::toupper(c);                                                                     

    return 0;
}

Uses C++ compilation addressing namespaces, ant that is what @numzero's answer talks about.

Now both will compile, but it is up to you and your own risk to use C function.