why and how does rand() exist both in global and std namespace in cstdlib?

Question

I understand that rand(), as an example function from <cstdlib>, exists both in the global and the std namespace.

In effect the following will compile without errors, i.e. both calls to std::rand() and rand() will be legit.

#include <cstdlib>    
int main() {
    std::rand();
    rand();    
}

What is the use for this and how exactly is it implemented (the function being in both namespaces)?

Answer 1

The behavior is Uspecified behavior as per standard.
As per the standard including cstdlib imports the symbol names in std namespace and possibly in Global namespace. If you rely on symbol names being included in global namespace then your program is non portable and you are relying on behavior of a specific implementation.

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To not rely on the implementatio behavior you must:

Include cstdlib and use fully qualified name for rand.

std::rand()

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References:

C++11 Standard: D.5 C standard library headers
Para 3:

[ Example: The header <cstdlib> assuredly provides its declarations and definitions within the namespace std. It may also provide these names within the global namespace. The header <stdlib.h> assuredly provides the same declarations and definitions within the global namespace, much as in the C Standard. It may also provide these names within the namespace std. —end example ]

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Good Read:
Should I include <xxxx.h> or <cxxxx> in C++ programs?