The C++ library includes the entire C standard library (from the 1990 C standard, plus Amendment 1), in which each C header, such as <stdio.h>, is wrapped as a C++ header (e.g., <cstdio>). Being part of the C++ standard, all types, functions, and objects are declared in the std namespace.
The external names are also reserved in the global namespace. Thus, proper practice is to use the names in the std namespace (e.g., std::strlen), but realize that these names are also reserved in the global namespace, so you cannot write your own ::strlen function.
The C standard permits macros to be defined to mask function names. In the C++ wrappers for these headers, the names must be declared as functions, not macros. Thus, the C <stdio.h> header might contain the following:
extern int printf(const char* fmt, ...); #define printf printf
In C++, the printf macro is not permitted, so the <cstdio> header must declare the printf function in the std namespace, so you can use it as std::printf.
A deprecated feature of C++ is that the C standard headers are also available as their original C names (e.g., <stdio.h>). When used in this fashion, their names are in the global namespace, as though a using declaration were applied to each name (e.g., using std::printf). Otherwise, the old style headers are equivalent to the new headers. The old C header names are deprecated; new code should use the <cstdio>, etc., style C headers.