Why memory functions such as memset, memchr… are in string.h, but not in stdlib.h with another mem functions?

Question

I wonder, why such function as:
-memset
-memmov
-memchr
-memcpy

Exist in string.h header file, but not in stdlib.h file, where there are ot

Answer 1

In Pre-Standard C, these functions were indeed defined somewhere else, but neither in stdlib.h nor in any of the other standard headers, but in memory.h. It still might exist on your system, it certainly still does on OS X (as of today).

memory.h on OS X 10.11 (without license header):

#include 

The whole file is only #include'ing string.h, to retain backwards compatibility with Pre-Standard C programs.