Why was the reallocarray function not added to glibc?

[Paul Eggert]

The patch should use gcc builtins (when available) to detect integer 
overflow instead of trying to do it by hand. You can do this by using 
the INT_MULTIPLY_OVERFLOW macro of Gnulib, documented here:

https://www.gnu.org/software/gnulib/manual/html_node/Integer-Type-Overflow.html

The easist way to use that macro is to copy gnulib intprops.h as-is into 
glibc, and then use "#include <intprops.h>". It need not be a public 
include file. Here's the current source:

http://git.savannah.gnu.org/cgit/gnulib.git/tree/lib/intprops.h