what does the weak_alias function do and where is it defined

from https://github.com/lattera/glibc/blob/master/include/libc-symbols.h

/* Define ALIASNAME as a weak alias for NAME.
   If weak aliases are not available, this defines a strong alias.  */
# define weak_alias(name, aliasname) _weak_alias (name, aliasname)
# define _weak_alias(name, aliasname) \
  extern __typeof (name) aliasname __attribute__ ((weak, alias (#name)));

About weak symbol:

https://en.wikipedia.org/wiki/Weak_symbol

It is a macro that does the following:

It declares a weak function, if you didnt provide a strong symbol name for that function it will call the function you have laised it to. for example

int _foo(){ return 1;}

//And weak alias
int __attribute__((weak, alias("_foo"))) foo();

So if you haven't provided actual implementation for foo it will basically use _foo and return 1.