b$.$ Consider the program below, where $""$ represents code that updates a shared data structure.

$```$

void $*$thread$\_$func$($void $*$arg$)\{$

pthread$\_$mutex$\_$t mymutex;

int rv $=$ pthread$\_$mutex$\_$init$($&mymutex,NULL$)$; $//$ default attributes

$...$

rv $=$ pthread$\_$mutex$\_$lock$($&mymutex$)$;

assert$(!$rv$)$;

$//$ critical section

rv $=$ pthread$\_$mutex$\_$unlock$($&mymutex$)$;

assert$(!$rv$)$;

$...$

return NULL;

$\}$

$```$

The main program creates and executes $4$ threads running thread$\_$func$().$ What is wrong with the above code? $($Hint: it has nothing to do with the variable rv or the call to assert.$)$