Given the code below,

$1)$ What are the two different semantics for fork$()$ system call on multithreading? Please explain each case.

$2)$ For each case, draw the process tree $($use P$,$ SP$,$ SSP$,...$ to indicate the main process, sub$-$process, sub$-$sub$-$process, $...$; use P$($n$)$ to indicate a process with n extra threads.$)$;

$3)$ For each case, how many new processes have been created? and how many extra threads have been created?

pid$\_$t pid;

pid $=$ fork$()$;

if $($pid $==0)\{/*$ child process $*/$

fork$()$;

thread$\_$create$(...)$;

$\}$

else $\{$

thread$\_$create$(...)$;

$\}$

fork$()$;