Problem $9.(8$ points$)$:

Concurrency and sharing. Consider a concurrent C program with two threads, where each thread execut

the following line of code:

$/*$ Increment the shared global variable cnt $*/$

$cnt=cnt+1$;

Suppose that this line of $C$ code compiles to the following assembly language instructions:

At runtime, the operating system kernel will choose some ordering of these instructions. Since we are n

explicitly synchonizing the threads, some of these orderings will produce the correct value for $cnt$ and othe

will not.

Let $L_i$ denote the execution of instruction $L$ by thread $i$; similarly for $U_i$ and $S_i.$ Each of the sequenc

shown below gives a possible ordering of the instructions when the two threads execute. Assuming that $cr$

is initially zero, what is the value of cnt in memory after each of the sequences completes?

A$.,$ cnt $=0$;$,L_1,U_1,S_1,L_2,U_2,S_2,$ cnt $==$

B$.cnt=0$;$,L_1,U_1,L_2,S_1,U_2,S_2,cnt==$

C$.,cnt=0$;$,L_2,U_2,S_2,L_1,U_1,S_1,$ cnt $==$

D$.,$ cnt $=0$;$,L_1,L_2,U_1,S_1,U_2,S_2,$ cnt $==$The following $C$ program and declarations are part of the next problem. For each part, the three comment

lines

$\frac{?}{?}**LINE1*\frac{*}{?}$

$\frac{?}{?}**LINE2*\frac{*}{?}$

will be replaced with different fragments of code. $($For space reasons, we are not checking error return

codes, so assume that all functions return normally.$)$

int counter $-2$

void foo

counter $++$

printf $("\%$id$",$ counter$)$:

int maino $\{$

pthread$\_$t tid$[2]$:

int $i$ :

for

$\frac{?}{?}**LINE1*\frac{*}{?}$

$\frac{?}{?}**LINE2*\frac{*}{?}$

$1**LINE3**$

$\}$

counter$++$

counter"$+$: printf$("\%$g$",$ counter$)$:

$($Feel free to remove this page from your exam packet for easy reference.$)$

Problem $7.(8$ points$)$:

This problem tests your understanding of the differences between processes and threads.

Part $1$

Suppose the following code replaces the three comment lines in the program on the previous page:

LINE $1$

LINE $2$

Pthread$\_$create$($&tid $[$i$],0,$ foo, $0)$:

What is the first number that gets printed on stdout? Circle only one answer.

$3$

$45$

Could be elther $3$ or $4$ or $5$

Could be elther $3$ or $4$

Part $2$

Suppose the following code replaces the three comment lines.

LINE $1$: Pthread$\_$create $($&tid $[$i$],0,$ foo, $0)$:

LINE $2$: Pthread$\_$join$($tid $[$i$],$ O$)$:

What is the first number that gets printed on stdout? Circle only one answer.

$3$

$345$

Could be elther $3$ or $4$ or $5$

Could be elther $3$ or $4$