released: 02/17, 11:00
due: 02/23, 23:59

Answer the following questions.
Submit your answers to Canvas assignments. There is an entry for this homework.


1. Threads vs. Processes (2 points)

A thread within a process has its own: (Choose all that apply)

    A. stack
    B. main() function
    C. registers
    D. global variables
    E. program code
    F. heap



2. Race conditions (3 points)

Below is a function that adds two input values and assigns the sum
to the first variable.

  void add(int *a, int *b) {
    *a += *b;
  }

Given "a" and "b" are two int variables that initially are 1,
what might happen when one thread runs add(&a,&b) while another runs add(&b,&a)?

Assume:
  -- memory satisfies sequential consistency.
  -- C code "X += Y" will be compiled as four instructions:
     (1) move X from memory to a register %rbx
     (2) move Y from memory to a register %rcx
     (3) add %rbx and %rcx and assign the sum to %rbx
     (4) move the value of %rbx to X in memory

Write down all possible final values of "a" and "b".



3. multi-threading counter

Below is a piece of code that is intended to do the following:
  - there will be 100 threads;
  - each thread increases a global counter, "counter", by 1 each time;
  - each thread does increase-by-1 10000 times;
  - after 100 threads finish, the program print the final value of counter.

Here is the incomplete code with two TODOs:

"""
// file name: counter.c
#include "pthread.h"
#include "stdio.h"

int counter = 0; // global variable

void *count(void *ignored) {
    for (int i=0; i<10000; i++)
        counter++;
    return NULL;
}

int main(){
    pthread_t tid[100];   // 100 threads
    for (int i=0; i<100; i++) {
        // TODO1: your code here
        // you should create threads to run function "count"
    }
    for (int i=0; i<100; i++) {
        // TODO2: your code here
        // you should "wait" for all 100 threads to finish
    }
    printf("counter  = %d\n", counter);
}
"""

3.a. fill "TODO1" to create threads; write your solution below. (2 points)
[hint: "$man pthread_create" to see how to create threads using pthread.]





3.b  fill "TODO2" to wait all threads; write your solution below. (2 points)
[hint: "$man pthread_join" to see how to wait threads using pthread.]



3.c. compile and run the code; write down the output you've seen. (1 point)
[hint: "$ gcc counter.c -lpthead" to compile with pthread library]