> 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
> 

[answer: A, C]


> 
> 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".
> 


[answer:
  a=3, b=2
  a=2, b=3
  a=2, b=2
]

> 
> 
> 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.]
> 

[answer: something like:
        pthread_create(&tid[i], NULL, count, NULL);
]


> 
> 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.]
> 

[answer: something like:
        pthread_join(tid[i], NULL);
]

> 
> 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]


[answer: anything between [1, 1000000] could happen.
    In fact, can you imagine when "1" can be the output?
    hint: think of a gigantic machine with 1M cores.
]