From afb09551d41f2c0ac666c68dc50c14b243e2784d Mon Sep 17 00:00:00 2001
From: loshprung <lshprung@scu.edu>
Date: Wed, 4 Mar 2020 11:36:12 -0800
Subject: Post-class 03/04

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+[\<- Notes 02/21](02-21.md)
+
+---
+
+# Threading Continued
+
+## Locking
+
+- Sharing Variables
+	- Requires mutual exclusion
+	- Lock/unlock to avoid a race condition
+- Have one lock for each independent critical region
+
+- Bad Example (no synchronization):
+
+```
+int count = 0;
+
+void *update(){ //returns a pointer with no given data type
+	int i;
+
+	for(i = 0; i < 1000; i++){
+		count++;
+	}
+}
+```
+
+- Good Example (using lock):
+
+```
+int count = 0;
+
+void *update(){ //returns a pointer with no given data type
+	int i;
+
+	for(i = 0; i < 1000; i++){
+		//lock
+		count++;
+		//unlock
+	}
+}
+```
+
+- More real world example:
+
+```
+int balance = 100; //$100
+
+//process 1: withdraw $20
+	//read balance (i.e $100)
+	//write new balance (i.e $80)
+
+//process 2: withdraw $30
+	//read balance (i.e $80)
+	//write new balance (i.e $50)
+```
+
+- In the above example, you need to use lock to avoid one process reading the balance before the other process finishes writing
+
+## pthread library
+
+- We will use the Linux pthread library
+	- Main information
+		- `man pthread`
+	- There are man pages for specific functions
+	- Include ".h" file as shown in the man page: `#include <pthread.h>`
+	- Compile using `-lpthread`
+
+- Operations:
+
+```
+#include <pthread.h>
+
+pthread_t thread;
+
+int pthread_create(
+	pthread_t *restrict thread,          //thread id
+	const pthread_attr_t *restrict attr, //attributes
+	void *(*start_routine)(void *),      //function
+	void *restrict arg);                 //argument
+```
+
+- More operations:
+	- `void pthread_exit(void *value_ptr);`
+	- `int pthread_join(pthread_t thread, void **value_ptr);`
+
+- Example using pthreads
+	- Alternating threads
+		- Creates 3 threads
+		- Let the system execute them in a round-robin fashion
+		- Wait for them to finish at main
+
+```
+#include <stdio.h>
+#include <pthread.h> //this is the library that allows for threading
+
+void *loopThread(void *arg);
+
+int main(){
+	int i;
+	pthread_t thr[3];
+
+	for(i = 0; i < 3; i++){
+		pthread_create(&thr[i], NULL, loopThread, (void *)i);
+	}
+	for(i = 0; i < 3; i++){
+		pthread_join(thr[i], NULL);
+	}
+}
+
+void *loopThread(void *arg){
+	int i, j;
+	int threadNo = (int)arg; //cast the void pointer
+
+	for(i = 0; i < 20; i++){
+		printf("Thread %d\n", threadNo);
+		sleep(1);
+	}
+}
+```
+
+- Functions:
+
+```
+pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER
+
+//INIT
+int pthread_mutex_init(pthread_mutex_t *restrict mutex, const pthread_mutexattr_t *restrict attr);
+
+//LOCK
+int pthread_mutex_lock(pthread_mutex_t *mutex);
+
+//UNLOCK
+int pthread_mutex_unlock(pthread_mutex_t *mutex);
+```
+
+- Example using mutex lock:
+
+```
+//Using mutex lock
+//pthread_mutex_init called in main
+
+int count = 0; 
+pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
+
+void *update(){
+	int i;
+
+	while(i = 0; i < 1000; i++){
+		pthread_mutex_lock(&mutex);
+		count++;
+		pthread_mutex_unlock(&mutex);
+	}
+}
+```
+
+- Danger!
+	- When one or more threads are waiting for one another and no thread can proceed
+- Example:
+	- One thread is waiting for itself
+	- Two threads are waiting for each other
+	- Several threads are waiting for one another in a cycle
+
+## Performance Considerations
+
+- Thread creation is expensive
+- Each thread has its own stack
+- Lock contention requires skills to keep many threads as possible running
+
+---
+
+[-> Notes 03/04](03-04.md)
-- 
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