auto commit

notes/计算机操作系统.md

@@ -273,15 +273,15 @@ down 和 up 操作需要被设计成原语，不可分割，通常的做法是
 typedef int semaphore;
 semaphore mutex = 1;
 void P1() {
-    down(mutex);
+    down(&mutex);
     // 临界区
-    up(mutex);
+    up(&mutex);
 }
 
 void P2() {
-    down(mutex);
+    down(&mutex);
     // 临界区
-    up(mutex);
+    up(&mutex);
 }
 ```
 
@@ -305,21 +305,21 @@ semaphore full = 0;
 void producer() {
     while(TRUE){
         int item = produce_item();
-        down(empty);
-        down(mutex);
+        down(&empty);
+        down(&mutex);
         insert_item(item);
-        up(mutex);
-        up(full);
+        up(&mutex);
+        up(&full);
     }
 }
 
 void consumer() {
     while(TRUE){
-        down(full);
-        down(mutex);
+        down(&full);
+        down(&mutex);
         int item = remove_item();
-        up(mutex);
-        up(empty);
+        up(&mutex);
+        up(&empty);
         consume_item(item);
     }
 }
@@ -452,23 +452,23 @@ int count = 0;
 
 void reader() {
     while(TRUE) {
-        down(count_mutex);
+        down(&count_mutex);
         count++;
-        if(count == 1) down(data_mutex); // 第一个读者需要对数据进行加锁，防止写进程访问
-        up(count_mutex);
+        if(count == 1) down(&data_mutex); // 第一个读者需要对数据进行加锁，防止写进程访问
+        up(&count_mutex);
         read();
-        down(count_mutex);
+        down(&count_mutex);
         count--;
-        if(count == 0) up(data_mutex);
-        up(count_mutex);
+        if(count == 0) up(&data_mutex);
+        up(&count_mutex);
     }
 }
 
 void writer() {
     while(TRUE) {
-        down(data_mutex);
+        down(&data_mutex);
         write();
-        up(data_mutex);
+        up(&data_mutex);
     }
 }
 ```
@@ -483,44 +483,70 @@ void writer() {
 
 ```c
 #define N 5
-#define LEFT (i + N - 1) % N
-#define RIGHT (i + N) % N
-typedef int semaphore;
-semaphore chopstick[N];
 
 void philosopher(int i) {
-    while(TURE){
+    while(TRUE) {
         think();
-        down(chopstick[LEFT[i]]);
-        down(chopstick[RIGHT[i]]);
+        take(i);       // 拿起左边的筷子
+        take((i+1)%N); // 拿起右边的筷子
         eat();
-        up(chopstick[RIGHT[i]]);
-        up(chopstick[LEFT[i]]);
+        put(i);
+        put((i+1)%N);
     }
 }
 ```
 
-为了防止死锁的发生，可以加一点限制，只允许同时拿起左右两边的筷子。方法是引入一个互斥量，对拿起两个筷子的那段代码加锁。
+为了防止死锁的发生，可以加一点限制，一个哲学家只有在两个邻居都没有进餐的情况下才允许进餐。
 
 ```c
-semaphore mutex = 1;
+#define N 5
+#define LEFT (i + N - 1) % N // 左邻居
+#define RIGHT (i + 1) % N    // 右邻居
+#define THINKING 0
+#define HUNGRY   1
+#define EATING   2
+typedef int semaphore;
+int state[N];                // 跟踪每个哲学家的状态
+semaphore mutex = 1;         // 临界区的互斥
+semaphore s[N];              // 每个哲学家一个信号量
 
 void philosopher(int i) {
-    while(TURE){
+    while(TRUE) {
         think();
-        down(mutex);
-        down(chopstick[LEFT[i]]);
-        down(chopstick[RIGHT[i]]);
-        up(mutex);
+        take_two(i);
         eat();
-        down(mutex);
-        up(chopstick[RIGHT[i]]);
-        up(chopstick[LEFT[i]]);
-        up(mutex);
+        put_tow(i);
+    }
+}
+
+void take_two(int i) {
+    down(&mutex);
+    state[i] = HUNGRY;
+    test(i);
+    up(&mutex);
+    down(&s[i]);
+}
+
+void put_tow(i) {
+    down(&mutex);
+    state[i] = THINKING;
+    text(LEFT);
+    test(RIGHT);
+    up(&mutex);
+}
+
+void test(i) {     // 尝试拿起两把筷子
+    if(state[i] == HUNGRY && state[LEFT] != EATING
+      && state[RIGHT] !=EATING) {
+        state[i] = EATING;
+        up(&s[i]);
     }
 }
 ```
 
+
+
+
 # 第三章 死锁
 
 ## 死锁的必要条件