互斥鎖Mutex
執行緒安全
多個執行緒搶同一個資源，就會發生問題，以下程式碼
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#include <iostream>
#include <unistd.h>
#include <thread>
using namespace std;
void func(string msg) {
//每一秒印1次，這裡是執行10秒
for (int i = 0; i < 10; i++) {
cout << "i = " << i << ", msg =" << msg << endl;
sleep(1);//停1秒鐘
}
}
int main() {
//建立執行緒t1
thread t1(func, "test test");
//建立執行緒t2
thread t2(func, "abcdefg abcdefg");
//執行緒t1被記憶體釋放
t1.join();
//執行緒t2被記憶體釋放
t2.join();
return 0;
}
i = 0, msg =test test
i = 0, msg =abcdefg abcdefg
i = 1, msg =test test
i = 1, msg =abcdefg abcdefg
i = 2, msg =abcdefg abcdefg
i = 2, msg =test test
i = 3i = 3, msg =test test, msg =abcdefg abcdefg
i = 4i = 4, msg =abcdefg abcdefg, msg =test test
i = 5, msg =test test
i = 5, msg =abcdefg abcdefg
i = 6, msg =test test
i = 6, msg =abcdefg abcdefg
i = 7, msg =test test
i = 7, msg =abcdefg abcdefg
i = 8, msg =abcdefg abcdefg
i = 8, msg =test test
i = 9, msg =abcdefg abcdefg
i = 9, msg =test test
從執行結果來看，以下二句是二個執行緒搶cout(全域變數)這個資源，導致輸出的結果不正常。
i = 3i = 3, msg =test test, msg =abcdefg abcdefg
i = 4i = 4, msg =abcdefg abcdefg, msg =test test
互斥鎖Mutex
同步(Concurrency Control)是指，多個執行緒協調如何使用同一個資源。
如下圖，每一個人代表一個執行緒，他們都在排隊上同一間廁所(同一個資源)，當一個人(一個執行緒)進入廁所，就要把門鎖上，當上完廁所，再把門打開，輪下一個人(下一個執行緒)進去上廁所(同一個資源)。
互斥鎖保證同一個時間只有一個人(一個執行緒)使用廁所(同一個資源)，當一個執行緒把門鎖上，其它執行緒就在門外排隊等候，等待廁所中的執行緒把門打開(開鎖)，輪下一個執行緒進去把門鎖上。
匯入標頭檔
#include <mutex>
互斥鎖全域變數
mutex mtx;
lock() 鎖上
可以把mutex想像成是廁所那個門，若是門是鎖起來的狀態，代表有人(一個thread)在廁所裡面，其它人(threads)就要在門外排隊等待，直到門開鎖，沒有鎖起來的狀態，執行緒呼叫mutex.lock()會自行判斷鎖起來的狀態，若沒鎖上，執行緒會得到鎖，若鎖上，執行緒就要排隊等待。
unlock() 開鎖
只有在廁所中的執行緒才有開鎖的功能，其它在廁所門外排隊的執行緒是沒辦法開鎖。
程式碼
先前的程式，把cout想成廁所，程式碼修改如下
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#include <iostream>
#include <unistd.h>
#include <thread>
#include <mutex>
using namespace std;
//建立mutex互斥鎖
mutex mtx;
void func(string msg) {
//每一秒印1次，這裡是執行10秒
for (int i = 0; i < 10; i++) {
mtx.lock();//把門鎖上
cout << "i = " << i << ", msg =" << msg << endl;
sleep(1);//持有鎖後，再休眠1秒
mtx.unlock();//使用完畢，把門開鎖
}
}
i = 0, msg =test test
i = 1, msg =test test
i = 2, msg =test test
i = 3, msg =test test
i = 4, msg =test test
i = 5, msg =test test
i = 0, msg =abcdefg abcdefg
i = 1, msg =abcdefg abcdefg
i = 2, msg =abcdefg abcdefg
i = 3, msg =abcdefg abcdefg
i = 4, msg =abcdefg abcdefg
i = 5, msg =abcdefg abcdefg
i = 6, msg =abcdefg abcdefg
i = 7, msg =abcdefg abcdefg
i = 8, msg =abcdefg abcdefg
i = 9, msg =abcdefg abcdefg
i = 6, msg =test test
i = 7, msg =test test
i = 8, msg =test test
i = 9, msg =test test
判斷是那個thread加鎖/解鎖/申請鎖
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void func(string msg) {
//每一秒印1次，這裡是執行10秒
for (int i = 0; i < 10; i++) {
cout << "申請鎖的thread = " << this_thread::get_id() << endl;
mtx.lock();
cout << "鎖上的thread = " << this_thread::get_id() << endl;
cout << "i = " << i << ", msg =" << msg << endl;
sleep(1);//持有鎖後，再休眠1秒
cout << "開鎖的thread = " << this_thread::get_id() << endl;
mtx.unlock();
}
}
類別與mutex
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class Student {
//私有成員變數互斥鎖
mutex m_mutex;
public:
void func(string msg) {
for (int i = 0; i < 10; i++) {
m_mutex.lock();
cout << "msg = " << msg << endl;
m_mutex.unlock();
}
}
};
int main() {
Student student;//建立物件
thread t1(&Student::func, &student, "student1");
thread t2(&Student::func, &student, "student2");
thread t3(&Student::func, &student, "student3");
thread t4(&Student::func, &student, "student4");
thread t5(&Student::func, &student, "student5");
t1.join();
t2.join();
t3.join();
t4.join();
t5.join();
return 0;
}
msg = student1
msg = student3
msg = student5
msg = student5
msg = student5
msg = student5
msg = student5
msg = student5
msg = student5
msg = student5
msg = student5
msg = student5
以下截斷
recursive_mutex
以下程式碼，func4()呼叫func3()，但執行結果只印出func4。 若是鎖中有鎖，會有死鎖(Dead lock)問題，mutex必須解鎖才能加鎖。
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mutex mtx;
void func3() {
mtx.lock();//加鎖
cout << "func3" << endl;
mtx.unlock();//解鎖
}
void func4() {
mtx.lock();//加鎖
cout << "func4" << endl;
func3();//呼叫func3
mtx.unlock();//解鎖
}
int main() {
//建立執行緒t1
thread t1(func4, "test test");
//執行緒t1被記憶體釋放
t1.join();
return 0;
}
func4
改成使用recursive_mutex，就不會有死鎖問題。
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recursive_mutex r_mtx;
void func3() {
r_mtx.lock();//加鎖
cout << "func3" << endl;
r_mtx.unlock();//解鎖
}
void func4() {
r_mtx.lock();//加鎖
cout << "func4" << endl;
func3();//呼叫func3
r_mtx.unlock();//解鎖
}
int main() {
//建立執行緒t1
thread t1(func4);
//執行緒t1被記憶體釋放
t1.join();
return 0;
}
func4
func3
lock_guard
lock_guard是mutex的樣板，只要管加鎖，不用管解鎖，它會在離開有效範圍(Scope)與生命週期(Lifetime)，自動解鎖。
語法
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lock_guard<mutex類別名> mlock(mtx物件名);
把之前的程式碼修改成lock_guard
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void func2(string msg) {
//每一秒印1次，這裡是執行10秒
for (int i = 0; i < 10; i++) {
cout << "申請鎖的thread = " << this_thread::get_id() << endl;
//有效範圍scope 開始
{
lock_guard<mutex> mlock(mtx);//把門鎖上
cout << "鎖上的thread = " << this_thread::get_id() << endl;
cout << "i = " << i << ", msg =" << msg << endl;
sleep(1);//持有鎖後，再休眠1秒
cout << "開鎖的thread = " << this_thread::get_id() << endl;
}
//有效範圍scope 結束
}
}
int main() {
//建立執行緒t1
thread t1(func2, "test test");
//建立執行緒t2
thread t2(func2, "abcdefg abcdefg");
//執行緒t1被記憶體釋放
t1.join();
//執行緒t2被記憶體釋放
t2.join();
return 0;
}
申請鎖的thread = 0x700005e2a000
鎖上的thread = 0x700005e2a000
i = 0, msg =test test
申請鎖的thread = 0x700005ead000
開鎖的thread = 0x700005e2a000
申請鎖的thread = 0x700005e2a000
鎖上的thread = 0x700005e2a000
i = 1, msg =test test
開鎖的thread = 0x700005e2a000
申請鎖的thread = 0x700005e2a000
鎖上的thread = 0x700005e2a000
i = 2, msg =test test
開鎖的thread = 0x700005e2a000
申請鎖的thread = 0x700005e2a000
鎖上的thread = 0x700005e2a000
i = 3, msg =test test
開鎖的thread = 0x700005e2a000
申請鎖的thread = 0x700005e2a000
鎖上的thread = 0x700005ead000
i = 0, msg =abcdefg abcdefg
開鎖的thread = 0x700005ead000
申請鎖的thread = 0x700005ead000
鎖上的thread = 0x700005ead000
i = 1, msg =abcdefg abcdefg
開鎖的thread = 0x700005ead000
申請鎖的thread = 0x700005ead000
鎖上的thread = 0x700005ead000
i = 2, msg =abcdefg abcdefg
開鎖的thread = 0x700005ead000
申請鎖的thread = 0x700005ead000
鎖上的thread = 0x700005ead000
i = 3, msg =abcdefg abcdefg
開鎖的thread = 0x700005ead000
申請鎖的thread = 0x700005ead000
鎖上的thread = 0x700005ead000
i = 4, msg =abcdefg abcdefg
開鎖的thread = 0x700005ead000
以下截斷