### 使用 `pthread_mutex_t` 和 `pthread_cond_t` 实现线程同步
#### 1. 线程互斥量 `pthread_mutex_t`
`pthread_mutex_t` 是一种用于保护共享资源的机制，防止多个线程同时访问同一数据而导致的数据不一致问题。它通过加锁和解锁来控制对临界区的访问。
- **初始化**
可以使用静态初始化或者动态初始化两种方式创建互斥锁。
静态初始化可以直接赋值为常量 `PTHREAD_MUTEX_INITIALIZER`[^1]；而动态初始化则调用函数 `pthread_mutex_init
()` 来完成设置特定属性的需求。
```c
#include <pthread.h>
// 动态初始化
pthread_mutex_t mutex;
if
(pthread_mutex_init
(&mutex, NULL
) != 0
) {
perror
("Mutex initialization failed"
);
}
```
- **锁定与解锁**
加锁操作由 `pthread_mutex_lock
()` 完成，在成功获取锁之前会阻塞当前线程直到其他持有该锁的线程释放为止。当不再需要独占访问时应立即解除锁定状态以便让其它等待中的线程继续运行下去。
```c
pthread_mutex_lock
(&mutex
);
// Critical Section Code Here...
pthread_mutex_unlock
(&mutex
);
```
#### 2. 条件变量 `pthread_cond_t`
条件变量允许一个或多个线程处于休眠状态直至某个指定事件发生才唤醒它们重新执行后续逻辑流程。通常配合互斥锁一起工作形成更复杂的同步模式比如生产者消费者模型等场景下非常有用。
- **初始化与销毁**
同样支持静态和动态两种方法来进行
实例化处理。如果采用后者形式，则需提供额外参数指明可能存在的特殊配置选项
(一般传入NULL表示接受默认行为即可
)[^2].
```c
pthread_cond_t cond;
// Dynamic Initialization with Attributes
if
(pthread_cond_init
(&cond, NULL
) != 0
){
perror
("Condition Variable Init Failed!"
);
}
// Destruction when no longer needed.
if
(pthread_cond_destroy
(&cond
)!=0
){
perror
("Destroy Condition Var Error."
);
}
```
- **信号发送与接收**
当满足某些预定条件下可以通知单个
(`pthread_cond_signal`
)或多组
(`pthread_cond_broadcast`
)正在睡眠监听此对象变化情况下的目标进程恢复活动状态并尝试再次竞争进入受控区域的机会.
```c
// Signal one waiting thread
to proceed.
pthread_cond_signal
(&cond
);
// Broadcast signal so all waiters can check predicate again.
pthread_cond_broadcast
(&cond
);
```
- **等待条件成立**
调用 `pthread_cond_wait
()` 或其超时版本会使当前线程放弃持有的互斥锁并挂起自己知道被另一个地方发出相应消息之后才会自动重获先前丢失掉的那个锁然后再返回给调用方进一步判断是否真正达到了预期目的再做下一步动作[^3].
```c
while
(!condition_met
()) { // Predicate checking loop inside critical section.
pthread_cond_wait
(&cond,&mutex
);
}
```
####
综合示例：生产者/消费者问题解决方案
下面给出一段简单的代码片段展示如何利用上述工具解决经典的生产和消费之间协调工作的例子：
```c
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#define BUFFER_SIZE 5
typedef struct {
int buffer[BUFFER_SIZE];
int count, in, out;
pthread_mutex_t lock;
pthread_cond_t full, empty;
} Buffer;
void initBuffer
(Buffer* b
) {
b
->count = b
->in = b
->out = 0;
pthread_mutex_init
(&
(b
->lock
), NULL
);
pthread_cond_init
(&
(b
->full
), NULL
);
pthread_cond_init
(&
(b
->empty
), NULL
);
}
void insert_item
(Buffer* b, int item
) {
pthread_mutex_lock
(&
(b
->lock
));
while
(b
->count >= BUFFER_SIZE
){
printf
("Producer waits.\n"
);
pthread_cond_wait
(&
(b
->empty
), &
(b
->lock
));
}
b
->buffer[b
->in] = item;
b
->in =
(b
->in + 1
)%BUFFER_SIZE;
++
(b
->count
);
pthread_cond_signal
(&
(b
->full
));
pthread_mutex_unlock
(&
(b
->lock
));
}
int remove_item
(Buffer* b
) {
int item=
-1;
pthread_mutex_lock
(&
(b
->lock
));
while
(b
->count<=0
){
printf
("Consumer waits.\n"
);
pthread_cond_wait
(&
(b
->full
),&
(b
->lock
));
}
item=b
->buffer[b
->out];
b
->out=
(b
->out+1
)%BUFFER_SIZE;
--(b
->count
);
pthread_cond_signal
(&
(b
->empty
));
pthread_mutex_unlock
(&
(b
->lock
));
return item;
}
void *producer
(void *arg
) {
Buffer *buf = arg;
static int counter=0;
while
(1
){
sleep
(rand
()%3+1
);
insert_item
(buf,++counter
);
printf
("Produced:%d\n",counter
);
}
}
void *consumer
(void *arg
) {
Buffer *buf = arg;
while
(1
){
sleep
(rand
()%4+1
);
int consumedItem=remove_item
(buf
);
if
(consumedItem!=
-1
)
printf
("Consumed :%d\n",consumedItem
);
}
}
int main
(){
srand
(time
(NULL
));
Buffer buf;
initBuffer
(&buf
);
pthread_t prod_thread, cons_thread;
pthread_create
(&prod_thread,NULL,
(void*
)&producer,
(void*
)&buf
);
pthread_create
(&cons_thread,NULL,
(void*
)&consumer,
(void*
)&buf
);
pthread_join
(prod_thread,NULL
);
pthread_join
(cons_thread,NULL
);
return EXIT_SUCCESS;
}
```
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