为什么另一个线程，比如 B 运行当 pthread_mutex 在线程 A 中被锁定和解锁时？

Question

根据手册页

pthread_mutex_lock locks the given mutex. If the mutex is currently unlocked, it becomes locked and owned by the calling thread, and pthread_mutex_lock returns immediately. If the mutex is already locked by another thread, pthread_mutex_lock suspends the calling thread until the mutex is unlocked.

我的理解是当 line 3 执行 main thread 时拥有 mtx 的所有权。然后它执行其关键区域操作，然后到达 line 4 并解锁 mtx。我的问题是 -

当mtx被锁定时，另一个线程是否可以并发运行？
line 2有什么用，因为newThread只能在line 4执行后才能解锁mtx，从而使line 2变得多余？

如果 line 1 取消注释会怎样？

#include<stdio.h>
#include<pthread.h>
#include<semaphore.h>
#include<unistd.h>


sem_t bin_sem;
pthread_mutex_t mtx;
char message[100];


void * thread_function(void * arg)
{   
    int x;
    char message2[10];
    while(1)
    {   

//      pthread_mutex_lock(&mtx); //line 1
        printf("thread2 : waiting..\n\n");
        sem_wait(&bin_sem);     
        printf("hi i am the new thread waiting inside critical..\n");
        scanf("%s",message);
        printf("You entered in newthread:%s\n\n",message);
        sem_post(&bin_sem);
        pthread_mutex_unlock(&mtx); //line 2

    }

}

int main(void)
{
    pthread_t athread;
    pthread_attr_t ta;
    char message2[10];
    int x;

    sem_init(&bin_sem,0,1);

    pthread_mutex_init(&mtx,NULL);

    pthread_attr_init(&ta);
    pthread_attr_setschedpolicy(&ta,SCHED_RR);                                                                                                                                                                                                       

    pthread_create(&athread,&ta,thread_function,NULL);
    while(1)
    {   
        pthread_mutex_lock(&mtx); //line 3
        printf("main waiting..\n\n");
        sem_wait(&bin_sem); 
        printf("hi i am the main thread waiting inside critical..\n");
        scanf("%s",message);
        printf("You entered in main:%s\n\n",message);
        sem_post(&bin_sem);
        pthread_mutex_unlock(&mtx); //line 4
    }
    sleep(5);       
}

Answer 1

互斥是一种实现临界区的机制。

pthread_mutex_lock(x) 和 pthread_mutex_unlock(x) 调用之间的任何代码在任何给定时间都将仅在一个线程中执行。就这些了。

所以...

1. Can the other thread concurrently run when mtx is locked?

如果它没有锁定 mtx，那当然可以。

2. What is the use of line 2 since newThread can only unlock mtx when line 4 has been executed, and thus makes line 2 redundant?

互斥体变得无用，并且您还会得到 UB，因为您在未锁定它的线程中解锁它：

If the mutex type is PTHREAD_MUTEX_DEFAULT...
Attempting to unlock the mutex if it was not locked by the calling thread results in undefined behavior.

（默认情况下你得到互斥类型PTHREAD_MUTEX_DEFAULT）

3. What would happen if line 1 is uncommented?

你得到 thread starvation，因为互斥量几乎一直处于锁定状态，并且在解锁后立即重新锁定（POSIX 不能保证互斥量的公平性）。

A POSIX semaphore does provide fairness in some cases (when you use SCHED_FIFO or SCHED_RR schedulers), but is heavier.

我不太明白你想达到什么目的（应用程序看起来做作）。在实际应用程序中，任一线程需要执行的操作可能有一些逻辑顺序。因此，如果信号量适合您，我会保留它并删除互斥量。

为什么另一个线程，比如 B 运行当 pthread_mutex 在线程 A 中被锁定和解锁时？

Why is another thread, say B running when pthread_mutex is being locked and unlocked inside thread A?

c

linux

multithreading

mutex

pthreads

为什么另一个线程，比如 B 运行 当 pthread_mutex 在线程 A 中被锁定和解锁时？

Why is another thread, say B running when pthread_mutex is being locked and unlocked inside thread A?

c

linux

multithreading

mutex

pthreads

为什么另一个线程，比如 B 运行当 pthread_mutex 在线程 A 中被锁定和解锁时？