Question

#include <pthread.h>
#include <time.h>
#include "errors.h"


    typedef struct alarm_tag {
struct alarm_tag    *link;
int                 seconds;
time_t              time;   /* seconds from EPOCH */
char                message[64];
    } alarm_t;

    pthread_mutex_t alarm_mutex = PTHREAD_MUTEX_INITIALIZER;
alarm_t *alarm_list = NULL;


void *alarm_thread (void *arg)
{
alarm_t *alarm;
int sleep_time;
time_t now;
int status;


while (1) {
    status = pthread_mutex_lock (&alarm_mutex);
    if (status != 0)
        err_abort (status, "Lock mutex");
    alarm = alarm_list;

    /*
     * If the alarm list is empty, wait for one second. This
     * allows the main thread to run, and read another
     * command. If the list is not empty, remove the first
     * item. Compute the number of seconds to wait -- if the
     * result is less than 0 (the time has passed), then set
     * the sleep_time to 0.
     */
    if (alarm == NULL)
        sleep_time = 1;
    else {
        alarm_list = alarm->link;
        now = time (NULL);
        if (alarm->time <= now)
            sleep_time = 0;
        else
            sleep_time = alarm->time - now;
#ifdef DEBUG
        printf ("[waiting: %d(%d)\"%s\"]\n", alarm->time,
            sleep_time, alarm->message);
#endif
        }

    /*
     * Unlock the mutex before waiting, so that the main
     * thread can lock it to insert a new alarm request. If
     * the sleep_time is 0, then call sched_yield, giving
     * the main thread a chance to run if it has been
     * readied by user input, without delaying the message
     * if there's no input.
     */
    status = pthread_mutex_unlock (&alarm_mutex);
    if (status != 0)
        err_abort (status, "Unlock mutex");
    if (sleep_time > 0)
        sleep (sleep_time);
    else
        sched_yield ();

    /*
     * If a timer expired, print the message and free the
     * structure.
     */
    if (alarm != NULL) {
        printf ("(%d) %s\n", alarm->seconds, alarm->message);
        free (alarm);
    }
}
}

int main (int argc, char *argv[])
{
        int status;
    char line[128];
    alarm_t *alarm, **last, *next;
    pthread_t thread;

status = pthread_create (
    &thread, NULL, alarm_thread, NULL);
if (status != 0)
    err_abort (status, "Create alarm thread");
while (1) {
    printf ("alarm> ");
    if (fgets (line, sizeof (line), stdin) == NULL) exit (0);
    if (strlen (line) <= 1) continue;
    alarm = (alarm_t*)malloc (sizeof (alarm_t));
    if (alarm == NULL)
        errno_abort ("Allocate alarm");

    /*
     * Parse input line into seconds (%d) and a message
     * (%64[^\n]), consisting of up to 64 characters
     * separated from the seconds by whitespace.
     */
    if (sscanf (line, "%d %64[^\n]", 
        &alarm->seconds, alarm->message) < 2) {
        fprintf (stderr, "Bad command\n");
        free (alarm);
    } else {
        status = pthread_mutex_lock (&alarm_mutex);
        if (status != 0)
            err_abort (status, "Lock mutex");
        alarm->time = time (NULL) + alarm->seconds;

        /*
         * Insert the new alarm into the list of alarms,
         * sorted by expiration time.
         */
        last = &alarm_list;
        next = *last;
        while (next != NULL) {
            if (next->time >= alarm->time) {
                alarm->link = next;
                *last = alarm;
                break;
            }
            last = &next->link;
            next = next->link;
        }
        /*
         * If we reached the end of the list, insert the new
         * alarm there. ("next" is NULL, and "last" points
         * to the link field of the last item, or to the
         * list header).
         */
        if (next == NULL) {
            *last = alarm;
            alarm->link = NULL;
        }
#ifdef DEBUG
        printf ("[list: ");
        for (next = alarm_list; next != NULL; next = next->link)
            printf ("%d(%d)[\"%s\"] ", next->time,
                next->time - time (NULL), next->message);
        printf ("]\n");
#endif
        status = pthread_mutex_unlock (&alarm_mutex);
        if (status != 0)
            err_abort (status, "Unlock mutex");
    }
}
    }

嗨，这是我的代码，任何人都可以告诉我，因为互斥锁未在结构中声明。因此，当互斥锁锁定和解锁时，实际更改的数据是否有人可以启发我？

Answer 1

这组数据受互斥锁保护？

互斥对象是alarm_mutex。其中“受保护”的数据不必在代码中明确提及;如在，不需要是语义连接。互斥体是一种低级线程原语，因此用户需要围绕它构建自己的逻辑。在您的情况下，内存中的一个位置用于阻止代码的其他部分，即访问实际数据的部分，来自干扰。

以这种方式思考：std::atomic<int> x;表达操作的原子性。 int x; mutex m;要求访问x的每段代码都正确查看m以确保程序的正确性。这个低级别的访问是我们在你的例子中看到的。

Answer 2

pthread_mutex_t alarm_mutex = PTHREAD_MUTEX_INITIALIZER;创建一个共享的互斥对象，用于锁定/解锁。

pthread_mutex_lock会在互斥锁可用时立即锁定互斥锁。执行此行后，它将变为所有其他线程不可用。 pthread_mutex_unlock解锁互斥锁，使其可用于其他线程（解锁另一个线程的pthread_mutex_lock）

Answer 3

互斥锁并不知道它在保护什么。程序员的工作就是知道并且只有在互斥锁被锁定时才更改它所保护的数据。

在这种特定情况下，似乎alarm列表是被锁定的数据。

互斥锁/被锁定的互斥锁数据是什么？

3 个答案: