mq_notify不通知事件（Linux编程）

Question

我使用mq_notify来通知有关消息队列上的事件，但我的注册通知程序功能未被调用。我错过了什么？

我粘贴下面我的代码片段：

---

static void sigNotifier(union sigval sv) 
{ 
    printf ("I'm called.\n"); 
}  

int main() 
{ 
    mqd_t queueID = 0; 
    message_t msg; 
    int retval; 
    struct mq_attr attr; 
    struct sigevent sev; 

    attr.mq_msgsize = MSG_SIZE; 
    attr.mq_maxmsg = 30; 

    errno = 0; 

    queueID = mq_open(MSG_QUEUE_NAME, O_RDONLY, 0666, &attr); 

    if (queueID == -1) { 
     printf ("Message queue open failed: %d\n", errno); 
    } 

    sev.sigev_notify = SIGEV_THREAD; 
    sev.sigev_notify_function = sigNotifier; 
    sev.sigev_notify_attributes = NULL; 
    sev.sigev_value.sival_ptr = &queueID; 

    retval = mq_notify(queueID, &sev); 
    if (retval < 0) { 
     printf ("Notification failed: %d\n", errno); 
    } 

    while (1);  
} 

Answer 1

由于代码是我要问的手册页副本：

1. 你确定你发送消息正确排队？
2. 请先尝试阅读使用阻止阅读，看看你是否得到了什么。

在Linux下，您可以使用select/poll/epoll等待有关队列的通知以及 mqd_t是普通文件描述符。

Answer 2

同时检查队列是否已经有消息。如果消息存在于队列中，那么mq_notify（）将不会收到通知，直到队列为空，然后新消息进来。 由于队列在程序运行期间是持久的，因此您希望确保mq_unlink（）已在在打开之前排队。

Answer 3

手册页mq_notify：

1）消息通知，只有当新的消息到达和队列以前为空发生。如果在调用mq_notify（）时该队列不是空的，则只有在队列被清空并且有新消息到达之后才会发生通知。

2）通知发生一次：通知发送后，通知注册被删除，另一个进程可以注册消息通知。如果通知的进程希望收到下一个通知，它可以使用mq_notify（）来请求进一步的通知。

3）只有一个进程可以注册接收来自消息队列的通知。

所以：

1）清空MSGQ与mq_notify注册​​后立即RECV在读取器进程新消息（）。

2）重新注册通知功能，接收下一条消息。

3）寄存器只有1用于从Q.

接收消息

这里是在C++中的简单消息队列读取的代码：

#include <iostream> 
#include <mqueue.h> 
#include <string.h> 
#include <sstream> 
#include <unistd.h> 
#include <errno.h> 

using namespace std; 

#define MSG_Q_NAME "/MY_MSGQ_3" 

static void      /* Thread start function */ 
tfunc(union sigval sv) 
{ 
    mqd_t msq_id = *(static_cast<mqd_t*>(sv.sival_ptr)); 

    struct mq_attr attr; 
    if(mq_getattr(msq_id, &attr) < 0) 
    { 
    cout << "Error in mq_getattr " << strerror(errno) << endl; 
    return; 
    } 

    // Reregister for new messages on Q 
    struct sigevent sev; 
    sev.sigev_notify = SIGEV_THREAD; 
    sev.sigev_notify_function = tfunc; 
    sev.sigev_notify_attributes = NULL; 
    sev.sigev_value.sival_ptr = sv.sival_ptr; 
    if (mq_notify(msq_id, &sev) < 0) 
    { 
    cout << "Error during Reregister in msq_notify : " 
     << strerror(errno) << endl; 
    exit(EXIT_FAILURE); 
    } 

    // Read new message on the Q 
    char* arr = new char[attr.mq_msgsize]; 
    memset(arr, 0, attr.mq_msgsize); 
    if(mq_receive(msq_id, arr, attr.mq_msgsize, 0) < 0) 
    { 
    if(errno != EAGAIN) 
    { 
     cout << "Error in mq_receive " << strerror(errno) << endl; 
     exit(EXIT_FAILURE); 
    } 
    } 
    else 
    { 
    cout << "Msg rcvd " << arr << endl; 
    } 
} 

int main() 
{ 
    mqd_t msq_id = mq_open(MSG_Q_NAME, O_RDONLY | O_NONBLOCK | O_CREAT, 0666, 0); 
    if(msq_id == (mqd_t) -1) 
    { 
    cout << "Error on msg Q creation: " << strerror(errno) << endl; 
    exit(EXIT_FAILURE); 
    } 

    // The process is registered for notification for new message on the Q 
    struct sigevent sev; 
    sev.sigev_notify = SIGEV_THREAD; 
    sev.sigev_notify_function = tfunc; 
    sev.sigev_notify_attributes = NULL; 
    sev.sigev_value.sival_ptr = &msq_id; 

    if (mq_notify(msq_id, &sev) < 0) 
    { 
    cout << "Error on msg Q notify : " << strerror(errno) << endl; 
    exit(EXIT_FAILURE); 
    } 
    else 
    { 
    cout << "Notify for msg Q reception " << MSG_Q_NAME << endl; 
    } 

    // Man Page mq_notify: Message notification occurs only when a new 
    // message arrives and the queue was previously empty. If the queue was 
    // not empty at the time mq_notify() was called, then a notification will 
    // occur only after the queue is emptied and a new message arrives. 
    // 
    // So emptying the Q to recv new messages 
    ssize_t n = 0; 
    struct mq_attr attr; 
    if(mq_getattr(msq_id, &attr) < 0) 
    { 
    cout << "Error in mq_getattr " << strerror(errno) << endl; 
    exit(EXIT_FAILURE); 
    } 
    char* arr = new char[attr.mq_msgsize]; 
    memset(arr, 0, attr.mq_msgsize); 
    while((n = mq_receive(msq_id, arr, attr.mq_msgsize, 0) >= 0)) 
    { 
    cout << "Empty the Q. Msg rcvd " << arr << endl; 
    } 

    while(1) 
    ; 

    mq_close(msq_id); 
} 

下面是一个简单的消息Q作家代码：

#include <iostream> 
#include <mqueue.h> 
#include <string.h> 
#include <sstream> 
#include <unistd.h> 
#include <errno.h> 
#include <sys/stat.h> 

using namespace std; 

#define MSG_Q_NAME "/MY_MSGQ_3" 

int main() 
{ 
    struct mq_attr attr; 
    memset(&attr, 0, sizeof attr); 
    attr.mq_msgsize = 8192; 
    attr.mq_flags = 0; 
    attr.mq_maxmsg = 10; 

    mqd_t msq_id = mq_open(MSG_Q_NAME, O_RDWR | O_CREAT | O_NONBLOCK, 
         0777, &attr); 
    if(msq_id == (mqd_t) -1) 
    { 
    cout << "Error on msg Q creation: " << strerror(errno) << endl; 
    exit(1); 
    } 

    // Write 5 msgs on message Q 
    for(int i = 0; i < 5; ++i) 
    { 
    stringstream s; 
    s << "My Msg " << i; 

    if(mq_send(msq_id, s.str().c_str(), strlen(s.str().c_str()), 0) < 0) 
    { 
     if(errno != EAGAIN) 
     { 
     cout << "Error on sending msg on MsgQ " << strerror(errno); 
     mq_close(msq_id); 
     exit(1); 
     } 
    } 
    else 
    { 
     cout << "Sent msg " << s.str() << endl; 
    } 

    sleep(1); // Easily see the received message in reader 
    } 

    mq_close(msq_id); 
}