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| .TH MQ_NOTIFY 3 2020-06-09 "Linux" "Linux Programmer's Manual" |
| .SH NAME |
| mq_notify \- register for notification when a message is available |
| .SH SYNOPSIS |
| .nf |
| .B #include <mqueue.h> |
| .PP |
| .BI "int mq_notify(mqd_t " mqdes ", const struct sigevent *" sevp ); |
| .fi |
| .PP |
| Link with \fI\-lrt\fP. |
| .SH DESCRIPTION |
| .BR mq_notify () |
| allows the calling process to register or unregister for delivery of |
| an asynchronous notification when a new message arrives on |
| the empty message queue referred to by the message queue descriptor |
| .IR mqdes . |
| .PP |
| The |
| .I sevp |
| argument is a pointer to a |
| .I sigevent |
| structure. |
| For the definition and general details of this structure, see |
| .BR sigevent (7). |
| .PP |
| If |
| .I sevp |
| is a non-null pointer, then |
| .BR mq_notify () |
| registers the calling process to receive message notification. |
| The |
| .I sigev_notify |
| field of the |
| .I sigevent |
| structure to which |
| .I sevp |
| points specifies how notification is to be performed. |
| This field has one of the following values: |
| .TP |
| .B SIGEV_NONE |
| A "null" notification: the calling process is registered as the target |
| for notification, but when a message arrives, no notification is sent. |
| .\" When is SIGEV_NONE useful? |
| .TP |
| .B SIGEV_SIGNAL |
| Notify the process by sending the signal specified in |
| .IR sigev_signo . |
| See |
| .BR sigevent (7) |
| for general details. |
| The |
| .I si_code |
| field of the |
| .I siginfo_t |
| structure will be set to |
| .BR SI_MESGQ . |
| In addition, |
| .\" I don't know of other implementations that set |
| .\" si_pid and si_uid -- MTK |
| .I si_pid |
| will be set to the PID of the process that sent the message, and |
| .I si_uid |
| will be set to the real user ID of the sending process. |
| .TP |
| .B SIGEV_THREAD |
| Upon message delivery, invoke |
| .I sigev_notify_function |
| as if it were the start function of a new thread. |
| See |
| .BR sigevent (7) |
| for details. |
| .PP |
| Only one process can be registered to receive notification |
| from a message queue. |
| .PP |
| If |
| .I sevp |
| is NULL, and the calling process is currently registered to receive |
| notifications for this message queue, then the registration is removed; |
| another process can then register to receive a message notification |
| for this queue. |
| .PP |
| Message notification occurs only when a new message arrives and |
| the queue was previously empty. |
| If the queue was not empty at the time |
| .BR mq_notify () |
| was called, then a notification will occur only after |
| the queue is emptied and a new message arrives. |
| .PP |
| If another process or thread is waiting to read a message |
| from an empty queue using |
| .BR mq_receive (3), |
| then any message notification registration is ignored: |
| the message is delivered to the process or thread calling |
| .BR mq_receive (3), |
| and the message notification registration remains in effect. |
| .PP |
| Notification occurs once: after a notification is delivered, |
| the notification registration is removed, |
| and another process can register for message notification. |
| If the notified process wishes to receive the next notification, |
| it can use |
| .BR mq_notify () |
| to request a further notification. |
| This should be done before emptying all unread messages from the queue. |
| (Placing the queue in nonblocking mode is useful for emptying |
| the queue of messages without blocking once it is empty.) |
| .SH RETURN VALUE |
| On success |
| .BR mq_notify () |
| returns 0; on error, \-1 is returned, with |
| .I errno |
| set to indicate the error. |
| .SH ERRORS |
| .TP |
| .B EBADF |
| The message queue descriptor specified in |
| .I mqdes |
| is invalid. |
| .TP |
| .B EBUSY |
| Another process has already registered to receive notification |
| for this message queue. |
| .TP |
| .B EINVAL |
| .I sevp\->sigev_notify |
| is not one of the permitted values; or |
| .I sevp\->sigev_notify |
| is |
| .B SIGEV_SIGNAL |
| and |
| .I sevp\->sigev_signo |
| is not a valid signal number. |
| .TP |
| .B ENOMEM |
| Insufficient memory. |
| .PP |
| POSIX.1-2008 says that an implementation |
| .I may |
| generate an |
| .B EINVAL |
| .\" Linux does not do this |
| error if |
| .I sevp |
| is NULL, and the caller is not currently registered to receive |
| notifications for the queue |
| .IR mqdes . |
| .SH ATTRIBUTES |
| For an explanation of the terms used in this section, see |
| .BR attributes (7). |
| .TS |
| allbox; |
| lb lb lb |
| l l l. |
| Interface Attribute Value |
| T{ |
| .BR mq_notify () |
| T} Thread safety MT-Safe |
| .TE |
| .sp 1 |
| .SH CONFORMING TO |
| POSIX.1-2001. |
| .SH NOTES |
| .\" |
| .SS C library/kernel differences |
| In the glibc implementation, the |
| .BR mq_notify () |
| library function is implemented on top of the system call of the same name. |
| When |
| .I sevp |
| is NULL, or specifies a notification mechanism other than |
| .BR SIGEV_THREAD , |
| the library function directly invokes the system call. |
| For |
| .BR SIGEV_THREAD , |
| much of the implementation resides within the library, |
| rather than the kernel. |
| (This is necessarily so, |
| since the thread involved in handling the notification is one |
| that must be managed by the C library POSIX threads implementation.) |
| The implementation involves the use of a raw |
| .BR netlink (7) |
| socket and creates a new thread for each notification that is |
| delivered to the process. |
| .SH EXAMPLES |
| The following program registers a notification request for the |
| message queue named in its command-line argument. |
| Notification is performed by creating a thread. |
| The thread executes a function which reads one message from the |
| queue and then terminates the process. |
| .SS Program source |
| .EX |
| #include <pthread.h> |
| #include <mqueue.h> |
| #include <stdio.h> |
| #include <stdlib.h> |
| #include <unistd.h> |
| |
| #define handle_error(msg) \e |
| do { perror(msg); exit(EXIT_FAILURE); } while (0) |
| |
| static void /* Thread start function */ |
| tfunc(union sigval sv) |
| { |
| struct mq_attr attr; |
| ssize_t nr; |
| void *buf; |
| mqd_t mqdes = *((mqd_t *) sv.sival_ptr); |
| |
| /* Determine max. msg size; allocate buffer to receive msg */ |
| |
| if (mq_getattr(mqdes, &attr) == \-1) |
| handle_error("mq_getattr"); |
| buf = malloc(attr.mq_msgsize); |
| if (buf == NULL) |
| handle_error("malloc"); |
| |
| nr = mq_receive(mqdes, buf, attr.mq_msgsize, NULL); |
| if (nr == \-1) |
| handle_error("mq_receive"); |
| |
| printf("Read %zd bytes from MQ\en", nr); |
| free(buf); |
| exit(EXIT_SUCCESS); /* Terminate the process */ |
| } |
| |
| int |
| main(int argc, char *argv[]) |
| { |
| mqd_t mqdes; |
| struct sigevent sev; |
| |
| if (argc != 2) { |
| fprintf(stderr, "Usage: %s <mq\-name>\en", argv[0]); |
| exit(EXIT_FAILURE); |
| } |
| |
| mqdes = mq_open(argv[1], O_RDONLY); |
| if (mqdes == (mqd_t) \-1) |
| handle_error("mq_open"); |
| |
| sev.sigev_notify = SIGEV_THREAD; |
| sev.sigev_notify_function = tfunc; |
| sev.sigev_notify_attributes = NULL; |
| sev.sigev_value.sival_ptr = &mqdes; /* Arg. to thread func. */ |
| if (mq_notify(mqdes, &sev) == \-1) |
| handle_error("mq_notify"); |
| |
| pause(); /* Process will be terminated by thread function */ |
| } |
| .EE |
| .SH SEE ALSO |
| .BR mq_close (3), |
| .BR mq_getattr (3), |
| .BR mq_open (3), |
| .BR mq_receive (3), |
| .BR mq_send (3), |
| .BR mq_unlink (3), |
| .BR mq_overview (7), |
| .BR sigevent (7) |