November 29, 2005
mq_receive, mq_timedreceive - receive a message from message queue (REALTIME)
.In mqueue.h ssize_t
.Fo mq_receive mqd_t mqdes char *msg_ptr size_t msg_len unsigned *msg_prio
.Fo mq_timedreceive mqd_t mqdes char *msg_ptr size_t msg_len unsigned *msg_prio const struct timespec *abs_timeout
mq_receivesystem call receives oldest of the highest priority message(s) from the message queue specified by mqdes. If the size of the buffer in bytes, specified by the msg_len argument, is less than the mq_msgsize attribute of the message queue, the system call will fail and return an error. Otherwise, the selected message will be removed from the queue and copied to the buffer pointed to by the msg_ptr argument.
If the argument msg_prio is not
NULL, the priority of the selected message will be stored in the location referenced by msg_prio. If the specified message queue is empty and
O_NONBLOCKis not set in the message queue description associated with mqdes,
mq_receivewill block until a message is enqueued on the message queue or until
mq_receiveis interrupted by a signal. If more than one thread is waiting to receive a message when a message arrives at an empty queue and the Priority Scheduling option is supported, then the thread of highest priority that has been waiting the longest will be selected to receive the message. Otherwise, it is unspecified which waiting thread receives the message. If the specified message queue is empty and
O_NONBLOCKis set in the message queue description associated with mqdes, no message will be removed from the queue, and
mq_receivewill return an error.
mq_timedreceivesystem call will receive the oldest of the highest priority messages from the message queue specified by mqdes as described for the
mq_receivesystem call. However, if
O_NONBLOCKwas not specified when the message queue was opened via the
mq_opensystem call, and no message exists on the queue to satisfy the receive, the wait for such a message will be terminated when the specified timeout expires. If
O_NONBLOCKis set, this system call is equivalent to
The timeout expires when the absolute time specified by abs_timeout passes, as measured by the clock on which timeouts are based (that is, when the value of that clock equals or exceeds abs_timeout), or if the absolute time specified by abs_timeout has already been passed at the time of the call.
The timeout is based on the
Upon successful completion, the
mq_timedreceivesystem calls return the length of the selected message in bytes and the message is removed from the queue. Otherwise, no message is removed from the queue, the system call returns a value of -1, and the global variable errno is set to indicate the error.
mq_timedreceivesystem calls will fail if:
|The mqdes argument is not a valid message queue descriptor open for reading.|
|The specified message buffer size, msg_len, is less than the message size attribute of the message queue.|
|The process or thread would have blocked, and the abs_timeout parameter specified a nanoseconds field value less than zero or greater than or equal to 1000 million.|
mq_timedreceivesystem calls conform to -p1003.1-2004.
Support for POSIX message queues first appeared in
.Fx 7.0 .
.Fx 7.0 .
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2004 Edition, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2004 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html.