Copyright (c) 2005 David Xu <davidxu@FreeBSD.org> All rights reserved. Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met: 1. Redistributions of source code must retain the above copyright notice(s), this list of conditions and the following disclaimer as the first lines of this file unmodified other than the possible addition of one or more copyright notices. 2. Redistributions in b...
NAMEmq_receive , mq_timedreceive - receive a message from message queue (REALTIME)
SYNOPSISIn mqueue.h Ft ssize_t Fo mq_receive Fa mqd_t mqdes Fa char *msg_ptr Fa size_t msg_len Fa unsigned *msg_prio Fc Ft ssize_t Fo mq_timedreceive Fa mqd_t mqdes Fa char *msg_ptr Fa size_t msg_len Fa unsigned *msg_prio Fa const struct timespec *abs_timeout Fc
DESCRIPTIONThe Fn mq_receive system call receives oldest of the highest priority message(s) from the message queue specified by Fa mqdes . If the size of the buffer in bytes, specified by the Fa 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 Fa msg_ptr argument.
If the argument Fa msg_prio is not NULL the priority of the selected message will be stored in the location referenced by Fa msg_prio . If the specified message queue is empty and O_NONBLOCK is not set in the message queue description associated with Fa mqdes , Fn mq_receive will block until a message is enqueued on the message queue or until Fn mq_receive is 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_NONBLOCK is set in the message queue description associated with Fa mqdes , no message will be removed from the queue, and Fn mq_receive will return an error.
The Fn mq_timedreceive system call will receive the oldest of the highest priority messages from the message queue specified by Fa mqdes as described for the Fn mq_receive system call. However, if O_NONBLOCK was not specified when the message queue was opened via the Fn mq_open system 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_NONBLOCK is set, this system call is equivalent to Fn mq_receive .
The timeout expires when the absolute time specified by Fa abs_timeout passes, as measured by the clock on which timeouts are based (that is, when the value of that clock equals or exceeds Fa abs_timeout ) , or if the absolute time specified by Fa abs_timeout has already been passed at the time of the call.
The timeout is based on the CLOCK_REALTIME clock.
RETURN VALUESUpon successful completion, the Fn mq_receive and Fn mq_timedreceive system 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.
ERRORSThe Fn mq_receive and Fn mq_timedreceive system calls will fail if:
- Bq Er EAGAIN
- O_NONBLOCK flag is set in the message queue description associated with Fa mqdes , and the specified message queue is empty.
- Bq Er EBADF
- The Fa mqdes argument is not a valid message queue descriptor open for reading.
- Bq Er EMSGSIZE
- The specified message buffer size, Fa msg_len , is less than the message size attribute of the message queue.
- Bq Er EINTR
- The Fn mq_receive or Fn mq_timedreceive operation was interrupted by a signal.
- Bq Er EINVAL
- The process or thread would have blocked, and the Fa abs_timeout parameter specified a nanoseconds field value less than zero or greater than or equal to 1000 million.
- Bq Er ETIMEDOUT
- The O_NONBLOCK flag was not set when the message queue was opened, but no message arrived on the queue before the specified timeout expired.