mq_receive , mq_timedreceive
- receive a message from message queue (REALTIME)
LIBRARY
Lb librt
SYNOPSIS
#include <mqueue.h> 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
DESCRIPTION
The
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 ,
mq_receive ();
will block until a message is enqueued on the message queue or until
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
mq_receive ();
will return an error.
The
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
mq_receive ();
system call.
However, if
O_NONBLOCK
was not specified when the message queue was opened via the
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
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 VALUES
Upon successful completion, the
mq_receive ();
and
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.
ERRORS
The
mq_receive ();
and
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
mq_receive ();
or
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.
SEE ALSO
mq_open2,
mq_send2,
mq_timedsend2
STANDARDS
The
mq_receive ();
and
mq_timedreceive ();
system calls conform to
St -p1003.1-2004 .
HISTORY
Support for
POSIX
message queues first appeared in
Fx 7.0 .
COPYRIGHT
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.
