freebsd-skq/lib/libc/sys/mq_receive.2

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.Dd November 29, 2005
.Dt MQ_SEND 2
.Os
.Sh NAME
.Nm mq_receive , mq_timedreceive
.Nd "receive a message from message queue (REALTIME)"
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In 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_timereceive
.Fa "mqd_t mqdes"
.Fa "char *msg_ptr"
.Fa "size_t msg_len"
.Fa "unsigned *msg_prio"
.Fa "const struct timespec *abs_timeout"
.Fc
.Sh DESCRIPTION
The
.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
.Fa 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.
.Pp
If the value of 
.Fa msg_len
is greater than
.Brq Dv SSIZE_MAX ,
the result is implementation-defined.
.Pp
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
.Dv 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
.Dv O_NONBLOCK
is set in the message queue description associated with mqdes, no message
will be removed from the queue, and
.Fn mq_receive
will return an error.
.Pp
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
.Dv 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
.Dv O_NONBLOCK
is set, this system call is equivalent to
.Fn mq_receive .
.Pp
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.
.Pp
The timeout is based on the CLOCK_REALTIME clock.
.Pp
.Sh RETURN VALUES
Upon 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 return a value of -1, and set
.Va errno
to indicate the error.
.Sh ERRORS
The
.Fn mq_receive
and
.Fn mq_timedreceive
system calls
will fail if:
.Bl -tag -width Er
.It Bq Er EAGAIN
.Dv O_NONBLOCK
flag is set in the message queue description associated with
.Fa mqdes ,
and the specified message queue is empty.
.It Bq Er EBADF
The
.Fa mqdes
argument is not a valid message queue descriptor open for reading.
.It Bq Er EMSGSIZE
The specified message buffer size,
.Fa msg_len ,
is less than the message size attribute of the message queue.
.It Bq Er EINTR
The
.Fn mq_receive
or
.Fn mq_timedreceive
operation was interrupted by a signal.
.It 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.
.It Bq Er ETIMEDOUT
The
.Dv O_NONBLOCK
flag was not set when the message queue was opened, but no message arrived
on the queue before the specified timeout expired.
.El
.Sh SEE ALSO
.Xr mq_open 2 ,
.Xr mq_send 2 ,
.Xr mq_timesend 2
.Sh STANDARDS
The
.Fn mq_receive
and
.Fn mq_timereceive
system calls conform to
.St -p1003.1-2004 .
.Sh HISTORY
Support for POSIX message queue first appeared in
.Fx 7.0 .