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

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.Dd November 29, 2005
.Dt MQ_SEND 2
.Os
.Sh NAME
.Nm mq_send , mq_timedsend
.Nd "send a message to message queue (REALTIME)"
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In mqueue.h
.Ft int
.Fo mq_send
.Fa "mqd_t mqdes"
.Fa "const char *msg_ptr"
.Fa "size_t msg_len"
.Fa "unsigned msg_prio"
.Fc
.Ft int
.Fo mq_timedsend
.Fa "mqd_t mqdes"
.Fa "const char *msg_ptr"
.Fa "size_t msg_len"
.Fa "unsigned msg_prio"
.Fa "const struct timespec *abs_timeout"
.Fc
.Sh DESCRIPTION
The
.Fn mq_send
system call adds the message pointed to by the argument
.Fa msg_ptr
to the message queue specified by
.Fa mqdes .
The
.Fa msg_len
argument specifies the length of the message, in bytes, pointed to by
.Fa msg_ptr .
The value of
.Fa msg_len
should be less than or equal to the
.Va mq_msgsize
attribute of the message queue, or
.Fn mq_send
will fail.
.Pp
If the specified message queue is not full,
.Fn mq_send
will behave as if the message is inserted into the message queue at
the position indicated by the
.Fa msg_prio
argument.
A message with a larger numeric value of
.Fa msg_prio
will be inserted before messages with lower values of
.Fa msg_prio .
A message will be inserted after other messages in the queue, if any,
with equal
.Fa msg_prio .
The value of
.Fa msg_prio
should be less than
.Brq Dv MQ_PRIO_MAX .
.Pp
If the specified message queue is full and
.Dv O_NONBLOCK
is not set in the message queue description associated with
.Fa mqdes ,
.Fn mq_send
will block until space becomes available to enqueue the
message, or until
.Fn mq_send
is interrupted by a signal.
If more than one thread is
waiting to send when space becomes available in the message queue and
the Priority Scheduling option is supported, then the thread of the
highest priority that has been waiting the longest will be unblocked
to send its message.
Otherwise, it is unspecified which waiting thread
is unblocked.
If the specified message queue is full and
.Dv O_NONBLOCK
is set in the message queue description associated with
.Fa mqdes ,
the message will not be queued and
.Fn mq_send
will return an error.
.Pp
The
.Fn mq_timedsend
system call will add a message to the message queue specified by
.Fa mqdes
in the manner defined for the
.Fn mq_send
system call.
However, if the specified message queue is full and
.Dv O_NONBLOCK
is not set in the message queue description associated with
.Fa mqdes ,
the wait for sufficient room in the queue will be terminated when
the specified timeout expires.
If
.Dv O_NONBLOCK
is set in the message queue description, this system call is
equivalent to
.Fn mq_send .
.Pp
The timeout will expire 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
.Dv CLOCK_REALTIME
clock.
.Sh RETURN VALUES
Upon successful completion, the
.Fn mq_send
and
.Fn mq_timedsend
system calls return a value of zero.
Otherwise, no message will be
enqueued, the system calls return \-1, and
the global variable
.Va errno
is set to indicate the error.
.Sh ERRORS
The
.Fn mq_send
and
.Fn mq_timedsend
system calls
will fail if:
.Bl -tag -width Er
.It Bq Er EAGAIN
The
.Dv O_NONBLOCK
flag is set in the message queue description associated with
.Fa mqdes ,
and the specified message queue is full.
.It Bq Er EBADF
The
.Fa mqdes
argument is not a valid message queue descriptor open for writing.
.It Bq Er EINTR
A signal interrupted the call to
.Fn mq_send
or
.Fn mq_timedsend .
.It Bq Er EINVAL
The value of
.Fa msg_prio
was outside the valid range.
.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 EMSGSIZE
The specified message length,
.Fa msg_len ,
exceeds the message size attribute of the message queue.
.It Bq Er ETIMEDOUT
The
.Dv O_NONBLOCK
flag was not set when the message queue was opened, but the timeout
expired before the message could be added to the queue.
.El
.Sh SEE ALSO
.Xr mq_open 2 ,
.Xr mq_receive 2 ,
.Xr mq_setattr 2 ,
.Xr mq_timedreceive 2
.Sh STANDARDS
The
.Fn mq_send
and
.Fn mq_timedsend
system calls conform to
.St -p1003.1-2004 .
.Sh HISTORY
Support for
.Tn POSIX
message queues first appeared in
.Fx 7.0 .