.\"	$NetBSD: msgrcv.2,v 1.22 2013/07/24 11:54:04 skrll Exp $
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.\" Copyright (c) 1995 Frank van der Linden
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.Dd July 24, 2013
.Dt MSGRCV 2
.Os
.Sh NAME
.Nm msgrcv
.Nd receive a message from a message queue
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/msg.h
.Ft ssize_t
.Fn msgrcv "int msqid" "void *msgp" "size_t msgsz" "long msgtyp" "int msgflg"
.Sh DESCRIPTION
The
.Fn msgrcv
function receives a message from the message queue specified in
.Fa msqid ,
and places it into the user-defined structure pointed to by
.Fa msgp .
This structure must contain a first field of type
.Sy long
that will indicate the user-defined type of the message.
The remaining fields will contain the contents of the message.
The following is an example of what this user-defined structure might
look like:
.Bd -literal
struct mymsg {
    long mtype;    /* message type */
    char mtext[1]; /* body of message */
};
.Ed
.Pp
.Va mtype
is an integer greater than 0 that can be used to select messages.
.Va mtext
is an array of bytes, with size up to the system limit
.Dv MSGMAX .
.Pp
The value of
.Fa msgtyp
has one of the following meanings:
.Bl -bullet
.It
.Fa msgtyp
is greater than 0.
The first message of type
.Fa msgtyp
will be received.
.It
.Fa msgtyp
is equal to 0.
The first message on the queue will be received.
.It
.Fa msgtyp
is less than 0.
The first message of the lowest message type that is
less than or equal to the absolute value of
.Fa msgtyp
will be received.
.El
.Pp
The argument
.Fa msgsz
specifies the size in bytes of
.Va mtext .
If the received message has a length greater than
.Fa msgsz
it will be silently truncated if the
.Dv MSG_NOERROR
flag is set in
.Fa msgflg ,
otherwise an error will be returned.
.Pp
If no matching message is present on the message queue specified by
.Fa msqid ,
the behaviour of
.Fn msgrcv
depends on whether the
.Dv IPC_NOWAIT
flag is set in
.Fa msgflg
or not.
If
.Dv IPC_NOWAIT
is set, then
.Fn msgrcv
will immediately return a value of \-1 and set
.Va errno
to
.Er EAGAIN .
If
.Dv IPC_NOWAIT
is not set, the calling process will block until:
.Bl -bullet
.It
A message of the requested type becomes available on the message queue.
.It
The message queue is removed, in which case \-1 will be returned and
.Va errno
set to
.Er EIDRM .
.It
A signal is received and caught.
\-1 is returned and
.Va errno
is set to
.Er EINTR .
.El
.Pp
If a message is successfully received, the data structure associated with
.Fa msqid
is updated as follows:
.Bl -bullet
.It
.Va msg_lrpid
is set to the pid of the caller.
.It
.Va msg_lrtime
is set to the current time.
.It
.Va msg_qnum
is decremented by 1.
.El
.Sh RETURN VALUES
Upon successful completion,
.Fn msgrcv
returns the number of bytes received and placed into the
.Va mtext
field of the structure pointed to by
.Fa msgp .
Otherwise, \-1 is returned, and
.Va errno
set to indicate the error.
.Sh ERRORS
.Fn msgrcv
will fail if:
.Bl -tag -width Er
.It Bq Er E2BIG
A matching message was received, but its size was greater than
.Fa msgsz
and the
.Dv MSG_NOERROR
flag was not set in
.Fa msgflg .
.It Bq Er EACCES
The calling process does not have read access to the message queue.
.It Bq Er EAGAIN
There is no message of the requested type available on the message queue,
and
.Dv IPC_NOWAIT
is set in
.Fa msgflg .
.It Bq Er EFAULT
.Fa msgp
points to an invalid address.
.It Bq Er EIDRM
The message queue identifier
.Fa msqid
is removed from the system.
.It Bq Er EINTR
The system call was interrupted by the delivery of a signal.
.It Bq Er EINVAL
.Fa msqid
is not a valid message queue identifier
.Pp
The message queue was removed while
.Fn msgrcv
was waiting for a message of the requested type to become available in it.
.Pp
.Fa msgsz
is greater than
.Dv SSIZE_MAX .
.It Bq Er ENOMSG
The queue does not contain a message of the desired type and
.Dv IPC_NOWAIT
is set.
.El
.Sh SEE ALSO
.Xr msgctl 2 ,
.Xr msgget 2 ,
.Xr msgsnd 2
.Sh STANDARDS
The
.Nm
system call conforms to
.St -xsh5 .
.Sh HISTORY
Message queues appeared in the first release of
.At V .