libc/sys/msgrcv.2

.\"	$NetBSD: msgrcv.2,v 1.1 1995/10/16 23:49:20 jtc Exp $
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.\" Copyright (c) 1995 Frank van der Linden
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.\"      by Frank van der Linden
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.\" $FreeBSD: src/lib/libc/gen/msgrcv.3,v 1.8.2.7 2003/03/15 15:11:05 trhodes Exp $
.\"
.Dd January 4, 2014
.Dt MSGRCV 2
.Os
.Sh NAME
.Nm msgrcv
.Nd receive a message from a message queue
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/types.h
.In sys/ipc.h
.In sys/msg.h
.Ft int
.Fn msgrcv "int msqid" "void *msgp" "size_t msgsz" "long msgtyp" "int msgflg"
.Sh DESCRIPTION
The
.Fn msgrcv
system call receives a message from the message queue specified in
.Fa msqid ,
and places it into the structure pointed to by
.Fa msgp .
This structure should consist of the following members:
.Bd -literal
    long mtype;    /* message type */
    char mtext[1]; /* body of message */
.Ed
.Pp
.Va mtype
is an integer greater than 0 that can be used for selecting messages,
.Va mtext
is an array of bytes, with a size up to that of the system limit
.Pf ( Dv MSGMAX ) .
.Pp
The value of
.Fa msgtyp
has one of the following meanings:
.Bl -bullet
.It
The
.Fa msgtyp
argument
is greater than 0. The first message of type
.Fa msgtyp
will be received.
.It
The
.Fa msgtyp
argument
is equal to 0. The first message on the queue will be received.
.It
The
.Fa msgtyp
argument
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
.Fa msgsz
argument
specifies the maximum length of the requested message.
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 behavior of
.Fn msgrcv
depends on whether the
.Dv IPC_NOWAIT
flag is set in
.Fa msgflg
or not.
If
.Dv IPC_NOWAIT
is set,
.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 be blocked
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 EINVAL .
.It
A signal is received and caught. -1 is returned, and
.Va errno
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_cbytes
is decremented by the size of the message.
.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 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 ENVIRONMENT
The XSI Interprocess Communication family of functions is also available
as an implementation in userspace.
To use it, the
.Xr sysvipcd 8
daemon has to be running.
.Pp
If the
.Ev USR_SYSVIPC
variable is set in a process' environment, the process and its children
will use the userspace implementation.
.Sh ERRORS
The
.Fn msgrcv
system call will fail if:
.Bl -tag -width Er
.It Bq Er EINVAL
The
.Fa msqid
argument
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 on it.
.Pp
The
.Fa msgsz
argument
is less than 0.
.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 EFAULT
The
.Fa msgp
argument
points to an invalid address.
.It Bq Er EINTR
The system call was interrupted by the delivery of a signal.
.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 .
.El
.Sh SEE ALSO
.Xr msgctl 2 ,
.Xr msgget 2 ,
.Xr msgsnd 2
.Sh HISTORY
Message queues appeared in the first release of
.At V .
.Sh AUTHORS
.An -nosplit
The
.Dx
specific userspace implementation (see
.Sx ENVIRONMENT )
was written by
.An Larisa Grigore .
.Sh BUGS
.Nx ,
.Dx ,
and
.Fx
do not define the
.Er EIDRM
error value, which should be used in
the case of a removed message queue, nor the
.Er ENOMSG
value, which
should be used when no suitable message is available and
.Dv IPC_NOWAIT
is set.