.\" Copyright 1993 Giorgio Ciucci <giorgio@crcc.it>
.\"
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.\" Modified Tue Oct 22 16:40:11 1996 by Eric S. Raymond <esr@thyrsus.com>
.TH MSGOP 2 "November 1, 1993" "Linux 0.99.13" "Linux Programmer's Manual" 
.SH NAME
msgop \- message operations
.SH SYNOPSIS
.nf
.B
# include <sys/types.h>
.br
.B
# include <sys/ipc.h>
.br
.B
# include <sys/msg.h>
.fi
.sp
.BI "int msgsnd ( int " msqid ,
.BI "struct msgbuf *" msgp ,
.BI "int " msgsz ,
.BI "int " msgflg " )"
.sp
.BI "int msgrcv ( int " msqid ,
.BI "struct msgbuf *" msgp ,
.BI "int " msgsz ,
.BI  "long " msgtyp ,
.BI "int " msgflg " )"
.SH DESCRIPTION
To send or receive a message, the calling process allocates a structure
that looks like the following
.sp
.B
	struct msgbuf {
.br
.B
		long	mtype;	
/* message type, must be > 0 */
.br
.B
		char	mtext[1];	
/* message data */
.br
.B
	};
.sp
but with an array
.B mtext
of size
.IR msgsz ,
a non-negative integer value.
The structure member
.B mtype
must have a strictly positive integer value that can be
used by the receiving process for message selection
(see the section about
.BR msgrcv ).
.PP
The calling process must have write access permissions to send
and read access permissions to receive a message on the queue.
.PP
The
.B msgsnd
system call enqueue a copy of the message pointed to by the
.I msgp
argument
on the message queue whose identifier is specified by the value
of the
.I msqid
argument.
.PP
The argument
.I msgflg
specifies the system call behaviour if enqueuing the new message
will require more than
.B msg_qbytes
in the queue.
Asserting
.B IPC_NOWAIT
the message will not be sent and the system call fails returning with
.B errno
set to
.BR EAGAIN .
Otherwise the process is suspended until the condition for the
suspension no longer exists (in which case the message is sent and the
system call succeeds),
or the queue is removed (in which case the system call fails
with
.B errno
set to
.BR EIDRM ),
or the process receives a signal that has to be
caught (in which case the system call fails
with
.B errno
set to
.BR EINTR ).
.PP
Upon successful completion the message queue data structure is updated
as follows:
.IP
.B msg_lspid
is set to the process-\ID of the calling process.
.IP
.B msg_qnum
is incremented by 1.
.IP
.B msg_stime
is set to the current time.
.PP
The system call
.B msgrcv
reads a message from the message queue specified by
.I msqid
into the
.B msgbuf
pointed to by the
.I msgp
argument removing from the queue, on success, the read message.
.PP
The argument
.I msgsz
specifies the maximum size in bytes for the member
.B mtext
of the structure pointed to by the
.I msgp
argument.
If the message text has length greater than
.IR msgsz ,
then if the
.I msgflg
argument asserts
.BR MSG_NOERROR ,
the message text will be truncated (and the truncated part will be
lost), otherwise the message isn't removed from the queue and
the system call fails returning with
.B errno
set to
.BR E2BIG .
.PP
The argument
.I msgtyp
specifies the type of message requested as follows:
.IP
If
.I msgtyp
is
.BR 0 ,
then the message on the queue's front is read.
.IP
If
.I msgtyp
is greater than
.BR 0 ,
then the first message on the queue of type
.I msgtyp
is read if
.B MSG_EXCEPT
isn't asserted by the
.I msgflg
argument, otherwise
the first message on the queue of type not equal to
.I msgtyp
will be read.
.IP
If
.I msgtyp
is less than
.BR 0 ,
then the first message on the queue with the lowest type less than or
equal to the absolute value of
.I msgtyp
will be read.
.PP
The
.I msgflg
argument asserts none, one or more (or\-ing them) among the following
flags:
.IP
.B IPC_NOWAIT
For immediate return if no message of the requested type is on the queue.
The system call fails with errno set to
.BR ENOMSG .
.IP
.B MSG_EXCEPT
Used with
.I msgtyp
greater than
.B 0
to read the first message on the queue with message type that differs
from
.IR msgtyp .
.IP
.B MSG_NOERROR
To truncate the message text if longer than
.I msgsz
bytes.
.PP
If no message of the requested type is available and
.B IPC_NOWAIT
isn't asserted in
.IR msgflg ,
the calling process is blocked until one of the following conditions occurs:
.IP
A message of the desired type is placed on the queue.
.IP
The message queue is removed from the system.
In such a case the system call fails with
.B errno
set to
.BR EIDRM .
.IP
The calling process receives a signal that has to be caught.
In such a case the system call fails with
.B errno
set to
.BR EINTR .
.PP
Upon successful completion the message queue data structure is updated
as follows:
.IP
.B msg_lrpid
is set to the process-\ID of the calling process.
.IP
.B msg_qnum
is decremented by 1.
.IP
.B msg_rtime
is set to the current time.
.SH "RETURN VALUE"
On a failure both functions return
.B \-1
with
.B errno
indicating the error,
otherwise
.B msgsnd
returns
.B 0
and
.B msgrvc
returns the number of bytes actually copied into the
.B mtext
array.
.SH ERRORS
When
.B msgsnd
fails, at return
.B errno
will be set to one among the following values:
.TP 11
.B EAGAIN
The message can't be sent due to the
.B msg_qbytes
limit for the queue and
.B IPC_NOWAIT
was asserted in
.IR mgsflg .
.TP
.B EACCES
The calling process has no write access permissions on the message queue.
.TP
.B EFAULT
The address pointed to by
.I msgp
isn't accessible.
.TP
.B EIDRM
The message queue was removed.
.TP
.B EINTR
Sleeping on a full message queue condition, the process received a signal
that had to be caught.
.TP
.B EINVAL
Invalid
.I msqid
value, or nonpositive
.I mtype
value, or
invalid
.I msgsz
value (less than 0 or greater than the system value
.BR MSGMAX ).
.TP
.B ENOMEM
The system has not enough memory to make a copy of the supplied
.BR msgbuf .
.PP
When
.B msgrcv
fails, at return
.B errno
will be set to one among the following values:
.TP 11
.B E2BIG
The message text length is greater than
.I msgsz
and
.B MSG_NOERROR
isn't asserted in
.IR msgflg .
.TP
.B EACCES
The calling process has no read access permissions on the message queue.
.TP
.B EFAULT
The address pointed to by
.I msgp
isn't accessible.
.TP
.B EIDRM
While the process was sleeping to receive a message,
the message queue was removed.
.TP
.B EINTR
While the process was sleeping to receive a message,
the process received a signal that had to be caught.
.TP
.B EINVAL
Illegal
.I msgqid
value, or
.I msgsz
less than
.BR 0 .
.TP
.B ENOMSG
.B IPC_NOWAIT
was asserted in
.I msgflg
and no message of the requested type existed on the message queue.
.SH NOTES
The followings are system limits affecting a
.B msgsnd
system call:
.TP 11
.B MSGMAX
Maximum size for a message text: the implementation set this value to
4080 bytes.
.TP
.B MSGMNB
Default maximum size in bytes of a message queue: policy dependent.
The super\-user can increase the size of a message queue beyond
.B MSGMNB
by a
.B msgctl
system call.
.PP
The implementation has no intrinsic limits for the system wide maximum
number of message headers
.RB ( MSGTQL )
and for the system wide maximum size in bytes of the message pool
.RB ( MSGPOOL ).
.SH "CONFORMING TO"
SVr4, SVID.
.SH "SEE ALSO"
.BR ipc (5),
.BR msgctl (2),
.BR msgget (2),
.BR msgrcv (2),
.BR msgsnd (2)