man-pages/man2/msgop.2

.\" 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>
.\" Modified Mon Jul 10 21:09:59 2000 by aeb
.\" Modified 1 Jun 2002, Michael Kerrisk <mtk-manpages@gmx.net>
.\"	Language clean-ups.
.\"	Enhanced and corrected information on msg_qbytes, MSGMNB and MSGMAX
.\"	Added note on restart behaviour of msgsnd and msgrcv
.\"	Formatting clean-ups (argument and field names marked as .I 
.\"		instead of .B)
.\" Modified, 27 May 2004, Michael Kerrisk <mtk-manpages@gmx.net>
.\"     Added notes on capability requirements
.\"
.TH MSGOP 2 2004-05-27 "Linux 2.6.6" "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 "size_t " msgsz ,
.BI "int " msgflg );
.sp
.BI "ssize_t msgrcv(int " msqid ,
.BI "struct msgbuf *" msgp ,
.BI "size_t " msgsz ,
.BI "long " msgtyp ,
.BI "int " msgflg );
.SH DESCRIPTION
To send or receive a message, the calling process allocates a structure
of the following general form:
.sp
.B
	struct msgbuf {
.br
.B
		long	mtype;	
/* message type, must be > 0 */
.br
.B
		char	mtext[1];	
/* message data */
.br
.B
	};
.sp
The
.I mtext
field is an array (or other structure) whose size is specified by
.IR msgsz ,
a non-negative integer value.
Messages of zero length (i.e., no
.I mtext
field) are permitted.
The
.I mtype
field 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 permission to send
and read permission to receive a message on the queue.
.PP
The
.B msgsnd
system call appends a copy of the message pointed to by
.I msgp
to the message queue whose identifier is specified
by
.IR msqid .
.PP
If sufficient space is available on the queue,
.B msgsnd
succeeds immediately.
(The queue capacity is defined by the
.I msg_bytes
field in the associated data structure for the message queue.
During queue creation this field is initialised to
.B MSGMNB
bytes, but this limit can be modified using
.BR msgctl .)
If insufficient space is available on the queue, then the default
behaviour of 
.B msgsnd
is to block until space becomes available.
If
.B IPC_NOWAIT
is asserted in
.B msgflg
then the call instead fails with the error
.BR EAGAIN .

A blocked 
.B msgsnd
call may also fail if the queue is removed 
(in which case the system call fails with
.I errno
set to
.BR EIDRM ),
or a signal is caught (in which case the system call fails
with
.I errno
set to
.BR EINTR ).
.RB ( msgsnd " and " msgrcv
are never automatically restarted after being interrupted by a 
signal handler, regardless of the  setting  of the
.B SA_RESTART
flag when establishing a signal handler.)
.PP
Upon successful completion the message queue data structure is updated
as follows:
.IP
.I msg_lspid
is set to the process ID of the calling process.
.IP
.I msg_qnum
is incremented by 1.
.IP
.I 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
.I msgbuf
pointed to by the
.I msgp
argument, removing the read message from the queue.
.PP
The argument
.I msgsz
specifies the maximum size in bytes for the member
.I 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
.I 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 first message in the queue is read.
.IP
If
.I msgtyp
is greater than
.BR 0 ,
then the first message on the queue of type
.I msgtyp
is read, unless
.B MSG_EXCEPT
was asserted in
.IR msgflg ,
in which case
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) of 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 this case the system call fails with
.I errno
set to
.BR EIDRM .
.IP
The calling process catches a signal.
In this case the system call fails with
.I errno
set to
.BR EINTR .
.PP
Upon successful completion the message queue data structure is updated
as follows:
.IP
.I msg_lrpid
is set to the process ID of the calling process.
.IP
.I msg_qnum
is decremented by 1.
.IP
.I msg_rtime
is set to the current time.
.SH "RETURN VALUE"
On a failure both functions return
.B \-1
with
.I errno
indicating the error,
otherwise
.B msgsnd
returns
.B 0
and
.B msgrvc
returns the number of bytes actually copied into the
.I mtext
array.
.SH ERRORS
When
.B msgsnd
fails, at return
.I errno
will be set to one among the following values:
.TP
.B EACCES
The calling process does not have write permission on the message queue,
and does not have the
.BR CAP_IPC_OWNER
capability.
.TP 11
.B EAGAIN
The message can't be sent due to the
.I msg_qbytes
limit for the queue and
.B IPC_NOWAIT
was asserted in
.IR mgsflg .
.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 caught a signal.
.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
.IR msgbuf .
.PP
When
.B msgrcv
fails, at return
.I 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 does not have read permission on the message queue,
and does not have the
.BR CAP_IPC_OWNER
capability.
.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
8192 bytes.
.TP
.B MSGMNB
Default maximum size in bytes of a message queue: 16384 bytes.
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 NOTE
The pointer argument is declared as \fIstruct msgbuf *\fP with
libc4, libc5, glibc 2.0, glibc 2.1. It is declared as \fIvoid *\fP
(\fIconst void *\fP for \fImsgsnd()\fP) with glibc 2.2, following the SUSv2.
.SH "SEE ALSO"
.BR msgctl (2),
.BR msgget (2),
.BR msgrcv (2),
.BR msgsnd (2),
.BR ipc (5),
.BR capabilities (7)