.\" 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@gmail.com>
.\"	Language clean-ups.
.\"	Enhanced and corrected information on msg_qbytes, MSGMNB and MSGMAX
.\"	Added note on restart behavior 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@gmail.com>
.\"     Added notes on capability requirements
.\" Modified, 11 Nov 2004, Michael Kerrisk <mtk.manpages@gmail.com>
.\"	Language and formatting clean-ups
.\"	Added notes on /proc files
.\" FIXME . Add example programs to this page.
.\"
.TH MSGOP 2 2008-04-23 "Linux" "Linux Programmer's Manual"
.SH NAME
msgrcv, msgsnd \- message operations
.SH SYNOPSIS
.nf
.B #include <sys/types.h>
.B #include <sys/ipc.h>
.B #include <sys/msg.h>
.sp
.BI "int msgsnd(int " msqid ", const void *" msgp ", size_t " msgsz \
", int " msgflg );
.sp
.BI "ssize_t msgrcv(int " msqid ", void *" msgp ", size_t " msgsz \
", long " msgtyp ,
.BI "               int " msgflg );
.fi
.SH DESCRIPTION
The
.BR msgsnd ()
and
.BR msgrcv ()
system calls are used, respectively, to send messages to,
and receive messages from, a message queue.
The calling process must have write permission on the message queue
in order to send a message, and read permission to receive a message.
.PP
The
.I msgp
argument is a pointer to caller-defined structure
of the following general form:
.in +4n
.nf

struct msgbuf {
    long mtype;       /* message type, must be > 0 */
    char mtext[1];    /* message data */
};
.fi
.in
.PP
The
.I mtext
field is an array (or other structure) whose size is specified by
.IR msgsz ,
a nonnegative 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.
This value can be
used by the receiving process for message selection
(see the description of
.BR msgrcv ()
below).
.SS msgsnd()
The
.BR 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 in the queue,
.BR msgsnd ()
succeeds immediately.
(The queue capacity is defined by the
.I msg_qbytes
field in the associated data structure for the message queue.
During queue creation this field is initialized to
.B MSGMNB
bytes, but this limit can be modified using
.BR msgctl (2).)
If insufficient space is available in the queue, then the default
behavior of
.BR msgsnd ()
is to block until space becomes available.
If
.B IPC_NOWAIT
is specified in
.IR msgflg ,
then the call instead fails with the error
.BR EAGAIN .

A blocked
.BR msgsnd ()
call may also fail if:
.IP * 2
the queue is removed,
in which case the system call fails with
.I errno
set to
.BR EIDRM ;
or
.IP *
a signal is caught, in which case the system call fails
with
.I errno
set to
.BR EINTR ; see
.BR signal (7).
.RB ( msgsnd ()
is 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.
.SS msgrcv()
The
.BR msgrcv ()
system call removes a message from the queue specified by
.I msqid
and places it in the buffer
pointed to by
.IR msgp .
.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 the behavior depends on whether
.B MSG_NOERROR
is specified in
.IR msgflg .
If
.B MSG_NOERROR
is specified, then
the message text will be truncated (and the truncated part will be
lost); if
.B MSG_NOERROR
is not specified, then
the message isn't removed from the queue and
the system call fails returning \-1 with
.I errno
set to
.BR E2BIG .
.PP
The argument
.I msgtyp
specifies the type of message requested as follows:
.IP * 2
If
.I msgtyp
is 0,
then the first message in the queue is read.
.IP *
If
.I msgtyp
is greater than 0,
then the first message in the queue of type
.I msgtyp
is read, unless
.B MSG_EXCEPT
was specified in
.IR msgflg ,
in which case
the first message in the queue of type not equal to
.I msgtyp
will be read.
.IP *
If
.I msgtyp
is less than 0,
then the first message in 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 is a bit mask constructed by ORing together zero or more
of the following flags:
.TP
.B IPC_NOWAIT
Return immediately if no message of the requested type is in the queue.
The system call fails with
.I errno
set to
.BR ENOMSG .
.TP
.B MSG_EXCEPT
Used with
.I msgtyp
greater than 0
to read the first message in the queue with message type that differs
from
.IR msgtyp .
.TP
.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 specified in
.IR msgflg ,
the calling process is blocked until one of the following conditions occurs:
.IP * 2
A message of the desired type is placed in 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 .
.RB ( msgrcv ()
is 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_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 failure both functions return \-1
with
.I errno
indicating the error,
otherwise
.BR msgsnd ()
returns 0
and
.BR msgrcv ()
returns the number of bytes actually copied into the
.I mtext
array.
.SH ERRORS
When
.BR msgsnd ()
fails,
.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
.B CAP_IPC_OWNER
capability.
.TP
.B EAGAIN
The message can't be sent due to the
.I msg_qbytes
limit for the queue and
.B IPC_NOWAIT
was specified in
.IR msgflg .
.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 does not have enough memory to make a copy of the
message pointed to by
.IR msgp .
.PP
When
.BR msgrcv ()
fails,
.I errno
will be set to one among the following values:
.TP
.B E2BIG
The message text length is greater than
.I msgsz
and
.B MSG_NOERROR
isn't specified in
.IR msgflg .
.TP
.B EACCES
The calling process does not have read permission on the message queue,
and does not have the
.B CAP_IPC_OWNER
capability.
.TP
.B EAGAIN
No message was available in the queue and
.B IPC_NOWAIT
was specified in
.IR msgflg .
.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 caught a signal; see
.BR signal (7).
.TP
.B EINVAL
.I msgqid
was invalid, or
.I msgsz
was less than 0.
.TP
.B ENOMSG
.B IPC_NOWAIT
was specified in
.I msgflg
and no message of the requested type existed on the message queue.
.SH "CONFORMING TO"
SVr4, POSIX.1-2001.
.SH NOTES
The
.I msgp
argument is declared as \fIstruct msgbuf *\fP with
libc4, libc5, glibc 2.0, glibc 2.1.
It is declared as \fIvoid *\fP
with glibc 2.2 and later, as required by SUSv2 and SUSv3.

The following limits on message queue resources affect the
.BR msgsnd ()
call:
.TP
.B MSGMAX
Maximum size for a message text: 8192 bytes
(on Linux, this limit can be read and modified via
.IR /proc/sys/kernel/msgmax ).
.TP
.B MSGMNB
Default maximum size in bytes of a message queue: 16384 bytes
(on Linux, this limit can be read and modified via
.IR /proc/sys/kernel/msgmnb ).
The superuser can increase the size of a message queue beyond
.B MSGMNB
by a
.BR msgctl (2)
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 "SEE ALSO"
.BR msgctl (2),
.BR msgget (2),
.BR capabilities (7),
.BR mq_overview (7),
.BR svipc (7)
.SH COLOPHON
This page is part of release 3.27 of the Linux
.I man-pages
project.
A description of the project,
and information about reporting bugs,
can be found at
http://www.kernel.org/doc/man-pages/.