+++ /dev/null
-.\" Copyright 1993 Giorgio Ciucci <giorgio@crcc.it>
-.\"
-.\" %%%LICENSE_START(VERBATIM)
-.\" Permission is granted to make and distribute verbatim copies of this
-.\" manual provided the copyright notice and this permission notice are
-.\" preserved on all copies.
-.\"
-.\" Permission is granted to copy and distribute modified versions of this
-.\" manual under the conditions for verbatim copying, provided that the
-.\" entire resulting derived work is distributed under the terms of a
-.\" permission notice identical to this one.
-.\"
-.\" Since the Linux kernel and libraries are constantly changing, this
-.\" manual page may be incorrect or out-of-date. The author(s) assume no
-.\" responsibility for errors or omissions, or for damages resulting from
-.\" the use of the information contained herein. The author(s) may not
-.\" have taken the same level of care in the production of this manual,
-.\" which is licensed free of charge, as they might when working
-.\" professionally.
-.\"
-.\" Formatted or processed versions of this manual, if unaccompanied by
-.\" the source, must acknowledge the copyright and authors of this work.
-.\" %%%LICENSE_END
-.\"
-.\" 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 2014-08-19 "Linux" "Linux Programmer's Manual"
-.SH NAME
-msgrcv, msgsnd \- System V message queue 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 System\ V 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 a 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 governed 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).
-A message queue is considered to be full if either of the following
-conditions is true:
-.IP * 2
-Adding a new message to the queue would cause the total number of bytes
-in the queue to exceed the queue's maximum size (the
-.I msg_qbytes
-field).
-.IP *
-Adding another message to the queue would cause the total number of messages
-in the queue to exceed the queue's maximum size (the
-.I msg_qbytes
-field).
-This check is necessary to prevent an unlimited number of zero-length
-messages being placed on the queue.
-Although such messages contain no data,
-they nevertheless consume (locked) kernel memory.
-.PP
-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
-Unless
-.B MSG_COPY
-is specified in
-.IR msgflg
-(see below),
-the
-.I msgtyp
-argument 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
-.BR MSG_COPY " (since Linux 3.8)"
-.\" commit 4a674f34ba04a002244edaf891b5da7fc1473ae8
-Nondestructively fetch a copy of the message at the ordinal position
-in the queue specified by
-.I msgtyp
-(messages are considered to be numbered starting at 0).
-
-This flag must be specified in conjunction with
-.BR IPC_NOWAIT ,
-with the result that, if there is no message available at the given position,
-the call fails immediately with the error
-.BR ENOMSG .
-Because they alter the meaning of
-.I msgtyp
-in orthogonal ways,
-.BR MSG_COPY
-and
-.BR MSG_EXCEPT
-may not both be specified in
-.IR msgflg .
-
-The
-.BR MSG_COPY
-flag was added for the implementation of
-the kernel checkpoint-restore facility and
-is available only if the kernel was built with the
-.B CONFIG_CHECKPOINT_RESTORE
-option.
-.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
-.BR EINVAL " (since Linux 3.14)"
-.I msgflg
-specified
-.BR MSG_COPY ,
-but not
-.BR IPC_NOWAIT .
-.TP
-.BR EINVAL " (since Linux 3.14)"
-.I msgflg
-specified both
-.BR MSG_COPY
-and
-.BR MSG_EXCEPT .
-.TP
-.B ENOMSG
-.B IPC_NOWAIT
-was specified in
-.I msgflg
-and no message of the requested type existed on the message queue.
-.TP
-.B ENOMSG
-.B IPC_NOWAIT
-and
-.B MSG_COPY
-were specified in
-.I msgflg
-and the queue contains less than
-.I msgtyp
-messages.
-.TP
-.BR ENOSYS " (since Linux 3.8)"
-.I MSG_COPY
-was specified in
-.IR msgflg ,
-and this kernel was configured without
-.BR CONFIG_CHECKPOINT_RESTORE .
-.SH CONFORMING TO
-SVr4, POSIX.1-2001.
-
-The
-.B MSG_EXCEPT
-and
-.B MSG_COPY
-flags are Linux-specific;
-their definitions can be obtained by defining the
-.B _GNU_SOURCE
-.\" MSG_COPY since glibc 2.18
-feature test macro.
-.SH NOTES
-The inclusion of
-.I <sys/types.h>
-and
-.I <sys/ipc.h>
-isn't required on Linux or by any version of POSIX.
-However,
-some old implementations required the inclusion of these header files,
-and the SVID also documented their inclusion.
-Applications intended to be portable to such old systems may need
-to include these header files.
-.\" Like Linux, the FreeBSD man pages still document
-.\" the inclusion of these header files.
-
-The
-.I msgp
-argument is declared as \fIstruct msgbuf\ *\fP in
-glibc 2.0 and 2.1.
-It is declared as \fIvoid\ *\fP
-in 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 ).
-A privileged process
-(Linux: a process with the
-.B CAP_SYS_RESOURCE
-capability)
-can increase the size of a message queue beyond
-.B MSGMNB
-by a
-.BR msgctl (2)
-system call.
-.PP
-The implementation has no intrinsic system-wide limits on the
-number of message headers
-.RB ( MSGTQL )
-and the number of bytes in the message pool
-.RB ( MSGPOOL ).
-.SH BUGS
-In Linux 3.13 and earlier,
-if
-.BR msgrcv ()
-was called with the
-.BR MSG_COPY
-flag, but without
-.BR IPC_NOWAIT ,
-and the message queue contained less than
-.I msgtyp
-messages, then the call would block until the next message is written
-to the queue.
-.\" http://marc.info/?l=linux-kernel&m=139048542803605&w=2
-At that point, the call would return a copy of the message,
-.I regardless
-of whether that message was at the ordinal position
-.IR msgtyp .
-This bug is fixed
-.\" commit 4f87dac386cc43d5525da7a939d4b4e7edbea22c
-in Linux 3.14.
-
-Specifying both
-.B MSG_COPY
-and
-.B MSC_EXCEPT
-in
-.I msgflg
-is a logical error (since these flags impose different interpretations on
-.IR msgtyp ).
-In Linux 3.13 and earlier,
-.\" http://marc.info/?l=linux-kernel&m=139048542803605&w=2
-this error was not diagnosed by
-.BR msgrcv ().
-This bug is fixed
-.\" commit 4f87dac386cc43d5525da7a939d4b4e7edbea22c
-in Linux 3.14.
-.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 3.79 of the Linux
-.I man-pages
-project.
-A description of the project,
-information about reporting bugs,
-and the latest version of this page,
-can be found at
-\%http://www.kernel.org/doc/man\-pages/.
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