mirror of https://github.com/mkerrisk/man-pages
315 lines
7.3 KiB
Groff
315 lines
7.3 KiB
Groff
'\" t
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.\" Copyright (C) 2006 Michael Kerrisk <mtk.manpages@gmail.com>
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.\"
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.\" %%%LICENSE_START(VERBATIM)
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of this
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.\" manual under the conditions for verbatim copying, provided that the
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.\" entire resulting derived work is distributed under the terms of a
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.\" permission notice identical to this one.
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.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume no
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.\" responsibility for errors or omissions, or for damages resulting from
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.\" the use of the information contained herein. The author(s) may not
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.\" have taken the same level of care in the production of this manual,
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.\" which is licensed free of charge, as they might when working
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.\" professionally.
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.\"
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and authors of this work.
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.\" %%%LICENSE_END
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.\"
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.TH MQ_OPEN 3 2016-12-12 "Linux" "Linux Programmer's Manual"
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.SH NAME
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mq_open \- open a message queue
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.SH SYNOPSIS
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.nf
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.BR "#include <fcntl.h>" " /* For O_* constants */"
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.BR "#include <sys/stat.h>" " /* For mode constants */"
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.B #include <mqueue.h>
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.PP
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.BI "mqd_t mq_open(const char *" name ", int " oflag );
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.BI "mqd_t mq_open(const char *" name ", int " oflag ", mode_t " mode ,
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.BI " struct mq_attr *" attr );
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.fi
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.PP
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Link with \fI\-lrt\fP.
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.SH DESCRIPTION
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.BR mq_open ()
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creates a new POSIX message queue or opens an existing queue.
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The queue is identified by
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.IR name .
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For details of the construction of
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.IR name ,
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see
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.BR mq_overview (7).
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.PP
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The
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.I oflag
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argument specifies flags that control the operation of the call.
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(Definitions of the flags values can be obtained by including
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.IR <fcntl.h> .)
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Exactly one of the following must be specified in
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.IR oflag :
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.TP
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.B O_RDONLY
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Open the queue to receive messages only.
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.TP
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.B O_WRONLY
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Open the queue to send messages only.
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.TP
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.B O_RDWR
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Open the queue to both send and receive messages.
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.PP
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Zero or more of the following flags can additionally be
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.IR OR ed
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in
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.IR oflag :
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.TP
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.BR O_CLOEXEC " (since Linux 2.6.26)"
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.\" commit 269f21344b23e552c21c9e2d7ca258479dcd7a0a
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Set the close-on-exec flag for the message queue descriptor.
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See
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.BR open (2)
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for a discussion of why this flag is useful.
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.TP
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.B O_CREAT
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Create the message queue if it does not exist.
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The owner (user ID) of the message queue is set to the effective
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user ID of the calling process.
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The group ownership (group ID) is set to the effective group ID
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of the calling process.
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.\" In reality the filesystem IDs are used on Linux.
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.TP
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.B O_EXCL
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If
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.B O_CREAT
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was specified in
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.IR oflag ,
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and a queue with the given
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.I name
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already exists, then fail with the error
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.BR EEXIST .
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.TP
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.B O_NONBLOCK
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Open the queue in nonblocking mode.
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In circumstances where
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.BR mq_receive (3)
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and
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.BR mq_send (3)
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would normally block, these functions instead fail with the error
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.BR EAGAIN .
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.PP
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If
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.B O_CREAT
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is specified in
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.IR oflag ,
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then two additional arguments must be supplied.
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The
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.I mode
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argument specifies the permissions to be placed on the new queue,
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as for
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.BR open (2).
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(Symbolic definitions for the permissions bits can be obtained by including
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.IR <sys/stat.h> .)
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The permissions settings are masked against the process umask.
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.PP
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The fields of the
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.IR "struct mq_attr"
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pointed to
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.I attr
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specify the maximum number of messages and
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the maximum size of messages that the queue will allow.
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This structure is defined as follows:
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.PP
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.PP
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.in +4n
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.EX
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struct mq_attr {
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long mq_flags; /* Flags (ignored for mq_open()) */
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long mq_maxmsg; /* Max. # of messages on queue */
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long mq_msgsize; /* Max. message size (bytes) */
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long mq_curmsgs; /* # of messages currently in queue
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(ignored for mq_open()) */
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};
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.EE
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.in
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.PP
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Only the
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.I mq_maxmsg
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and
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.I mq_msgsize
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fields are employed when calling
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.BR mq_open ();
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the values in the remaining fields are ignored.
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.PP
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If
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.I attr
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is NULL, then the queue is created with implementation-defined
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default attributes.
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Since Linux 3.5, two
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.I /proc
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files can be used to control these defaults; see
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.BR mq_overview (7)
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for details.
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.SH RETURN VALUE
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On success,
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.BR mq_open ()
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returns a message queue descriptor for use by other
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message queue functions.
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On error,
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.BR mq_open ()
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returns
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.IR "(mqd_t)\ \-1",
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with
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.I errno
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set to indicate the error.
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.SH ERRORS
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.TP
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.B EACCES
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The queue exists, but the caller does not have permission to
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open it in the specified mode.
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.TP
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.B EACCES
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.I name
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contained more than one slash.
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.\" Note that this isn't consistent with the same case for sem_open()
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.TP
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.B EEXIST
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Both
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.B O_CREAT
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and
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.B O_EXCL
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were specified in
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.IR oflag ,
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but a queue with this
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.I name
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already exists.
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.TP
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.B EINVAL
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.\" glibc checks whether the name starts with a "/" and if not,
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.\" gives this error
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.I name
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doesn't follow the format in
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.BR mq_overview (7).
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.TP
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.B EINVAL
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.B O_CREAT
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was specified in
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.IR oflag ,
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and
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.I attr
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was not NULL, but
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.I attr\->mq_maxmsg
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or
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.I attr\->mq_msqsize
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was invalid.
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Both of these fields must be greater than zero.
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In a process that is unprivileged (does not have the
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.B CAP_SYS_RESOURCE
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capability),
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.I attr\->mq_maxmsg
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must be less than or equal to the
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.I msg_max
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limit, and
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.I attr\->mq_msgsize
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must be less than or equal to the
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.I msgsize_max
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limit.
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In addition, even in a privileged process,
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.I attr\->mq_maxmsg
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cannot exceed the
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.B HARD_MAX
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limit.
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(See
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.BR mq_overview (7)
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for details of these limits.)
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.TP
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.B EMFILE
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The per-process limit on the number of open file
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and message queue descriptors has been reached
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(see the description of
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.BR RLIMIT_NOFILE
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in
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.BR getrlimit (2)).
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.TP
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.B ENAMETOOLONG
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.I name
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was too long.
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.TP
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.B ENFILE
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The system-wide limit on the total number of open files
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and message queues has been reached.
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.TP
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.B ENOENT
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The
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.B O_CREAT
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flag was not specified in
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.IR oflag ,
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and no queue with this
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.I name
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exists.
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.TP
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.B ENOENT
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.I name
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was just "/" followed by no other characters.
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.\" Note that this isn't consistent with the same case for sem_open()
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.TP
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.B ENOMEM
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Insufficient memory.
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.TP
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.B ENOSPC
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Insufficient space for the creation of a new message queue.
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This probably occurred because the
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.I queues_max
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limit was encountered; see
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.BR mq_overview (7).
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.SH ATTRIBUTES
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For an explanation of the terms used in this section, see
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.BR attributes (7).
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.TS
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allbox;
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lb lb lb
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l l l.
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Interface Attribute Value
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T{
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.BR mq_open ()
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T} Thread safety MT-Safe
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.TE
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.SH CONFORMING TO
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POSIX.1-2001, POSIX.1-2008.
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.SH NOTES
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.SS C library/kernel differences
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The
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.BR mq_open ()
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library function is implemented on top of a system call of the same name.
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The library function performs the check that the
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.I name
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starts with a slash (/), giving the
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.B EINVAL
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error if it does not.
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The kernel system call expects
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.I name
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to contain no preceding slash,
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so the C library function passes
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.I name
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without the preceding slash (i.e.,
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.IR name+1 )
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to the system call.
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.SH BUGS
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In kernels before 2.6.14,
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the process umask was not applied to the permissions specified in
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.IR mode .
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.SH SEE ALSO
|
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.BR mq_close (3),
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.BR mq_getattr (3),
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.BR mq_notify (3),
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.BR mq_receive (3),
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.BR mq_send (3),
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.BR mq_unlink (3),
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.BR mq_overview (7)
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