mirror of https://github.com/mkerrisk/man-pages
147 lines
3.8 KiB
Groff
147 lines
3.8 KiB
Groff
'\" t
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.\" Hey Emacs! This file is -*- nroff -*- source.
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.\" Copyright (C) 2006 Michael Kerrisk <mtk.manpages@gmail.com>
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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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.TH MQ_GETATTR 3 2006-02-25 "Linux" "Linux Programmer's Manual"
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.SH NAME
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mq_getattr, mq_setattr \- get/set message queue attributes
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.SH SYNOPSIS
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.nf
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.B #include <mqueue.h>
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.sp
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.BI "mqd_t mq_getattr(mqd_t " mqdes ", struct mq_attr *" attr );
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.BI "mqd_t mq_setattr(mqd_t " mqdes ", struct mq_attr *" newattr ","
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.BI " struct mq_attr *" oldattr );
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.fi
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.sp
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Link with \fI\-lrt\fP.
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.SH DESCRIPTION
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.BR mq_getattr ()
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and
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.BR mq_setattr ()
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respectively retrieve and modify attributes of the message queue
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referred to by the descriptor
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.IR mqdes .
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.BR mq_getattr ()
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returns an
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.I mq_attr
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structure in the buffer pointed by
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.IR attr .
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This structure is defined as:
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.in +4n
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.nf
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struct mq_attr {
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long mq_flags; /* Flags: 0 or O_NONBLOCK */
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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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};
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.fi
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.in
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.PP
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The
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.I mq_flags
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field contains flags associated with the open message queue description.
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This field is initialized when the queue is created by
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.BR mq_open (3).
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The only flag that can appear in this field is
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.BR O_NONBLOCK .
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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 set when the message queue is created by
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.BR mq_open (3).
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The
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.I mq_maxmsg
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field is an upper limit on the number of messages
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that may be placed on the queue using
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.BR mq_send (3).
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The
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.I mq_msgsize
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field is an upper limit on the size of messages
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that may be placed on the queue.
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Both of these fields must have a value greater than zero.
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Two
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.I /proc
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files that place ceilings on the values for these fields are described in
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.BR mq_open (3).
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The
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.I mq_curmsgs
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field returns the number of messages currently held in the queue.
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.BR mq_setattr ()
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sets message queue attributes using information supplied in the
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.I mq_attr
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structure pointed to by
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.IR newattr .
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The only attribute that can be modified is the setting of the
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.B O_NONBLOCK
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flag in
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.IR mq_flags .
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The other fields in
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.I newattr
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are ignored.
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If the
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.I oldattr
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field is not NULL,
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then the buffer that it points to is used to return an
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.I mq_attr
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structure that contains the same information that is returned by
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.BR mq_getattr ().
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.SH RETURN VALUE
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On success
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.BR mq_getattr ()
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and
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.BR mq_setattr ()
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return 0; on error, \-1 is returned, 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 EBADF
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The descriptor specified in
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.I mqdes
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is invalid.
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.TP
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.B EINVAL
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.I newattr\->mq_flags
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contained set bits other than
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.BR O_NONBLOCK .
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.SH CONFORMING TO
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POSIX.1-2001.
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.SH "SEE ALSO"
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.BR mq_close (3),
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.BR mq_notify (3),
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.BR mq_open (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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