442 lines
10 KiB
Groff
442 lines
10 KiB
Groff
.\"
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.\" Copyright (c) 2003 Hiten Pandya <hmp@FreeBSD.org>
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.\" Copyright (c) 2009-2010 The FreeBSD Foundation
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.\" All rights reserved.
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.\"
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.\" Portions of this software were developed at the Centre for Advanced
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.\" Internet Architectures, Swinburne University of Technology, Melbourne,
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.\" Australia by Lawrence Stewart under sponsorship from the FreeBSD
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.\" Foundation.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions, and the following disclaimer,
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.\" without modification, immediately at the beginning of the file.
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.\" 2. The name of the author may not be used to endorse or promote products
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.\" derived from this software without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE FOR
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.\" ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.\"
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.Dd April 26, 2010
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.Dt ALQ 9
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.Os
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.Sh NAME
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.Nm alq ,
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.Nm alq_open_flags ,
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.Nm alq_open ,
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.Nm alq_writen ,
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.Nm alq_write ,
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.Nm alq_flush ,
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.Nm alq_close ,
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.Nm alq_getn ,
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.Nm alq_get ,
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.Nm alq_post_flags ,
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.Nm alq_post
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.Nd Asynchronous Logging Queues
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.Sh SYNOPSIS
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.In sys/alq.h
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.Ft int
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.Fo alq_open_flags
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.Fa "struct alq **app"
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.Fa "const char *file"
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.Fa "struct ucred *cred"
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.Fa "int cmode"
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.Fa "int size"
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.Fa "int flags"
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.Fc
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.Ft int
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.Fo alq_open
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.Fa "struct alq **app"
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.Fa "const char *file"
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.Fa "struct ucred *cred"
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.Fa "int cmode"
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.Fa "int size"
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.Fa "int count"
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.Fc
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.Ft int
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.Fn alq_writen "struct alq *alq" "void *data" "int len" "int flags"
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.Ft int
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.Fn alq_write "struct alq *alq" "void *data" "int flags"
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.Ft void
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.Fn alq_flush "struct alq *alq"
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.Ft void
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.Fn alq_close "struct alq *alq"
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.Ft struct ale *
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.Fn alq_getn "struct alq *alq" "int len" "int flags"
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.Ft struct ale *
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.Fn alq_get "struct alq *alq" "int flags"
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.Ft void
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.Fn alq_post_flags "struct alq *alq" "struct ale *ale" "int flags"
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.Ft void
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.Fn alq_post "struct alq *alq" "struct ale *ale"
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.Sh DESCRIPTION
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The
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.Nm
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facility provides an asynchronous fixed or variable length recording
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mechanism, known as Asynchronous Logging Queues.
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It can record to any
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.Xr vnode 9 ,
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thus providing the ability to journal logs to character
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devices as well as regular files.
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All functions accept a
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.Vt "struct alq"
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argument, which is an opaque type that maintains state information
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for an Asynchronous Logging Queue.
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The logging facility runs in a separate kernel thread, which services
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all log entry requests.
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.Pp
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An
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.Dq asynchronous log entry
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is defined as
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.Vt "struct ale" ,
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which has the following members:
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.Bd -literal -offset indent
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struct ale {
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intptr_t ae_bytesused; /* # bytes written to ALE. */
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char *ae_data; /* Write ptr. */
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int ae_pad; /* Unused, compat. */
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};
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.Ed
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.Pp
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An
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.Nm
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can be created in either fixed or variable length mode.
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A variable length
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.Nm
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accommodates writes of varying length using
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.Fn alq_writen
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and
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.Fn alq_getn .
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A fixed length
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.Nm
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accommodates a fixed number of writes using
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.Fn alq_write
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and
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.Fn alq_get ,
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each of fixed size (set at queue creation time).
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Fixed length mode is deprecated in favour of variable length mode.
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.Sh FUNCTIONS
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The
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.Fn alq_open_flags
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function creates a new variable length asynchronous logging queue.
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The
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.Fa file
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argument is the name of the file to open for logging.
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If the file does not yet exist,
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.Fn alq_open
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will attempt to create it.
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The
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.Fa cmode
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argument will be passed to
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.Fn vn_open
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as the requested creation mode, to be used if the file will be created by
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.Fn alq_open .
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Consumers of this API may wish to pass
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.Dv ALQ_DEFAULT_CMODE ,
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a default creation mode suitable for most applications.
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The
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.Fa cred
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argument specifies the credentials to use when opening and performing I/O on the file.
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The
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.Fa size
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argument sets the size (in bytes) of the underlying queue.
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The ALQ_ORDERED flag may be passed in via
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.Fa flags
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to indicate that the ordering of writer threads waiting for a busy
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.Nm
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to free up resources should be preserved.
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.Pp
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The deprecated
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.Fn alq_open
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function is implemented as a wrapper around
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.Fn alq_open_flags
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to provide backwards compatibility to consumers that have not been updated to
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utilise the newer
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.Fn alq_open_flags
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function.
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It passes all arguments through to
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.Fn alq_open_flags
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untouched except for
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.Fa size
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and
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.Fa count ,
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and sets
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.Fa flags
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to 0.
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To create a variable length mode
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.Nm ,
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the
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.Fa size
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argument should be set to the size (in bytes) of the underlying queue and the
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.Fa count
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argument should be set to 0.
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To create a fixed length mode
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.Nm ,
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the
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.Fa size
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argument should be set to the size (in bytes) of each write and the
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.Fa count
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argument should be set to the number of
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.Fa size
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byte chunks to reserve capacity for.
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.Pp
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The
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.Fn alq_writen
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function writes
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.Fa len
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bytes from
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.Fa data
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to the designated variable length mode queue
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.Fa alq .
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If
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.Fn alq_writen
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could not write the entry immediately and
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.Dv ALQ_WAITOK
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is set in
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.Fa flags ,
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the function will be allowed to
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.Xr msleep_spin 9
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with the
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.Dq Li alqwnord
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or
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.Dq Li alqwnres
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wait message.
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A write will automatically schedule the queue
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.Fa alq
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to be flushed to disk.
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This behaviour can be controlled by passing ALQ_NOACTIVATE via
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.Fa flags
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to indicate that the write should not schedule
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.Fa alq
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to be flushed to disk.
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.Pp
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The deprecated
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.Fn alq_write
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function is implemented as a wrapper around
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.Fn alq_writen
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to provide backwards compatibility to consumers that have not been updated to
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utilise variable length mode queues.
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The function will write
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.Fa size
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bytes of data (where
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.Fa size
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was specified at queue creation time) from the
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.Fa data
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buffer to the
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.Fa alq .
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Note that it is an error to call
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.Fn alq_write
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on a variable length mode queue.
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.Pp
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The
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.Fn alq_flush
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function is used for flushing
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.Fa alq
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to the log medium that was passed to
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.Fn alq_open .
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If
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.Fa alq
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has data to flush and is not already in the process of being flushed, the
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function will block doing IO.
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Otherwise, the function will return immediately.
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.Pp
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The
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.Fn alq_close
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function will close the asynchronous logging queue
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.Fa alq
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and flush all pending write requests to the log medium.
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It will free all resources that were previously allocated.
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.Pp
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The
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.Fn alq_getn
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function returns an asynchronous log entry from
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.Fa alq ,
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initialised to point at a buffer capable of receiving
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.Fa len
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bytes of data.
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This function leaves
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.Fa alq
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in a locked state, until a subsequent
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.Fn alq_post
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or
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.Fn alq_post_flags
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call is made.
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If
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.Fn alq_getn
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could not obtain
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.Fa len
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bytes of buffer immediately and
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.Dv ALQ_WAITOK
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is set in
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.Fa flags ,
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the function will be allowed to
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.Xr msleep_spin 9
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with the
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.Dq Li alqgnord
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or
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.Dq Li alqgnres
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wait message.
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The caller can choose to write less than
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.Fa len
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bytes of data to the returned asynchronous log entry by setting the entry's
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ae_bytesused field to the number of bytes actually written.
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This must be done prior to calling
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.Fn alq_post .
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.Pp
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The deprecated
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.Fn alq_get
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function is implemented as a wrapper around
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.Fn alq_getn
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to provide backwards compatibility to consumers that have not been updated to
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utilise variable length mode queues.
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The asynchronous log entry returned will be initialised to point at a buffer
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capable of receiving
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.Fa size
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bytes of data (where
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.Fa size
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was specified at queue creation time).
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Note that it is an error to call
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.Fn alq_get
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on a variable length mode queue.
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.Pp
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The
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.Fn alq_post_flags
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function schedules the asynchronous log entry
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.Fa ale
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(obtained from
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.Fn alq_getn
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or
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.Fn alq_get )
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for writing to
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.Fa alq .
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The ALQ_NOACTIVATE flag may be passed in via
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.Fa flags
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to indicate that the queue should not be immediately scheduled to be flushed to
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disk.
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This function leaves
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.Fa alq
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in an unlocked state.
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.Pp
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The
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.Fn alq_post
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function is implemented as a wrapper around
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.Fn alq_post_flags
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to provide backwards compatibility to consumers that have not been updated to
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utilise the newer
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.Fn alq_post_flags
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function.
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It simply passes all arguments through to
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.Fn alq_post_flags
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untouched, and sets
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.Fa flags
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to 0.
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.Sh IMPLEMENTATION NOTES
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The
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.Fn alq_writen
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and
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.Fn alq_write
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functions both perform a
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.Xr bcopy 3
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from the supplied
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.Fa data
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buffer into the underlying
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.Nm
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buffer.
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Performance critical code paths may wish to consider using
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.Fn alq_getn
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(variable length queues) or
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.Fn alq_get
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(fixed length queues) to avoid the extra memory copy. Note that a queue
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remains locked between calls to
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.Fn alq_getn
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or
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.Fn alq_get
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and
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.Fn alq_post
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or
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.Fn alq_post_flags ,
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so this method of writing to a queue is unsuitable for situations where the
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time between calls may be substantial.
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.Sh LOCKING
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Each asynchronous logging queue is protected by a spin mutex.
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.Pp
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Functions
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.Fn alq_flush
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and
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.Fn alq_open
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may attempt to acquire an internal sleep mutex, and should
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consequently not be used in contexts where sleeping is
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not allowed.
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.Sh RETURN VALUES
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The
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.Fn alq_open
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function returns one of the error codes listed in
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.Xr open 2 ,
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if it fails to open
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.Fa file ,
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or else it returns 0.
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.Pp
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The
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.Fn alq_writen
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and
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.Fn alq_write
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functions return
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.Er EWOULDBLOCK
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if
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.Dv ALQ_NOWAIT
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was set in
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.Fa flags
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and either the queue is full or the system is shutting down.
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.Pp
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The
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.Fn alq_getn
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and
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.Fn alq_get
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functions return
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.Dv NULL
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if
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.Dv ALQ_NOWAIT
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was set in
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.Fa flags
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and either the queue is full or the system is shutting down.
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.Pp
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NOTE: invalid arguments to non-void functions will result in
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undefined behaviour.
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.Sh SEE ALSO
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.Xr syslog 3 ,
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.Xr kproc 9 ,
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.Xr ktr 9 ,
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.Xr msleep_spin 9 ,
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.Xr vnode 9
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.Sh HISTORY
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The
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Asynchronous Logging Queues (ALQ) facility first appeared in
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.Fx 5.0 .
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.Sh AUTHORS
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.An -nosplit
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The
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.Nm
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facility was written by
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.An Jeffrey Roberson Aq Mt jeff@FreeBSD.org
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and extended by
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.An Lawrence Stewart Aq Mt lstewart@freebsd.org .
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.Pp
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This manual page was written by
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.An Hiten Pandya Aq Mt hmp@FreeBSD.org
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and revised by
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.An Lawrence Stewart Aq Mt lstewart@freebsd.org .
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