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ALQ(9)			 BSD Kernel Developer's	Manual			ALQ(9)

NAME
     alq, alq_open_flags, alq_open, alq_writen,	alq_write, alq_flush,
     alq_close,	alq_getn, alq_get, alq_post_flags, alq_post -- Asynchronous
     Logging Queues

SYNOPSIS
     #include <sys/alq.h>

     int
     alq_open_flags(struct alq **app, const char *file,	struct ucred *cred,
	 int cmode, int	size, int flags);

     int
     alq_open(struct alq **app,	const char *file, struct ucred *cred,
	 int cmode, int	size, int count);

     int
     alq_writen(struct alq *alq, void *data, int len, int flags);

     int
     alq_write(struct alq *alq,	void *data, int	flags);

     void
     alq_flush(struct alq *alq);

     void
     alq_close(struct alq *alq);

     struct ale	*
     alq_getn(struct alq *alq, int len,	int flags);

     struct ale	*
     alq_get(struct alq	*alq, int flags);

     void
     alq_post_flags(struct alq *alq, struct ale	*ale, int flags);

     void
     alq_post(struct alq *alq, struct ale *ale);

DESCRIPTION
     The alq facility provides an asynchronous fixed or	variable length
     recording mechanism, known	as Asynchronous	Logging	Queues.	 It can	record
     to	any vnode(9), thus providing the ability to journal logs to character
     devices as	well as	regular	files.	All functions accept a struct alq ar-
     gument, which is an opaque	type that maintains state information for an
     Asynchronous Logging Queue.  The logging facility runs in a separate ker-
     nel thread, which services	all log	entry requests.

     An	"asynchronous log entry" is defined as struct ale, which has the fol-
     lowing members:

	   struct ale {
		   intptr_t	   ae_bytesused;   /* #	bytes written to ALE. */
		   char		   *ae_data;	   /* Write ptr. */
		   int		   ae_pad;	   /* Unused, compat. */
	   };

     An	alq can	be created in either fixed or variable length mode.  A vari-
     able length alq accommodates writes of varying length using alq_writen()
     and alq_getn().  A	fixed length alq accommodates a	fixed number of	writes
     using alq_write() and alq_get(), each of fixed size (set at queue cre-
     ation time).  Fixed length	mode is	deprecated in favour of	variable
     length mode.

FUNCTIONS
     The alq_open_flags() function creates a new variable length asynchronous
     logging queue.  The file argument is the name of the file to open for
     logging.  If the file does	not yet	exist, alq_open() will attempt to cre-
     ate it.  The cmode	argument will be passed	to vn_open() as	the requested
     creation mode, to be used if the file will	be created by alq_open().
     Consumers of this API may wish to pass ALQ_DEFAULT_CMODE, a default cre-
     ation mode	suitable for most applications.	 The cred argument specifies
     the credentials to	use when opening and performing	I/O on the file.  The
     size argument sets	the size (in bytes) of the underlying queue.  The
     ALQ_ORDERED flag may be passed in via flags to indicate that the ordering
     of	writer threads waiting for a busy alq to free up resources should be
     preserved.

     The deprecated alq_open() function	is implemented as a wrapper around
     alq_open_flags() to provide backwards compatibility to consumers that
     have not been updated to utilise the newer	alq_open_flags() function.  It
     passes all	arguments through to alq_open_flags() untouched	except for
     size and count, and sets flags to 0.  To create a variable	length mode
     alq, the size argument should be set to the size (in bytes) of the	under-
     lying queue and the count argument	should be set to 0.  To	create a fixed
     length mode alq, the size argument	should be set to the size (in bytes)
     of	each write and the count argument should be set	to the number of size
     byte chunks to reserve capacity for.

     The alq_writen() function writes len bytes	from data to the designated
     variable length mode queue	alq.  If alq_writen() could not	write the en-
     try immediately and ALQ_WAITOK is set in flags, the function will be al-
     lowed to msleep_spin(9) with the "alqwnord" or "alqwnres" wait message.
     A write will automatically	schedule the queue alq to be flushed to	disk.
     This behaviour can	be controlled by passing ALQ_NOACTIVATE	via flags to
     indicate that the write should not	schedule alq to	be flushed to disk.

     The deprecated alq_write()	function is implemented	as a wrapper around
     alq_writen() to provide backwards compatibility to	consumers that have
     not been updated to utilise variable length mode queues.  The function
     will write	size bytes of data (where size was specified at	queue creation
     time) from	the data buffer	to the alq.  Note that it is an	error to call
     alq_write() on a variable length mode queue.

     The alq_flush() function is used for flushing alq to the log medium that
     was passed	to alq_open().	If alq has data	to flush and is	not already in
     the process of being flushed, the function	will block doing IO.  Other-
     wise, the function	will return immediately.

     The alq_close() function will close the asynchronous logging queue	alq
     and flush all pending write requests to the log medium.  It will free all
     resources that were previously allocated.

     The alq_getn() function returns an	asynchronous log entry from alq, ini-
     tialised to point at a buffer capable of receiving	len bytes of data.
     This function leaves alq in a locked state, until a subsequent alq_post()
     or	alq_post_flags() call is made.	If alq_getn() could not	obtain len
     bytes of buffer immediately and ALQ_WAITOK	is set in flags, the function
     will be allowed to	msleep_spin(9) with the	"alqgnord" or "alqgnres" wait
     message.  The caller can choose to	write less than	len bytes of data to
     the returned asynchronous log entry by setting the	entry's	ae_bytesused
     field to the number of bytes actually written.  This must be done prior
     to	calling	alq_post().

     The deprecated alq_get() function is implemented as a wrapper around
     alq_getn()	to provide backwards compatibility to consumers	that have not
     been updated to utilise variable length mode queues.  The asynchronous
     log entry returned	will be	initialised to point at	a buffer capable of
     receiving size bytes of data (where size was specified at queue creation
     time).  Note that it is an	error to call alq_get()	on a variable length
     mode queue.

     The alq_post_flags() function schedules the asynchronous log entry	ale
     (obtained from alq_getn() or alq_get()) for writing to alq.  The
     ALQ_NOACTIVATE flag may be	passed in via flags to indicate	that the queue
     should not	be immediately scheduled to be flushed to disk.	 This function
     leaves alq	in an unlocked state.

     The alq_post() function is	implemented as a wrapper around
     alq_post_flags() to provide backwards compatibility to consumers that
     have not been updated to utilise the newer	alq_post_flags() function.  It
     simply passes all arguments through to alq_post_flags() untouched,	and
     sets flags	to 0.

IMPLEMENTATION NOTES
     The alq_writen() and alq_write() functions	both perform a bcopy(3)	from
     the supplied data buffer into the underlying alq buffer.  Performance
     critical code paths may wish to consider using alq_getn() (variable
     length queues) or alq_get() (fixed	length queues) to avoid	the extra mem-
     ory copy.	Note that a queue remains locked between calls to alq_getn()
     or	alq_get() and alq_post() or alq_post_flags(), so this method of	writ-
     ing to a queue is unsuitable for situations where the time	between	calls
     may be substantial.

LOCKING
     Each asynchronous logging queue is	protected by a spin mutex.

     Functions alq_flush() and alq_open() may attempt to acquire an internal
     sleep mutex, and should consequently not be used in contexts where	sleep-
     ing is not	allowed.

RETURN VALUES
     The alq_open() function returns one of the	error codes listed in open(2),
     if	it fails to open file, or else it returns 0.

     The alq_writen() and alq_write() functions	return EWOULDBLOCK if
     ALQ_NOWAIT	was set	in flags and either the	queue is full or the system is
     shutting down.

     The alq_getn() and	alq_get() functions return NULL	if ALQ_NOWAIT was set
     in	flags and either the queue is full or the system is shutting down.

     NOTE: invalid arguments to	non-void functions will	result in undefined
     behaviour.

SEE ALSO
     syslog(3),	kproc(9), ktr(9), msleep_spin(9), vnode(9)

HISTORY
     The Asynchronous Logging Queues (ALQ) facility first appeared in
     FreeBSD 5.0.

AUTHORS
     The alq facility was written by Jeffrey Roberson <jeff@FreeBSD.org> and
     extended by Lawrence Stewart <lstewart@freebsd.org>.

     This manual page was written by Hiten Pandya <hmp@FreeBSD.org> and	re-
     vised by Lawrence Stewart <lstewart@freebsd.org>.

BSD				April 26, 2010				   BSD

NAME | SYNOPSIS | DESCRIPTION | FUNCTIONS | IMPLEMENTATION NOTES | LOCKING | RETURN VALUES | SEE ALSO | HISTORY | AUTHORS

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