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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 argument, which is an opaque type that maintains state information for an
     Asynchronous Logging Queue.  The logging facility runs in a separate kernel thread, which
     services all log entry requests.

     An “asynchronous log entry” is defined as struct ale, which has the following 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 variable 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 creation 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 create 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 creation 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 underlying 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 entry immediately and ALQ_WAITOK is set in
     flags, the function will be allowed 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.  Otherwise, 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, initialised 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 memory 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 writing 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 sleeping 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

     kproc(9), ktr(9), msleep_spin(9), syslog(3), 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 revised by Lawrence
     Stewart <lstewart@freebsd.org>.