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      alq, alq_open, alq_write, alq_flush, alq_close, alq_get, alq_post - Asyn‐
      chronous Logging Queues


      #include <sys/alq.h>
      alq_open(struct alq **app, const char *file, struct ucred *cred,
              int cmode, int size, int count);
      alq_write(struct alq *alq, void *data, int waitok);
      alq_flush(struct alq *alq);
      alq_close(struct alq *alq);
      struct ale *
      alq_get(struct alq *alq, int waitok);
      alq_post(struct alq *alq, struct ale *ale);


      The alq facility provides an asynchronous fixed length recording mecha‐
      nism, 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 Asyn‐
      chronous 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 fol‐
      lowing members:
            struct ale {
                    struct ale      *ae_next;       /* Next Entry */
                    char            *ae_data;       /* Entry buffer */
                    int             ae_flags;       /* Entry flags */
      The ae_flags field is for internal use, clients of the alq interface
      should not modify this field.  Behaviour is undefined if this field is


      The alq_open() function creates a new 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 appli‐
      cations.  The argument cred specifies the credentials to use when opening
      and performing I/O on the file.  The size of each entry in the queue is
      determined by size.  The count argument determines the number of items to
      be stored in the asynchronous queue over an approximate period of a disk
      write operation.
      The alq_write() function writes data to the designated queue, alq.  In
      the event that alq_write() could not write the entry immediately, and
      ALQ_WAITOK is passed to waitok, then alq_write() will be allowed to
      The alq_flush() function is used for flushing alq to the log medium that
      was passed to alq_open().
      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_get() function returns the next available asynchronous logging
      entry from the queue, alq.  This function leaves the queue in a locked
      state, until a subsequent alq_post() call is made.  In the event that
tsleep(9) with
      the “alqget” wait message.
      The alq_post() function schedules the asynchronous logging entry, ale,
      which is retrieved using the alq_get() function, for writing to the asyn‐
      chronous logging queue, alq.  This function leaves the queue, alq, in an
      unlocked state.
      The alq_write() function is a wrapper around the alq_get() and alq_post()
      functions; by using these functions separately, a call to bcopy() can be
      avoided for performance critical code paths.


      Each asynchronous queue is protected by a spin mutex.
      Functions alq_flush(), alq_open() and alq_post() may attempt to acquire
      an internal sleep mutex, and should consequently not be used in contexts
      where sleeping is not allowed.
      if it fails to open file, or else it returns 0.
      The alq_write() function returns EWOULDBLOCK if ALQ_NOWAIT was provided
      as a value to waitok and either the queue is full, or when the system is
      shutting down.
      The alq_get() function returns NULL, if ALQ_NOWAIT was provided as a
      value to waitok and either the queue is full, or when the system is shut‐
      ting down.
      NOTE: invalid arguments to non-void functions will result in undefined


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


      The alq facility was written by Jeffrey Roberson 〈jeff@FreeBSD.org〉.
      This manual page was written by Hiten Pandya 〈hmp@FreeBSD.org〉.


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