ALQ(9) MidnightBSD Kernel Developer’s Manual ALQ(9)


alq, alq_open, alq_write, alq_flush, alq_close, alq_get, alq_post — Asynchronous 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 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 {

struct ale


/* Next Entry */



/* Entry buffer */



/* 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 modified.


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 applications. 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 tsleep(9).

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 alq_get() could not retrieve an entry immediately, it will 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 asynchronous 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.


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_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 shutting down.

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


syslog(3), kthread(9), ktr(9), tsleep(9), vnode(9)


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


The alq facility was written by Jeffrey Roberson 〈〉.

This manual page was written by Hiten Pandya 〈〉.

MidnightBSD 0.3 May 16, 2003 MidnightBSD 0.3