man/man9/alq.9

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.Dd May 16, 2003
.Dt ALQ 9
.Os
.Sh NAME
.Nm alq ,
.Nm alq_open ,
.Nm alq_write ,
.Nm alq_flush ,
.Nm alq_close ,
.Nm alq_get ,
.Nm alq_post
.Nd Asynchronous Logging Queues
.Sh SYNOPSIS
.In sys/alq.h
.Ft int
.Fo alq_open
.Fa "struct alq **app"
.Fa "const char *file"
.Fa "struct ucred *cred"
.Fa "int size"
.Fa "int count"
.Fc
.Ft int
.Fn alq_write "struct alq *alq" "void *data" "int waitok"
.Ft void
.Fn alq_flush "struct alq *alq"
.Ft void
.Fn alq_close "struct alq *alq"
.Ft struct ale *
.Fn alq_get "struct alq *alq" "int waitok"
.Ft void
.Fn alq_post "struct alq *alq" "struct ale *ale"
.Sh DESCRIPTION
The
.Nm
facility provides an asynchronous fixed length recording
mechanism, known as Asynchronous Logging Queues.
It can record to any
.Xr vnode 9 ,
thus providing the ability to journal logs to character
devices as well as regular files.
All functions accept a
.Vt "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.
.Pp
An
.Dq asynchronous log entry
is defined as
.Vt "struct ale" ,
which has the following members:
.Bd -literal -offset indent
struct ale {
	struct ale	*ae_next;	/* Next Entry */
	char		*ae_data;	/* Entry buffer */
	int		ae_flags;	/* Entry flags */
};
.Ed
.Pp
The
.Va ae_flags
field is for internal use, clients of the
.Nm
interface should not modify this field.
Behaviour is undefined if this field is modified.
.Sh FUNCTIONS
The
.Fn alq_open
function creates a new logging queue.
The
.Fa file
argument is the name of the file to open for logging.
The argument
.Fa cred
specifies the credentials to use when opening the file.
The size of each entry in the queue is determined by
.Fa size .
The
.Fa count
argument determines the number of items to be stored in the
asynchronous queue over an approximate period of a disk
write operation.
.Pp
The
.Fn alq_write
function writes
.Fa data
to the designated queue,
.Fa alq .
In the event that
.Fn alq_write
could not write the entry immediately, and
.Dv ALQ_WAITOK
is passed to
.Fa waitok ,
then
.Fn alq_write
will be allowed to
.Xr tsleep 9 .
.Pp
The
.Fn alq_flush
function is used for flushing
.Fa alq
to the log medium that was passed to
.Fn alq_open .
.Pp
The
.Fn alq_close
function will close the asynchronous logging queue,
.Fa alq ,
and flush all pending write requests to the log medium.
It will free all resources that were previously allocated.
.Pp
The
.Fn alq_get
function returns the next available asynchronous logging entry
from the queue,
.Fa alq .
This function leaves the queue in a locked state, until a subsequent
.Fn alq_post
call is made.
In the event that
.Fn alq_get
could not retrieve an entry immediately, it will
.Xr tsleep 9
with the
.Dq Li alqget
wait message.
.Pp
The
.Fn alq_post
function schedules the asynchronous logging entry,
.Fa ale ,
which is retrieved using the
.Fn alq_get
function,
for writing to the asynchronous logging queue,
.Fa alq .
This function leaves the queue,
.Fa alq ,
in an unlocked state.
.Sh IMPLEMENTATION NOTES
The
.Fn alq_write
function is a wrapper around the
.Fn alq_get
and
.Fn alq_post
functions; by using these functions separately, a call
to
.Fn bcopy
can be avoided for performance critical code paths.
.Sh LOCKING
Each asynchronous queue is protected by a spin mutex.
.Pp
Functions
.Fn alq_flush ,
.Fn alq_open
and
.Fn alq_post
may attempt to acquire an internal sleep mutex, and should
consequently not be used in contexts where sleeping is
not allowed.
.Sh RETURN VALUES
The
.Fn alq_open
function returns one of the error codes listed in
.Xr open 2 ,
if it fails to open
.Fa file ,
or else it returns 0.
.Pp
The
.Fn alq_write
function returns
.Er EWOULDBLOCK
if
.Dv ALQ_NOWAIT
was provided as a value to
.Fa waitok
and either the queue is full, or when the system is shutting down.
.Pp
The
.Fn alq_get
function returns
.Dv NULL ,
if
.Dv ALQ_NOWAIT
was provided as a value to
.Fa waitok
and either the queue is full, or when the system is shutting down.
.Pp
NOTE: invalid arguments to non-void functions will result in
undefined behaviour.
.Sh SEE ALSO
.Xr syslog 3 ,
.Xr kthread 9 ,
.Xr ktr 9 ,
.Xr tsleep 9 ,
.Xr vnode 9
.Sh HISTORY
The
Asynchronous Logging Queues (ALQ) facility first appeared in
.Fx 5.0 .
.Sh AUTHORS
.An -nosplit
The
.Nm
facility was written by
.An Jeffrey Roberson Aq jeff@FreeBSD.org .
.Pp
This manual page was written by
.An Hiten Pandya Aq hmp@FreeBSD.org .