QIO(10.2) QIO(10.2)
NAME
qio: qget, qdiscard, qconsume, qpass, qproduce, qcopy,
qopen, qbread, qread, qbwrite, qwrite, qiwrite, qfree,
qclose, qhangup, qreopen, qlen, qwindow, qcanread,
qsetlimit, qnoblock, qflush, qfull - queued I/O for devices
SYNOPSIS
Queue* qopen(int limit,int msg, void (*kick)(void*),void
*arg)
void qhangup(Queue *q, char *reason)
void qclose(Queue *q)
void qreopen(Queue *q)
void qfree(Queue *q)
long qbwrite(Queue *q, Block *b)
long qwrite(Queue *q, void *buf, int len)
int qpass(Queue *q, Block *b)
int qpassnolim(Queue *q, Block *b)
int qproduce(Queue *q, void*buf, int len)
int qiwrite(Queue *q, void *buf, int len)
Block* qbread(Queue *q, int len)
long qread(Queue *q, void *buf, int len)
Block* qcopy(Queue *q, int len, ulong offset)
Block* qget(Queue *q)
int qconsume(Queue *q, void *buf, int len)
int qdiscard(Queue *q, int len)
void qflush(Queue *q)
int qlen(Queue *q)
int qwindow(Queue *q)
int qcanread(Queue *q)
Page 1 Plan 9 (printed 4/16/24)
QIO(10.2) QIO(10.2)
void qsetlimit(Queue *q, int limit)
void qnoblock(Queue *q, int nonblock)
int qfull(Queue *q);
DESCRIPTION
This suite of functions provides serial data buffering for
device drivers. Data is stored in a Queue structure as a
sequence of variable-sized Blocks; see allocb(10.2).
Qopen initialises and returns a pointer to a new Queue, con-
figuring it according to the following parameters:
limit Set the queue limit (high water mark) in bytes.
msg Set message mode if non-zero; otherwise, stream mode
(discussed below).
kick Optional flow-control function called by qbread to
restart writers, and by qbwrite (also qiwrite) to
restart readers.
arg Argument to pass to kick
Qhangup marks q as `hung up' for the given reason (Ehungup
by default). Subsequent attempts to write to the queue
raise an error(10.2). Qhangup does not flush the queue: sub-
sequent read requests are handled normally until the queue
empties. Qread and the other functions then return their
conventional values for a hungup stream: 0, -1 or a null
pointer, depending on the function. After a few such
attempts by any process, an error(10.2) is raised (typically
Ehungup) on each subsequent read.
If queued data is left unread, and not flushed by qflush or
qclose, the data will again be readable following a subse-
quent qreopen.
Qclose also marks a given q as `hung up', but removes and
frees any queued data Blocks. Qclose ignores calls when q
is null.
Qreopen makes a closed or hung up queue available for use
again. The queue's data limit is reset to the limit value
given when the queue was first created by qopen, cancelling
the effect of any previous call to qsetlimit.
Qfree closes q with qclose and frees it. The caller must
ensure that no references remain; these functions do not
keep a reference count.
Page 2 Plan 9 (printed 4/16/24)
QIO(10.2) QIO(10.2)
Flow control
The queue I/O routines provide a flow control mechanism to
coordinate producers and consumers. Each queue has a limit
on the number of bytes queued, its `high water mark', ini-
tially set when the queue is created, but adjustable by
qsetlimit, below. The low water mark is not set explicitly:
it is always half the current queue limit. When the high
water mark is exceeded, writes normally block until a reader
drains the queue below its low water mark; the writer is
then allowed to proceed. Conversely, readers normally block
when the queue is empty, until a writer arrives with data,
or the queue is closed.
A queue can be given a kick function when the queue is cre-
ated by qopen. The function is invoked by qread and qbread,
to prod an output routine when the queue falls below the
low-water mark, and by qwrite, qbwrite and qiwrite, to
notify a reader that a queue is no longer empty. Because
kick is called from the reading (or writing) process, or an
interrupt handler, it must not block.
Interrupt handlers must not sleep(10.2), and are therefore
restricted to using only the non-blocking functions
described below.
Stream mode and message mode
In stream mode, no read will return more than one block of
data, but a read can split a block that contains more data
than requested, leaving the remainder in a new block at the
front of the Queue. Writes of more than the maximum Block
size (currently 128k bytes) are split into as many Blocks as
required, each written separately to the queue, in order,
but with possible flow-control between them. The queue is
locked meanwhile, however, so that data from other writers
is not intermingled.
In message mode, by contrast, a read will return at most one
block's worth of data, but the remainder of a partially-read
block will be discarded, not returned to the queue. If a
write count exceeds the maximum Block size, the excess data
is discarded: at most a single block can be queued.
The mode of the queue should be taken into account in the
descriptions below of the following functions: qwrite,
qiwrite, qbread and qconsume. No other functions are aware
of the distinction.
Write operations (flow controlled)
Qwrite copies len bytes of data from buf into one or more
Blocks which it places on the q. Qwrite always returns len.
It can implement message mode.
Page 3 Plan 9 (printed 4/16/24)
QIO(10.2) QIO(10.2)
Qbwrite places the single Block b on the tail of q, waking
any sleeping reader. If the queue is full, the writing pro-
cess blocks until a reader has reduced the queued data to
the low-water mark; if the queue is non-blocking (see
qnoblock below), the data is discarded without notice.
Qbwrite normally returns len, but raises an error(10.2) if
the queue is closed (see qhangup and qclose). The block b is
always freed. Note that b can be empty (zero-length), to
punctuate the data in a queue. Qbwrite cannot handle a list
of Blocks; qpass must be used instead.
Non-blocking writes
Qproduce returns -1immediately if q is full. Otherwise, it
queues len bytes of data from buf in a single Block on q and
returns the number of bytes written.
Qpass attempts to place the list of Blocks headed by b on q,
returning the number of bytes written if successful. If q
was full, it frees the Block list b and returns -1.
Qpassnolim puts the Block list b on q regardless of flow
control; it returns the number of bytes in the list b.
Qiwrite is a variant of qwrite used exclusively by the ker-
nel print function, to allow printing by interrupt handlers;
qiwrite could be used with care by other routines, but
qproduce is preferable. Qiwrite writes the len bytes of
data at buf into the q without regard to flow control; the
writer never blocks. The queue is assumed to be open.
Qiwrite always returns len. It can implement message mode.
Read operations (flow controlled)
Qbread blocks until data arrives on q, then returns the
first Block; it limits the data returned to len bytes (in
the manner depending on the mode of q). It returns a null
pointer if the queue has hung up.
Qread reads a Block of up to len bytes from q using qbread,
and copies the data in the Block into buf, then frees the
Block and returns the number of bytes read. Qread returns 0
on end of file or error (hangup). It can implement message
mode.
Qcopy returns a Block with a copy of data from the queue
(the data remains on the queue). The copy begins offset
bytes into the queue's data and proceeds until len bytes
have been copied or no more data remains. The Block's read
and write pointers delimit the data copied into it. Qcopy
can be used by a reliable transport protocol to copy a
packet for transmission, leaving the data queued for possi-
ble retransmission, if unacknowledged.
Page 4 Plan 9 (printed 4/16/24)
QIO(10.2) QIO(10.2)
Non-blocking reads
Qconsume returns -1 immediately if q is empty. Otherwise,
it copies up to len bytes from the first Block on the queue
into buf, returning the number of bytes copied. It can
implement message mode.
Qget returns a null pointer immediately if q is empty or
closed. Otherwise, it returns the first Block on the queue.
Discard and flush
Qdiscard removes the first len data bytes from q; it returns
the number of bytes actually discarded, in case the queue is
shorter than len. If the queue drains below the low-water
mark, qdiscard wakes any sleeping writers. Since it does
not block, qdiscard can safely be called from interrupt han-
dlers. It is useful in transport protocol drivers to remove
data from the queue once acknowledged.
Qflush discards all data waiting on q, waking any waiting
writer.
Queue status
The following functions return a Queue's status. Note that
between a call to one of these functions and another opera-
tion, the state can change if a driver allows concurrent
access by either another process or an interrupt handler.
Qlen returns the number of bytes queued on q.
Qwindow returns the number of bytes that can be written
before reaching the queue's high-water mark. A return of 0
means that a write operation will certainly block; a non-
zero return gives no guarantees (see qfull, below).
Qcanread returns 1 if any data queued is queued. A subse-
quent read operation will not block.
Qfull returns non-zero if q is flow-controlled and a write
would block or a non-blocking write would return an error.
(Note that the implementation allows qwindow to return non-
zero yet qfull to return true.)
Queue control
Qsetlimit sets the high water mark for the queue to limit.
Note that qopen saves the initial queue limit. If the queue
is closed and reopened (by qreopen) that initial limit is
restored.
Qnoblock sets or resets non-blocking mode. If nonblock is
non-zero, the queue becomes non-blocking, and data written
to a queue beyond its high water mark is discarded by calls
that would otherwise block.
Page 5 Plan 9 (printed 4/16/24)
QIO(10.2) QIO(10.2)
SOURCE
/os/port/qio.c
/emu/port/qio.c
SEE ALSO
allocb(10.2), ref(10.2)
Page 6 Plan 9 (printed 4/16/24)