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)

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, configuring 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: subsequent 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 subsequent 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.

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', initially 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 created 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.

Qbwrite places the single Block b on the tail of q, waking any sleeping reader. If the queue is full, the writing process 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 kernel 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 possible retransmission, if unacknowledged.

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 handlers. 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 operation, 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 subsequent 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.

SOURCE

/os/port/qio.c
/emu/port/qio.c

SEE ALSO

allocb(10.2), ref(10.2)

QIO(10.2 )

Rev:  Tue Mar 31 02:42:39 GMT 2015