*
* Generally speaking, a fastbuf consists of a buffer and a set of callbacks.
* All front-end functions operate on the buffer and if the buffer becomes
- * empty or fills up, they ask the corresponding callback to solve the
+ * empty or fills up, they ask the corresponding callback to handle the
* situation. Back-ends then differ just in the definition of the callbacks.
- * The state of the fastbuf is represented by `struct fastbuf`, which
- * is a simple structure describing the state of the buffer and pointers
- * to the callback functions.
- *
- * When we are reading from the fastbuf, the buffer is laid out as follows:
- *
- * +----------------+---------------------------+
- * | read data | free space |
- * +----------------+---------------------------+
- * ^ ^ ^ ^
- * buffer bptr bstop bufend
- *
- * Here `bptr` points to the next character to be read. After the last character is
- * read, `bptr == bstop` and the `refill` callback gets called upon the next read
- * attempt to bring further data. This gives us an easy way how to implement @bungetc().
- *
- * When writing, the situation looks like:
- *
- * +--------+--------------+--------------------+
- * | unused | written data | free space |
- * +--------+--------------+--------------------+
- * ^ ^ ^ ^
- * buffer bstop bptr bufend
- *
- * In this case, the `bptr` points at the position where the next character
- * will be written to. When we want to write, but `bptr == bufend`, we call
- * the `spout` hook to flush the data and get an empty buffer.
- *
- * Several dirty tricks can be played:
- *
- * - The `spout`/`refill` hooks can change not only `bptr` and `bstop`, but also
- * the location and size of the buffer; the fb-mem back-end takes advantage of it.
- * - In some cases, the user of the `bdirect` interface can be allowed to modify
- * the data in the buffer to avoid unnecessary copying. If the back-end
- * allows such modifications, it can set `fastbuf->can_overwrite_buffer` accordingly:
- * * 0 if no modification is allowed,
- * * 1 if the user can modify the buffer on the condition that
- * the modifications will be undone before calling the next
- * fastbuf operation
- * * 2 if the user is allowed to overwrite the data in the buffer
- * if @bdirect_read_commit_modified() is called afterwards.
- * In this case, the back-end must be prepared for trimming
- * of the buffer which is done by the commit function.
+ *
+ * The state of the fastbuf is represented by a `struct fastbuf`, which
+ * is a simple structure describing the state of the buffer (the pointers
+ * `buffer`, `bufend`), two front-end cursors (`bptr`, `bstop`), position in the file (`pos`)
+ * and pointers to the callback functions.
+ *
+ * The buffer can be in one of the following states:
+ *
+ * 1. Flushed:
+ *
+ * +----------------+---------------------------+
+ * | unused | free space |
+ * +----------------+---------------------------+
+ * ^ ^ ^
+ * buffer <= bptr == bstop (pos) <= bufend
+ *
+ * * If `bptr == bstop`, then there is no cached data and
+ * the fastbuf is ready for any read or write operation.
+ * Position of the back-end's cursor equals the front-end's one.
+ * * The interval `[bstop, bufend]` can be used by front-ends
+ * for writing. If it is empty, the `spout` callback gets called
+ * upon the first write attempt to allocate a new buffer.
+ * * When a front-end needs to read something, it calls the `spout` callback.
+ * * Any of the pointers can be NULL.
+ *
+ * 2. Reading:
+ *
+ * +----------------+---------------------------+
+ * | read data | unused |
+ * +----------------+---------------------------+
+ * ^ ^ ^ ^
+ * buffer <= bptr <= bstop (pos) <= bufend
+ *
+ * * If we try to read something, we get to the reading mode.
+ * * No writing is allowed until a flush operation. But note that @bflush()
+ * will simply set `bptr` to `bstop` and it breaks the position of the front-end's cursor.
+ * * The interval `[buffer, bstop]` contains a block of data read by the back-end.
+ * `bptr` is the front-end's cursor which points to the next character to be read.
+ * After the last character is read, `bptr == bstop` and the `refill` callback
+ * gets called upon the next read attempt to bring further data.
+ * This gives us an easy way how to implement @bungetc().
+ *
+ * 3. Writing:
+ *
+ * +---------+--------------+-------------------+
+ * | unused | written data | free space |
+ * +---------+--------------+-------------------+
+ * ^ ^ ^ ^
+ * buffer <= bstop (pos) < bptr <= bufend
+ *
+ * * This schema corresponds to the situation after a write attempt.
+ * * No reading is allowed until a flush operation.
+ * * The `bptr` points at the position where the next character
+ * will be written to. When we want to write, but `bptr == bufend`, we call
+ * the `spout` hook to flush the data and get an empty buffer.
+ *
+ *
+ * Rules for back-ends:
+ *
+ * - Front-ends are only allowed to change the value of `bptr`, some flags
+ * and if a fatal error occurs, then also `bstop`.
+ * - `buffer <= bstop <= bufend`.
+ * - `pos` should be the position in the file corresponding of the location of `bstop` in the buffer.
+ * - Failed callbacks (except `close`) should use @bthrow().
+ * - Any callback pointers may be NULL in case the callback is not implemented.
+ * - Callbacks can change not only `bptr` and `bstop`, but also the location and size of the buffer;
+ * the fb-mem back-end takes advantage of it.
+ *
+ * - initialization:
+ * * out: `buffer <= bptr == bstop <= bufend` (flushed)
+ *
+ * - `refill`:
+ * * in: `buffer <= bptr == bstop <= bufend` (reading or flushed)
+ * * out: `buffer <= bptr < bstop <= bufend` (reading)
+ *
+ * - `spout`:
+ * * in: `buffer <= bstop <= bptr <= bufend` (writing or flushed)
+ * * out: `buffer <= bstop <= bufend` (flushed)
+ * * `bptr` is set automatically to `bstop`.
+ * * If the input `bptr` equals ` bstop`, then the resulting `bstop` must be lower than `bufend`.
+ *
+ * - `seek`:
+ * * in: `buffer <= bstop == bptr <= bufend` (flushed)
+ * * out: `buffer <= bstop <= bufend` (flushed)
+ * * `bptr` is set automatically to `bstop`.
+ *
+ * - `close`:
+ * * in: `buffer <= bptr == bstop <= bufend` (flushed)
+ * * `close` must always free all internal structures, even when it throws an exception.
*
***/
bputc_slow(f, c);
}
-static inline uns bavailr(struct fastbuf *f)
+static inline uns bavailr(struct fastbuf *f) /** Return the length of the cached data to be read. Do not use directly. **/
{
return f->bstop - f->bptr;
}
-static inline uns bavailw(struct fastbuf *f)
+static inline uns bavailw(struct fastbuf *f) /** Return the length of the buffer available for writing. Do not use directly. **/
{
return f->bufend - f->bptr;
}
void bbcopy_slow(struct fastbuf *f, struct fastbuf *t, uns l);
/**
* Copy @l bytes of data from fastbuf @f to fastbuf @t.
- * `~0U` means all data, even if more than `~0U` bytes remain.
+ * `UINT_MAX` (`~0U`) means all data, even if more than `UINT_MAX` bytes remain.
**/
static inline void bbcopy(struct fastbuf *f, struct fastbuf *t, uns l)
{
}
/*** === Direct I/O on buffers ***/
-// TODO Documentation -- what do they do?
-static inline uns
-bdirect_read_prepare(struct fastbuf *f, byte **buf)
+/**
+ * Begin direct reading from fastbuf's internal buffer to avoid unnecessary copying.
+ * The function returns a buffer @buf together with its length in bytes (zero means EOF)
+ * with cached data to be read.
+ *
+ * Some back-ends allow the user to modify the data in the returned buffer to avoid unnecessary.
+ * If the back-end allows such modifications, it can set `f->can_overwrite_buffer` accordingly:
+ *
+ * - 0 if no modification is allowed,
+ * - 1 if the user can modify the buffer on the condition that
+ * the modifications will be undone before calling the next
+ * fastbuf operation
+ * - 2 if the user is allowed to overwrite the data in the buffer
+ * if @bdirect_read_commit_modified() is called afterwards.
+ * In this case, the back-end must be prepared for trimming
+ * of the buffer which is done by the commit function.
+ *
+ * The reading must be ended by @bdirect_read_commit() or @bdirect_read_commit_modified(),
+ * unless the user did not read or modify anything.
+ **/
+static inline uns bdirect_read_prepare(struct fastbuf *f, byte **buf)
{
if (f->bptr == f->bstop && !f->refill(f))
{
return bavailr(f);
}
-static inline void
-bdirect_read_commit(struct fastbuf *f, byte *pos)
+/**
+ * End direct reading started by @bdirect_read_prepare() and move the cursor at @pos.
+ * Data in the returned buffer must be same as after @bdirect_read_prepare() and
+ * @pos must point somewhere inside the buffer.
+ **/
+static inline void bdirect_read_commit(struct fastbuf *f, byte *pos)
{
f->bptr = pos;
}
-static inline void
-bdirect_read_commit_modified(struct fastbuf *f, byte *pos)
+/**
+ * Similar to @bdirect_read_commit(), but accepts also modified data before @pos.
+ * Note that such modifications are supported only if `f->can_overwrite_buffer == 2`.
+ **/
+static inline void bdirect_read_commit_modified(struct fastbuf *f, byte *pos)
{
f->bptr = pos;
f->buffer = pos; /* Avoid seeking backwards in the buffer */
}
-static inline uns
-bdirect_write_prepare(struct fastbuf *f, byte **buf)
+/**
+ * Start direct writing to fastbuf's internal buffer to avoid copy overhead.
+ * The function returns the length of the buffer in @buf (at least one byte)
+ * where we can write to. The operation must be ended by @bdirect_write_commit(),
+ * unless nothing is written.
+ **/
+static inline uns bdirect_write_prepare(struct fastbuf *f, byte **buf)
{
if (f->bptr == f->bufend)
f->spout(f);
return bavailw(f);
}
-static inline void
-bdirect_write_commit(struct fastbuf *f, byte *pos)
+/**
+ * Commit the data written to the buffer returned by @bdirect_write_prepare().
+ * The length is specified by @pos which must point just after the written data.
+ * Also moves the cursor to @pos.
+ **/
+static inline void bdirect_write_commit(struct fastbuf *f, byte *pos)
{
f->bptr = pos;
}