* 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 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`)
+ * The state of the fastbuf is represented by a <<struct_fastbuf,`struct fastbuf`>>,
+ * which is a simple structure describing the state of the buffer (the pointers
+ * `buffer`, `bufend`), the front-end cursor (`bptr`), the back-end cursor (`bstop`),
+ * position of the back-end cursor in the file (`pos`), some flags (`flags`)
* 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
+ * +------------------------------------+---------------------------+
+ * | unused | free space |
+ * +------------------------------------+---------------------------+
+ * ^ ^ ^ ^
+ * buffer <= bstop (BE pos) <= bptr (FE 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
+ * * This schema describes a fastbuf after its initialization or bflush().
+ * * There is no cached data and we are ready for any read or write operation
+ * (well, only if the back-end supports it).
+ * * The interval `[bptr, 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.
+ * upon the first write attempt to allocate a new buffer. Otherwise
+ * the fastbuf silently comes to the writing mode.
+ * * When a front-end needs to read something, it calls the `refill` callback.
+ * * The pointers can be either all non-`NULL` or all NULL.
+ * * `bstop == bptr` in most back-ends, but it is not necessary. Some
+ * in-memory streams take advantage of this.
*
* 2. Reading:
*
- * +----------------+---------------------------+
- * | read data | unused |
- * +----------------+---------------------------+
- * ^ ^ ^ ^
- * buffer <= bptr <= bstop (pos) <= bufend
+ * +------------------------------------+---------------------------+
+ * | read data | unused |
+ * +------------------------------------+---------------------------+
+ * ^ ^ ^ ^
+ * buffer <= bptr (FE pos) <= bstop (BE 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.
+ * will simply set `bptr` to `bstop` before `spout`
+ * and it breaks the position of the front-end's cursor,
+ * so the user should seek afwards.
* * 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
*
* 3. Writing:
*
- * +---------+--------------+-------------------+
- * | unused | written data | free space |
- * +---------+--------------+-------------------+
- * ^ ^ ^ ^
- * buffer <= bstop (pos) < bptr <= bufend
+ * +-----------------------+----------------+-----------------------+
+ * | unused | written data | free space |
+ * +-----------------------+----------------+-----------------------+
+ * ^ ^ ^ ^
+ * buffer <= bstop (BE pos) < bptr (FE pos) <= 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.
+ * the `spout` hook to flush the witten data and get an empty buffer.
+ * * `bstop` usually points at the beginning of the written data,
+ * but it is not necessary.
*
*
* 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.
+ * and if a fatal error occurs, then also `bstop`. Back-ends can rely on it.
+ * - `buffer <= bstop <= bufend` and `buffer <= bptr <= bufend`.
+ * - `pos` should be the real position in the file corresponding to the location of `bstop` in the buffer.
+ * It can be modified by any back-end's callback, but the position of `bptr` (`pos + (bptr - bstop)`)
+ * must stay unchanged (except the `seek` callback of course).
* - Failed callbacks (except `close`) should use @bthrow().
- * - Any callback pointers may be NULL in case the callback is not implemented.
+ * - Any callback pointer 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)
+ * - Initialization:
+ * * out: `buffer <= bstop <= bptr <= bufend` (flushed).
*
* - `refill`:
- * * in: `buffer <= bptr == bstop <= bufend` (reading or flushed)
- * * out: `buffer <= bptr < bstop <= bufend` (reading)
+ * * in: `buffer <= bstop <= bptr <= bufend` (reading or flushed).
+ * * out: `buffer <= bptr <= bstop <= bufend` (reading).
+ * * Resulting `bptr == bstop` signals the end of file.
+ * The next reading attempt will again call `refill` which can succeed this time.
+ * * The callback must also return zero on EOF (iff `bptr == bstop`).
*
* - `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`.
+ * * in: `buffer <= bstop <= bptr <= bufend` (writing or flushed).
+ * * out: `buffer <= bstop <= bptr < bufend` (flushed).
*
* - `seek`:
- * * in: `buffer <= bstop == bptr <= bufend` (flushed)
- * * out: `buffer <= bstop <= bufend` (flushed)
- * * `bptr` is set automatically to `bstop`.
+ * * in: `buffer <= bstop <= bptr <= bufend` (flushed).
+ * * in: `(ofs >= 0 && whence == SEEK_SET) || (ofs <= 0 && whence == SEEK_END)`.
+ * * out: `buffer <= bstop <= bptr <= bufend` (flushed).
*
* - `close`:
- * * in: `buffer <= bptr == bstop <= bufend` (flushed)
+ * * in: `buffer <= bstop <= bptr <= bufend` (flushed or after @bthrow()).
* * `close` must always free all internal structures, even when it throws an exception.
- *
***/
/**
};
struct cf_section;
-extern struct cf_section fbpar_cf; /** Configuration section with which you can fill the `fb_params` **/
+extern struct cf_section fbpar_cf; /** Configuration section with which you can fill the `fb_params` **/
extern struct fb_params fbpar_def; /** The default `fb_params` **/
/**
projects / libucw.git / commitdiff