From 065a57426d46eda425a270415d5bcc3effa924eb Mon Sep 17 00:00:00 2001 From: Pavel Charvat Date: Thu, 6 Nov 2008 10:37:27 +0100 Subject: [PATCH] Doc: Described the interface for direct I/O on fastbufs. --- ucw/fastbuf.h | 81 +++++++++++++++++++++++++++++++-------------------- 1 file changed, 50 insertions(+), 31 deletions(-) diff --git a/ucw/fastbuf.h b/ucw/fastbuf.h index 5e3358e4..70fbac41 100644 --- a/ucw/fastbuf.h +++ b/ucw/fastbuf.h @@ -86,6 +86,8 @@ * - `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) @@ -106,26 +108,9 @@ * * `bptr` is set automatically to `bstop`. * * - `close`: - * * out: `buffer <= bptr == bstop <= bufend` (flushed) + * * in: `buffer <= bptr == bstop <= bufend` (flushed) * * `close` must always free all internal structures, even when it throws an exception. * - * - * 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. - * ***/ /** @@ -505,12 +490,12 @@ static inline void bputc(struct fastbuf *f, uns c) /** Write a single character 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; } @@ -656,10 +641,28 @@ static inline int bskip(struct fastbuf *f, uns len) /** Skip @len bytes without } /*** === 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 modified anything. + **/ +static inline uns bdirect_read_prepare(struct fastbuf *f, byte **buf) { if (f->bptr == f->bstop && !f->refill(f)) { @@ -670,21 +673,33 @@ bdirect_read_prepare(struct fastbuf *f, byte **buf) 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); @@ -692,8 +707,12 @@ bdirect_write_prepare(struct fastbuf *f, byte **buf) 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; } -- 2.39.2