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267 lines
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<title>Streaming Versions of API functions.</title>
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<a href="intro.html"><h2>Session Module C Interface</h2></a><h2>Streaming Versions of API functions.</h2><blockquote><pre>int sqlite3changeset_apply_strm(
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sqlite3 *db, /* Apply change to "main" db of this handle */
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int (*xInput)(void *pIn, void *pData, int *pnData), /* Input function */
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void *pIn, /* First arg for xInput */
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int(*xFilter)(
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void *pCtx, /* Copy of sixth arg to _apply() */
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const char *zTab /* Table name */
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),
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int(*xConflict)(
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void *pCtx, /* Copy of sixth arg to _apply() */
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int eConflict, /* DATA, MISSING, CONFLICT, CONSTRAINT */
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sqlite3_changeset_iter *p /* Handle describing change and conflict */
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),
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void *pCtx /* First argument passed to xConflict */
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);
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int sqlite3changeset_apply_v2_strm(
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sqlite3 *db, /* Apply change to "main" db of this handle */
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int (*xInput)(void *pIn, void *pData, int *pnData), /* Input function */
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void *pIn, /* First arg for xInput */
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int(*xFilter)(
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void *pCtx, /* Copy of sixth arg to _apply() */
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const char *zTab /* Table name */
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),
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int(*xConflict)(
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void *pCtx, /* Copy of sixth arg to _apply() */
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int eConflict, /* DATA, MISSING, CONFLICT, CONSTRAINT */
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sqlite3_changeset_iter *p /* Handle describing change and conflict */
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),
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void *pCtx, /* First argument passed to xConflict */
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void **ppRebase, int *pnRebase,
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int flags
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);
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int sqlite3changeset_concat_strm(
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int (*xInputA)(void *pIn, void *pData, int *pnData),
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void *pInA,
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int (*xInputB)(void *pIn, void *pData, int *pnData),
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void *pInB,
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int (*xOutput)(void *pOut, const void *pData, int nData),
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void *pOut
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);
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int sqlite3changeset_invert_strm(
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int (*xInput)(void *pIn, void *pData, int *pnData),
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void *pIn,
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int (*xOutput)(void *pOut, const void *pData, int nData),
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void *pOut
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);
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int sqlite3changeset_start_strm(
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sqlite3_changeset_iter **pp,
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int (*xInput)(void *pIn, void *pData, int *pnData),
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void *pIn
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);
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int sqlite3changeset_start_v2_strm(
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sqlite3_changeset_iter **pp,
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int (*xInput)(void *pIn, void *pData, int *pnData),
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void *pIn,
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int flags
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);
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int sqlite3session_changeset_strm(
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sqlite3_session *pSession,
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int (*xOutput)(void *pOut, const void *pData, int nData),
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void *pOut
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);
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int sqlite3session_patchset_strm(
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sqlite3_session *pSession,
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int (*xOutput)(void *pOut, const void *pData, int nData),
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void *pOut
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);
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int sqlite3changegroup_add_strm(sqlite3_changegroup*,
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int (*xInput)(void *pIn, void *pData, int *pnData),
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void *pIn
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);
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int sqlite3changegroup_output_strm(sqlite3_changegroup*,
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int (*xOutput)(void *pOut, const void *pData, int nData),
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void *pOut
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);
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int sqlite3rebaser_rebase_strm(
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sqlite3_rebaser *pRebaser,
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int (*xInput)(void *pIn, void *pData, int *pnData),
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void *pIn,
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int (*xOutput)(void *pOut, const void *pData, int nData),
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void *pOut
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);
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</pre></blockquote><p>
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The six streaming API xxx_strm() functions serve similar purposes to the
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corresponding non-streaming API functions:</p>
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<p><table border=1 style="margin-left:8ex;margin-right:8ex">
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<tr><th>Streaming function<th>Non-streaming equivalent</th>
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<tr><td>sqlite3changeset_apply_strm<td><a href="../session/sqlite3changeset_apply.html">sqlite3changeset_apply</a>
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<tr><td>sqlite3changeset_apply_strm_v2<td><a href="../session/sqlite3changeset_apply.html">sqlite3changeset_apply_v2</a>
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<tr><td>sqlite3changeset_concat_strm<td><a href="../session/sqlite3changeset_concat.html">sqlite3changeset_concat</a>
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<tr><td>sqlite3changeset_invert_strm<td><a href="../session/sqlite3changeset_invert.html">sqlite3changeset_invert</a>
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<tr><td>sqlite3changeset_start_strm<td><a href="../session/sqlite3changeset_start.html">sqlite3changeset_start</a>
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<tr><td>sqlite3session_changeset_strm<td><a href="../session/sqlite3session_changeset.html">sqlite3session_changeset</a>
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<tr><td>sqlite3session_patchset_strm<td><a href="../session/sqlite3session_patchset.html">sqlite3session_patchset</a>
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</table></p>
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<p>Non-streaming functions that accept changesets (or patchsets) as input
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require that the entire changeset be stored in a single buffer in memory.
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Similarly, those that return a changeset or patchset do so by returning
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a pointer to a single large buffer allocated using sqlite3_malloc().
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Normally this is convenient. However, if an application running in a
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low-memory environment is required to handle very large changesets, the
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large contiguous memory allocations required can become onerous.</p>
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<p>In order to avoid this problem, instead of a single large buffer, input
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is passed to a streaming API functions by way of a callback function that
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the sessions module invokes to incrementally request input data as it is
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required. In all cases, a pair of API function parameters such as</p>
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<p> <pre>
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int nChangeset,
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void *pChangeset,
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</pre></p>
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<p>Is replaced by:</p>
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<p> <pre>
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int (*xInput)(void *pIn, void *pData, int *pnData),
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void *pIn,
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</pre></p>
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<p>Each time the xInput callback is invoked by the sessions module, the first
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argument passed is a copy of the supplied pIn context pointer. The second
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argument, pData, points to a buffer (*pnData) bytes in size. Assuming no
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error occurs the xInput method should copy up to (*pnData) bytes of data
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into the buffer and set (*pnData) to the actual number of bytes copied
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before returning SQLITE_OK. If the input is completely exhausted, (*pnData)
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should be set to zero to indicate this. Or, if an error occurs, an SQLite
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error code should be returned. In all cases, if an xInput callback returns
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an error, all processing is abandoned and the streaming API function
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returns a copy of the error code to the caller.</p>
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<p>In the case of sqlite3changeset_start_strm(), the xInput callback may be
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invoked by the sessions module at any point during the lifetime of the
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iterator. If such an xInput callback returns an error, the iterator enters
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an error state, whereby all subsequent calls to iterator functions
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immediately fail with the same error code as returned by xInput.</p>
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<p>Similarly, streaming API functions that return changesets (or patchsets)
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return them in chunks by way of a callback function instead of via a
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pointer to a single large buffer. In this case, a pair of parameters such
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as:</p>
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<p> <pre>
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int *pnChangeset,
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void **ppChangeset,
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</pre></p>
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<p>Is replaced by:</p>
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<p> <pre>
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int (*xOutput)(void *pOut, const void *pData, int nData),
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void *pOut
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</pre></p>
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<p>The xOutput callback is invoked zero or more times to return data to
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the application. The first parameter passed to each call is a copy of the
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pOut pointer supplied by the application. The second parameter, pData,
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points to a buffer nData bytes in size containing the chunk of output
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data being returned. If the xOutput callback successfully processes the
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supplied data, it should return SQLITE_OK to indicate success. Otherwise,
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it should return some other SQLite error code. In this case processing
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is immediately abandoned and the streaming API function returns a copy
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of the xOutput error code to the application.</p>
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<p>The sessions module never invokes an xOutput callback with the third
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parameter set to a value less than or equal to zero. Other than this,
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no guarantees are made as to the size of the chunks of data returned.
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</p><p>See also lists of
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<a href="objlist.html">Objects</a>,
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<a href="constlist.html">Constants</a>, and
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<a href="funclist.html">Functions</a>.</p>
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