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+<!-- keywords: sqlite3_io_methods -->
+<div class=nosearch>
+<a href="../c3ref/intro.html"><h2>SQLite C Interface</h2></a>
+<h2>OS Interface File Virtual Methods Object</h2>
+</div>
+<blockquote><pre>
+typedef struct sqlite3_io_methods sqlite3_io_methods;
+struct sqlite3_io_methods {
+  int iVersion;
+  int (*xClose)(sqlite3_file*);
+  int (*xRead)(sqlite3_file*, void*, int iAmt, sqlite3_int64 iOfst);
+  int (*xWrite)(sqlite3_file*, const void*, int iAmt, sqlite3_int64 iOfst);
+  int (*xTruncate)(sqlite3_file*, sqlite3_int64 size);
+  int (*xSync)(sqlite3_file*, int flags);
+  int (*xFileSize)(sqlite3_file*, sqlite3_int64 *pSize);
+  int (*xLock)(sqlite3_file*, int);
+  int (*xUnlock)(sqlite3_file*, int);
+  int (*xCheckReservedLock)(sqlite3_file*, int *pResOut);
+  int (*xFileControl)(sqlite3_file*, int op, void *pArg);
+  int (*xSectorSize)(sqlite3_file*);
+  int (*xDeviceCharacteristics)(sqlite3_file*);
+  /* Methods above are valid for version 1 */
+  int (*xShmMap)(sqlite3_file*, int iPg, int pgsz, int, void volatile**);
+  int (*xShmLock)(sqlite3_file*, int offset, int n, int flags);
+  void (*xShmBarrier)(sqlite3_file*);
+  int (*xShmUnmap)(sqlite3_file*, int deleteFlag);
+  /* Methods above are valid for version 2 */
+  int (*xFetch)(sqlite3_file*, sqlite3_int64 iOfst, int iAmt, void **pp);
+  int (*xUnfetch)(sqlite3_file*, sqlite3_int64 iOfst, void *p);
+  /* Methods above are valid for version 3 */
+  /* Additional methods may be added in future releases */
+};
+</pre></blockquote>
+<p>
+Every file opened by the <a href="../c3ref/vfs.html#sqlite3vfsxopen">sqlite3_vfs.xOpen</a> method populates an
+<a href="../c3ref/file.html">sqlite3_file</a> object (or, more commonly, a subclass of the
+<a href="../c3ref/file.html">sqlite3_file</a> object) with a pointer to an instance of this object.
+This object defines the methods used to perform various operations
+against the open file represented by the <a href="../c3ref/file.html">sqlite3_file</a> object.</p>
+
+<p>If the <a href="../c3ref/vfs.html#sqlite3vfsxopen">sqlite3_vfs.xOpen</a> method sets the sqlite3_file.pMethods element
+to a non-NULL pointer, then the sqlite3_io_methods.xClose method
+may be invoked even if the <a href="../c3ref/vfs.html#sqlite3vfsxopen">sqlite3_vfs.xOpen</a> reported that it failed.  The
+only way to prevent a call to xClose following a failed <a href="../c3ref/vfs.html#sqlite3vfsxopen">sqlite3_vfs.xOpen</a>
+is for the <a href="../c3ref/vfs.html#sqlite3vfsxopen">sqlite3_vfs.xOpen</a> to set the sqlite3_file.pMethods element
+to NULL.</p>
+
+<p>The flags argument to xSync may be one of <a href="../c3ref/c_sync_dataonly.html">SQLITE_SYNC_NORMAL</a> or
+<a href="../c3ref/c_sync_dataonly.html">SQLITE_SYNC_FULL</a>.  The first choice is the normal fsync().
+The second choice is a Mac OS X style fullsync.  The <a href="../c3ref/c_sync_dataonly.html">SQLITE_SYNC_DATAONLY</a>
+flag may be ORed in to indicate that only the data of the file
+and not its inode needs to be synced.</p>
+
+<p>The integer values to xLock() and xUnlock() are one of
+<ul>
+<li> <a href="../c3ref/c_lock_exclusive.html">SQLITE_LOCK_NONE</a>,
+<li> <a href="../c3ref/c_lock_exclusive.html">SQLITE_LOCK_SHARED</a>,
+<li> <a href="../c3ref/c_lock_exclusive.html">SQLITE_LOCK_RESERVED</a>,
+<li> <a href="../c3ref/c_lock_exclusive.html">SQLITE_LOCK_PENDING</a>, or
+<li> <a href="../c3ref/c_lock_exclusive.html">SQLITE_LOCK_EXCLUSIVE</a>.
+</ul>
+xLock() upgrades the database file lock.  In other words, xLock() moves the
+database file lock in the direction NONE toward EXCLUSIVE. The argument to
+xLock() is always on of SHARED, RESERVED, PENDING, or EXCLUSIVE, never
+SQLITE_LOCK_NONE.  If the database file lock is already at or above the
+requested lock, then the call to xLock() is a no-op.
+xUnlock() downgrades the database file lock to either SHARED or NONE.
+to xUnlock() is a no-op.
+The xCheckReservedLock() method checks whether any database connection,
+either in this process or in some other process, is holding a RESERVED,
+PENDING, or EXCLUSIVE lock on the file.  It returns true
+if such a lock exists and false otherwise.</p>
+
+<p>The xFileControl() method is a generic interface that allows custom
+VFS implementations to directly control an open file using the
+<a href="../c3ref/file_control.html">sqlite3_file_control()</a> interface.  The second "op" argument is an
+integer opcode.  The third argument is a generic pointer intended to
+point to a structure that may contain arguments or space in which to
+write return values.  Potential uses for xFileControl() might be
+functions to enable blocking locks with timeouts, to change the
+locking strategy (for example to use dot-file locks), to inquire
+about the status of a lock, or to break stale locks.  The SQLite
+core reserves all opcodes less than 100 for its own use.
+A <a href="../c3ref/c_fcntl_begin_atomic_write.html">list of opcodes</a> less than 100 is available.
+Applications that define a custom xFileControl method should use opcodes
+greater than 100 to avoid conflicts.  VFS implementations should
+return <a href="../rescode.html#notfound">SQLITE_NOTFOUND</a> for file control opcodes that they do not
+recognize.</p>
+
+<p>The xSectorSize() method returns the sector size of the
+device that underlies the file.  The sector size is the
+minimum write that can be performed without disturbing
+other bytes in the file.  The xDeviceCharacteristics()
+method returns a bit vector describing behaviors of the
+underlying device:</p>
+
+<p><ul>
+<li> <a href="../c3ref/c_iocap_atomic.html">SQLITE_IOCAP_ATOMIC</a>
+<li> <a href="../c3ref/c_iocap_atomic.html">SQLITE_IOCAP_ATOMIC512</a>
+<li> <a href="../c3ref/c_iocap_atomic.html">SQLITE_IOCAP_ATOMIC1K</a>
+<li> <a href="../c3ref/c_iocap_atomic.html">SQLITE_IOCAP_ATOMIC2K</a>
+<li> <a href="../c3ref/c_iocap_atomic.html">SQLITE_IOCAP_ATOMIC4K</a>
+<li> <a href="../c3ref/c_iocap_atomic.html">SQLITE_IOCAP_ATOMIC8K</a>
+<li> <a href="../c3ref/c_iocap_atomic.html">SQLITE_IOCAP_ATOMIC16K</a>
+<li> <a href="../c3ref/c_iocap_atomic.html">SQLITE_IOCAP_ATOMIC32K</a>
+<li> <a href="../c3ref/c_iocap_atomic.html">SQLITE_IOCAP_ATOMIC64K</a>
+<li> <a href="../c3ref/c_iocap_atomic.html">SQLITE_IOCAP_SAFE_APPEND</a>
+<li> <a href="../c3ref/c_iocap_atomic.html">SQLITE_IOCAP_SEQUENTIAL</a>
+<li> <a href="../c3ref/c_iocap_atomic.html">SQLITE_IOCAP_UNDELETABLE_WHEN_OPEN</a>
+<li> <a href="../c3ref/c_iocap_atomic.html">SQLITE_IOCAP_POWERSAFE_OVERWRITE</a>
+<li> <a href="../c3ref/c_iocap_atomic.html">SQLITE_IOCAP_IMMUTABLE</a>
+<li> <a href="../c3ref/c_iocap_atomic.html">SQLITE_IOCAP_BATCH_ATOMIC</a>
+</ul></p>
+
+<p>The SQLITE_IOCAP_ATOMIC property means that all writes of
+any size are atomic.  The SQLITE_IOCAP_ATOMICnnn values
+mean that writes of blocks that are nnn bytes in size and
+are aligned to an address which is an integer multiple of
+nnn are atomic.  The SQLITE_IOCAP_SAFE_APPEND value means
+that when data is appended to a file, the data is appended
+first then the size of the file is extended, never the other
+way around.  The SQLITE_IOCAP_SEQUENTIAL property means that
+information is written to disk in the same order as calls
+to xWrite().</p>
+
+<p>If xRead() returns SQLITE_IOERR_SHORT_READ it must also fill
+in the unread portions of the buffer with zeros.  A VFS that
+fails to zero-fill short reads might seem to work.  However,
+failure to zero-fill short reads will eventually lead to
+database corruption.
+</p><p>See also lists of
+  <a href="../c3ref/objlist.html">Objects</a>,
+  <a href="../c3ref/constlist.html">Constants</a>, and
+  <a href="../c3ref/funclist.html">Functions</a>.</p>
+