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Improve comments and documentation of the asynchronous IO VFS module. (CVS 6543)
FossilOrigin-Name: 92bc6be2a86f8a68ceded2bc08fe7d6ff23b56fb
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danielk1977
@@ -5,32 +5,201 @@
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#define SQLITEASYNC_VFSNAME "sqlite3async"
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/*
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** Install the asynchronous IO VFS.
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** THREAD SAFETY NOTES:
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**
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** Of the four API functions in this file, the following are not threadsafe:
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**
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** sqlite3async_initialize()
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** sqlite3async_shutdown()
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**
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** Care must be taken that neither of these functions is called while
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** another thread may be calling either any sqlite3async_XXX() function
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** or an sqlite3_XXX() API function related to a database handle that
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** is using the asynchronous IO VFS.
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**
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** These functions:
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**
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** sqlite3async_run()
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** sqlite3async_control()
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**
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** are threadsafe. It is quite safe to call either of these functions even
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** if another thread may also be calling one of them or an sqlite3_XXX()
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** function related to a database handle that uses the asynchronous IO VFS.
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*/
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/*
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** Initialize the asynchronous IO VFS and register it with SQLite using
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** sqlite3_vfs_register(). If the asynchronous VFS is already initialized
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** and registered, this function is a no-op. The asynchronous IO VFS
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** is registered as "sqlite3async".
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**
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** The asynchronous IO VFS does not make operating system IO requests
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** directly. Instead, it uses an existing VFS implementation for all
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** required file-system operations. If the first parameter to this function
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** is NULL, then the current default VFS is used for IO. If it is not
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** NULL, then it must be the name of an existing VFS. In other words, the
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** first argument to this function is passed to sqlite3_vfs_find() to
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** locate the VFS to use for all real IO operations. This VFS is known
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** as the "parent VFS".
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**
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** If the second parameter to this function is non-zero, then the
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** asynchronous IO VFS is registered as the default VFS for all SQLite
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** database connections within the process. Otherwise, the asynchronous IO
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** VFS is only used by connections opened using sqlite3_open_v2() that
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** specifically request VFS "sqlite3async".
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**
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** If a parent VFS cannot be located, then SQLITE_ERROR is returned.
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** In the unlikely event that operating system specific initialization
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** fails (win32 systems create the required critical section and event
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** objects within this function), then SQLITE_ERROR is also returned.
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** Finally, if the call to sqlite3_vfs_register() returns an error, then
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** the error code is returned to the user by this function. In all three
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** of these cases, intialization has failed and the asynchronous IO VFS
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** is not registered with SQLite.
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**
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** Otherwise, if no error occurs, SQLITE_OK is returned.
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*/
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int sqlite3async_initialize(const char *zParent, int isDefault);
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/*
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** Uninstall the asynchronous IO VFS.
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** This function unregisters the asynchronous IO VFS using
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** sqlite3_vfs_unregister().
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**
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** On win32 platforms, this function also releases the small number of
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** critical section and event objects created by sqlite3async_initialize().
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*/
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void sqlite3async_shutdown();
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/*
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** Process events on the write-queue.
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** This function may only be called when the asynchronous IO VFS is
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** installed (after a call to sqlite3async_initialize()). It processes
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** zero or more queued write operations before returning. It is expected
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** (but not required) that this function will be called by a different
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** thread than those threads that use SQLite. The "background thread"
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** that performs IO.
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**
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** How many queued write operations are performed before returning
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** depends on the global setting configured by passing the SQLITEASYNC_HALT
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** verb to sqlite3async_control() (see below for details). By default
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** this function never returns - it processes all pending operations and
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** then blocks waiting for new ones.
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**
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** If multiple simultaneous calls are made to sqlite3async_run() from two
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** or more threads, then the calls are serialized internally.
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*/
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void sqlite3async_run();
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/*
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** Control/configure the asynchronous IO system.
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** This function may only be called when the asynchronous IO VFS is
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** installed (after a call to sqlite3async_initialize()). It is used
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** to query or configure various parameters that affect the operation
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** of the asynchronous IO VFS. At present there are three parameters
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** supported:
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**
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** * The "halt" parameter, which configures the circumstances under
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** which the sqlite3async_run() parameter is configured.
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**
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** * The "delay" parameter. Setting the delay parameter to a non-zero
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** value causes the sqlite3async_run() function to sleep for the
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** configured number of milliseconds between each queued write
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** operation.
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**
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** * The "lockfiles" parameter. This parameter determines whether or
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** not the asynchronous IO VFS locks the database files it operates
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** on. Disabling file locking can improve throughput.
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**
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** This function is always passed two arguments. When setting the value
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** of a parameter, the first argument must be one of SQLITEASYNC_HALT,
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** SQLITEASYNC_DELAY or SQLITEASYNC_LOCKFILES. The second argument must
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** be passed the new value for the parameter as type "int".
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**
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** When querying the current value of a paramter, the first argument must
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** be one of SQLITEASYNC_GET_HALT, GET_DELAY or GET_LOCKFILES. The second
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** argument to this function must be of type (int *). The current value
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** of the queried parameter is copied to the memory pointed to by the
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** second argument. For example:
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**
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** int eCurrentHalt;
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** int eNewHalt = SQLITEASYNC_HALT_IDLE;
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**
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** sqlite3async_control(SQLITEASYNC_HALT, eNewHalt);
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** sqlite3async_control(SQLITEASYNC_GET_HALT, &eCurrentHalt);
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** assert( eNewHalt==eCurrentHalt );
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**
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** See below for more detail on each configuration parameter.
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**
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** SQLITEASYNC_HALT:
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**
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** This is used to set the value of the "halt" parameter. The second
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** argument must be one of the SQLITEASYNC_HALT_XXX symbols defined
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** below (either NEVER, IDLE and NOW).
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**
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** If the parameter is set to NEVER, then calls to sqlite3async_run()
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** never return. This is the default setting. If the parameter is set
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** to IDLE, then calls to sqlite3async_run() return as soon as the
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** queue of pending write operations is empty. If the parameter is set
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** to NOW, then calls to sqlite3async_run() return as quickly as
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** possible, without processing any pending write requests.
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**
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** If an attempt is made to set this parameter to an integer value other
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** than SQLITEASYNC_HALT_NEVER, IDLE or NOW, then sqlite3async_control()
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** returns SQLITE_MISUSE and the current value of the parameter is not
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** modified.
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**
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** Modifying the "halt" parameter affects calls to sqlite3async_run()
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** made by other threads that are currently in progress.
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**
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** SQLITEASYNC_DELAY:
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**
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** This is used to set the value of the "delay" parameter. If set to
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** a non-zero value, then after completing a pending write request, the
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** sqlite3async_run() function sleeps for the configured number of
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** milliseconds.
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**
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** If an attempt is made to set this parameter to a negative value,
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** sqlite3async_control() returns SQLITE_MISUSE and the current value
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** of the parameter is not modified.
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**
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** Modifying the "delay" parameter affects calls to sqlite3async_run()
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** made by other threads that are currently in progress.
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**
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** SQLITEASYNC_LOCKFILES:
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**
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** This is used to set the value of the "lockfiles" parameter. This
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** parameter must be set to either 0 or 1. If set to 1, then the
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** asynchronous IO VFS uses the xLock() and xUnlock() methods of the
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** parent VFS to lock database files being read and/or written. If
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** the parameter is set to 0, then these locks are omitted.
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**
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** This parameter may only be set when there are no open database
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** connections using the VFS and the queue of pending write requests
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** is empty. Attempting to set it when this is not true, or to set it
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** to a value other than 0 or 1 causes sqlite3async_control() to return
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** SQLITE_MISUSE and the value of the parameter to remain unchanged.
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**
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** If this parameter is set to zero, then it is only safe to access the
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** database via the asynchronous IO VFS from within a single process. If
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** while writing to the database via the asynchronous IO VFS the database
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** is also read or written from within another process, or via another
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** connection that does not use the asynchronous IO VFS within the same
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** process, the results are undefined (and may include crashes or database
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** corruption).
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**
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** Alternatively, if this parameter is set to 1, then it is safe to access
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** the database from multiple connections within multiple processes using
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** either the asynchronous IO VFS or the parent VFS directly.
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*/
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int sqlite3async_control(int op, ...);
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/*
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** Values that can be used as the first argument to sqlite3async_control().
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*/
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#define SQLITEASYNC_HALT 1
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#define SQLITEASYNC_DELAY 2
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#define SQLITEASYNC_GET_HALT 3
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#define SQLITEASYNC_GET_DELAY 4
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#define SQLITEASYNC_HALT 1
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#define SQLITEASYNC_GET_HALT 2
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#define SQLITEASYNC_DELAY 3
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#define SQLITEASYNC_GET_DELAY 4
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#define SQLITEASYNC_LOCKFILES 5
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#define SQLITEASYNC_GET_LOCKFILES 6
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/*
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** If the first argument to sqlite3async_control() is SQLITEASYNC_HALT,
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