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diff --git a/www/c3ref/mutex_alloc.html b/www/c3ref/mutex_alloc.html new file mode 100644 index 0000000..f32efce --- /dev/null +++ b/www/c3ref/mutex_alloc.html @@ -0,0 +1,247 @@ +<!DOCTYPE html> +<html><head> +<meta name="viewport" content="width=device-width, initial-scale=1.0"> +<meta http-equiv="content-type" content="text/html; charset=UTF-8"> +<link href="../sqlite.css" rel="stylesheet"> +<title>Mutexes</title> +<!-- path=../ --> +</head> +<body> +<div class=nosearch> +<a href="../index.html"> +<img class="logo" src="../images/sqlite370_banner.gif" alt="SQLite" border="0"> +</a> +<div><!-- IE hack to prevent disappearing logo --></div> +<div class="tagline desktoponly"> +Small. Fast. 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Though they are intended for internal +use by SQLite, code that links against SQLite is +permitted to use any of these routines.</p> + +<p>The SQLite source code contains multiple implementations +of these mutex routines. An appropriate implementation +is selected automatically at compile-time. The following +implementations are available in the SQLite core:</p> + +<p><ul> +<li> SQLITE_MUTEX_PTHREADS +<li> SQLITE_MUTEX_W32 +<li> SQLITE_MUTEX_NOOP +</ul></p> + +<p>The SQLITE_MUTEX_NOOP implementation is a set of routines +that does no real locking and is appropriate for use in +a single-threaded application. The SQLITE_MUTEX_PTHREADS and +SQLITE_MUTEX_W32 implementations are appropriate for use on Unix +and Windows.</p> + +<p>If SQLite is compiled with the SQLITE_MUTEX_APPDEF preprocessor +macro defined (with "-DSQLITE_MUTEX_APPDEF=1"), then no mutex +implementation is included with the library. In this case the +application must supply a custom mutex implementation using the +<a href="../c3ref/c_config_covering_index_scan.html#sqliteconfigmutex">SQLITE_CONFIG_MUTEX</a> option of the sqlite3_config() function +before calling sqlite3_initialize() or any other public sqlite3_ +function that calls sqlite3_initialize().</p> + +<p>The sqlite3_mutex_alloc() routine allocates a new +mutex and returns a pointer to it. The sqlite3_mutex_alloc() +routine returns NULL if it is unable to allocate the requested +mutex. The argument to sqlite3_mutex_alloc() must one of these +integer constants:</p> + +<p><ul> +<li> SQLITE_MUTEX_FAST +<li> SQLITE_MUTEX_RECURSIVE +<li> SQLITE_MUTEX_STATIC_MAIN +<li> SQLITE_MUTEX_STATIC_MEM +<li> SQLITE_MUTEX_STATIC_OPEN +<li> SQLITE_MUTEX_STATIC_PRNG +<li> SQLITE_MUTEX_STATIC_LRU +<li> SQLITE_MUTEX_STATIC_PMEM +<li> SQLITE_MUTEX_STATIC_APP1 +<li> SQLITE_MUTEX_STATIC_APP2 +<li> SQLITE_MUTEX_STATIC_APP3 +<li> SQLITE_MUTEX_STATIC_VFS1 +<li> SQLITE_MUTEX_STATIC_VFS2 +<li> SQLITE_MUTEX_STATIC_VFS3 +</ul></p> + +<p>The first two constants (SQLITE_MUTEX_FAST and SQLITE_MUTEX_RECURSIVE) +cause sqlite3_mutex_alloc() to create +a new mutex. The new mutex is recursive when SQLITE_MUTEX_RECURSIVE +is used but not necessarily so when SQLITE_MUTEX_FAST is used. +The mutex implementation does not need to make a distinction +between SQLITE_MUTEX_RECURSIVE and SQLITE_MUTEX_FAST if it does +not want to. SQLite will only request a recursive mutex in +cases where it really needs one. If a faster non-recursive mutex +implementation is available on the host platform, the mutex subsystem +might return such a mutex in response to SQLITE_MUTEX_FAST.</p> + +<p>The other allowed parameters to sqlite3_mutex_alloc() (anything other +than SQLITE_MUTEX_FAST and SQLITE_MUTEX_RECURSIVE) each return +a pointer to a static preexisting mutex. Nine static mutexes are +used by the current version of SQLite. Future versions of SQLite +may add additional static mutexes. Static mutexes are for internal +use by SQLite only. Applications that use SQLite mutexes should +use only the dynamic mutexes returned by SQLITE_MUTEX_FAST or +SQLITE_MUTEX_RECURSIVE.</p> + +<p>Note that if one of the dynamic mutex parameters (SQLITE_MUTEX_FAST +or SQLITE_MUTEX_RECURSIVE) is used then sqlite3_mutex_alloc() +returns a different mutex on every call. For the static +mutex types, the same mutex is returned on every call that has +the same type number.</p> + +<p>The sqlite3_mutex_free() routine deallocates a previously +allocated dynamic mutex. Attempting to deallocate a static +mutex results in undefined behavior.</p> + +<p>The sqlite3_mutex_enter() and sqlite3_mutex_try() routines attempt +to enter a mutex. If another thread is already within the mutex, +sqlite3_mutex_enter() will block and sqlite3_mutex_try() will return +SQLITE_BUSY. The sqlite3_mutex_try() interface returns <a href="../rescode.html#ok">SQLITE_OK</a> +upon successful entry. Mutexes created using +SQLITE_MUTEX_RECURSIVE can be entered multiple times by the same thread. +In such cases, the +mutex must be exited an equal number of times before another thread +can enter. If the same thread tries to enter any mutex other +than an SQLITE_MUTEX_RECURSIVE more than once, the behavior is undefined.</p> + +<p>Some systems (for example, Windows 95) do not support the operation +implemented by sqlite3_mutex_try(). On those systems, sqlite3_mutex_try() +will always return SQLITE_BUSY. In most cases the SQLite core only uses +sqlite3_mutex_try() as an optimization, so this is acceptable +behavior. The exceptions are unix builds that set the +SQLITE_ENABLE_SETLK_TIMEOUT build option. In that case a working +sqlite3_mutex_try() is required.</p> + +<p>The sqlite3_mutex_leave() routine exits a mutex that was +previously entered by the same thread. The behavior +is undefined if the mutex is not currently entered by the +calling thread or is not currently allocated.</p> + +<p>If the argument to sqlite3_mutex_enter(), sqlite3_mutex_try(), +sqlite3_mutex_leave(), or sqlite3_mutex_free() is a NULL pointer, +then any of the four routines behaves as a no-op.</p> + +<p>See also: <a href="../c3ref/mutex_held.html">sqlite3_mutex_held()</a> and <a href="../c3ref/mutex_held.html">sqlite3_mutex_notheld()</a>. +</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> + |