diff options
Diffstat (limited to 'www/c3ref/mem_methods.html')
-rw-r--r-- | www/c3ref/mem_methods.html | 200 |
1 files changed, 200 insertions, 0 deletions
diff --git a/www/c3ref/mem_methods.html b/www/c3ref/mem_methods.html new file mode 100644 index 0000000..35a76fa --- /dev/null +++ b/www/c3ref/mem_methods.html @@ -0,0 +1,200 @@ +<!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>Memory Allocation Routines</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. Reliable.<br>Choose any three. +</div> +<div class="menu mainmenu"> +<ul> +<li><a href="../index.html">Home</a> +<li class='mobileonly'><a href="javascript:void(0)" onclick='toggle_div("submenu")'>Menu</a> +<li class='wideonly'><a href='../about.html'>About</a> +<li class='desktoponly'><a href="../docs.html">Documentation</a> +<li class='desktoponly'><a href="../download.html">Download</a> +<li class='wideonly'><a href='../copyright.html'>License</a> +<li class='desktoponly'><a href="../support.html">Support</a> +<li class='desktoponly'><a href="../prosupport.html">Purchase</a> +<li class='search' id='search_menubutton'> +<a href="javascript:void(0)" onclick='toggle_search()'>Search</a> +</ul> +</div> +<div class="menu submenu" id="submenu"> +<ul> +<li><a href='../about.html'>About</a> +<li><a href='../docs.html'>Documentation</a> +<li><a href='../download.html'>Download</a> +<li><a href='../support.html'>Support</a> +<li><a href='../prosupport.html'>Purchase</a> +</ul> +</div> +<div class="searchmenu" id="searchmenu"> +<form method="GET" action="../search"> +<select name="s" id="searchtype"> +<option value="d">Search Documentation</option> +<option value="c">Search Changelog</option> +</select> +<input type="text" name="q" id="searchbox" value=""> +<input type="submit" value="Go"> +</form> +</div> +</div> +<script> +function toggle_div(nm) { +var w = document.getElementById(nm); +if( w.style.display=="block" ){ +w.style.display = "none"; +}else{ +w.style.display = "block"; +} +} +function toggle_search() { +var w = document.getElementById("searchmenu"); +if( w.style.display=="block" ){ +w.style.display = "none"; +} else { +w.style.display = "block"; +setTimeout(function(){ +document.getElementById("searchbox").focus() +}, 30); +} +} +function div_off(nm){document.getElementById(nm).style.display="none";} +window.onbeforeunload = function(e){div_off("submenu");} +/* Disable the Search feature if we are not operating from CGI, since */ +/* Search is accomplished using CGI and will not work without it. */ +if( !location.origin || !location.origin.match || !location.origin.match(/http/) ){ +document.getElementById("search_menubutton").style.display = "none"; +} +/* Used by the Hide/Show button beside syntax diagrams, to toggle the */ +function hideorshow(btn,obj){ +var x = document.getElementById(obj); +var b = document.getElementById(btn); +if( x.style.display!='none' ){ +x.style.display = 'none'; +b.innerHTML='show'; +}else{ +x.style.display = ''; +b.innerHTML='hide'; +} +return false; +} +var antiRobot = 0; +function antiRobotGo(){ +if( antiRobot!=3 ) return; +antiRobot = 7; +var j = document.getElementById("mtimelink"); +if(j && j.hasAttribute("data-href")) j.href=j.getAttribute("data-href"); +} +function antiRobotDefense(){ +document.body.onmousedown=function(){ +antiRobot |= 2; +antiRobotGo(); +document.body.onmousedown=null; +} +document.body.onmousemove=function(){ +antiRobot |= 2; +antiRobotGo(); +document.body.onmousemove=null; +} +setTimeout(function(){ +antiRobot |= 1; +antiRobotGo(); +}, 100) +antiRobotGo(); +} +antiRobotDefense(); +</script> +<!-- keywords: sqlite3_mem_methods --> +<div class=nosearch> +<a href="../c3ref/intro.html"><h2>SQLite C Interface</h2></a> +<h2>Memory Allocation Routines</h2> +</div> +<blockquote><pre> +typedef struct sqlite3_mem_methods sqlite3_mem_methods; +struct sqlite3_mem_methods { + void *(*xMalloc)(int); /* Memory allocation function */ + void (*xFree)(void*); /* Free a prior allocation */ + void *(*xRealloc)(void*,int); /* Resize an allocation */ + int (*xSize)(void*); /* Return the size of an allocation */ + int (*xRoundup)(int); /* Round up request size to allocation size */ + int (*xInit)(void*); /* Initialize the memory allocator */ + void (*xShutdown)(void*); /* Deinitialize the memory allocator */ + void *pAppData; /* Argument to xInit() and xShutdown() */ +}; +</pre></blockquote> +<p> +An instance of this object defines the interface between SQLite +and low-level memory allocation routines.</p> + +<p>This object is used in only one place in the SQLite interface. +A pointer to an instance of this object is the argument to +<a href="../c3ref/config.html">sqlite3_config()</a> when the configuration option is +<a href="../c3ref/c_config_covering_index_scan.html#sqliteconfigmalloc">SQLITE_CONFIG_MALLOC</a> or <a href="../c3ref/c_config_covering_index_scan.html#sqliteconfiggetmalloc">SQLITE_CONFIG_GETMALLOC</a>. +By creating an instance of this object +and passing it to <a href="../c3ref/config.html">sqlite3_config</a>(<a href="../c3ref/c_config_covering_index_scan.html#sqliteconfigmalloc">SQLITE_CONFIG_MALLOC</a>) +during configuration, an application can specify an alternative +memory allocation subsystem for SQLite to use for all of its +dynamic memory needs.</p> + +<p>Note that SQLite comes with several <a href="../malloc.html#altalloc">built-in memory allocators</a> +that are perfectly adequate for the overwhelming majority of applications +and that this object is only useful to a tiny minority of applications +with specialized memory allocation requirements. This object is +also used during testing of SQLite in order to specify an alternative +memory allocator that simulates memory out-of-memory conditions in +order to verify that SQLite recovers gracefully from such +conditions.</p> + +<p>The xMalloc, xRealloc, and xFree methods must work like the +malloc(), realloc() and free() functions from the standard C library. +SQLite guarantees that the second argument to +xRealloc is always a value returned by a prior call to xRoundup.</p> + +<p>xSize should return the allocated size of a memory allocation +previously obtained from xMalloc or xRealloc. The allocated size +is always at least as big as the requested size but may be larger.</p> + +<p>The xRoundup method returns what would be the allocated size of +a memory allocation given a particular requested size. Most memory +allocators round up memory allocations at least to the next multiple +of 8. Some allocators round up to a larger multiple or to a power of 2. +Every memory allocation request coming in through <a href="../c3ref/free.html">sqlite3_malloc()</a> +or <a href="../c3ref/free.html">sqlite3_realloc()</a> first calls xRoundup. If xRoundup returns 0, +that causes the corresponding memory allocation to fail.</p> + +<p>The xInit method initializes the memory allocator. For example, +it might allocate any required mutexes or initialize internal data +structures. The xShutdown method is invoked (indirectly) by +<a href="../c3ref/initialize.html">sqlite3_shutdown()</a> and should deallocate any resources acquired +by xInit. The pAppData pointer is used as the only parameter to +xInit and xShutdown.</p> + +<p>SQLite holds the <a href="../c3ref/c_mutex_fast.html">SQLITE_MUTEX_STATIC_MAIN</a> mutex when it invokes +the xInit method, so the xInit method need not be threadsafe. The +xShutdown method is only called from <a href="../c3ref/initialize.html">sqlite3_shutdown()</a> so it does +not need to be threadsafe either. For all other methods, SQLite +holds the <a href="../c3ref/c_mutex_fast.html">SQLITE_MUTEX_STATIC_MEM</a> mutex as long as the +<a href="../c3ref/c_config_covering_index_scan.html#sqliteconfigmemstatus">SQLITE_CONFIG_MEMSTATUS</a> configuration option is turned on (which +it is by default) and so the methods are automatically serialized. +However, if <a href="../c3ref/c_config_covering_index_scan.html#sqliteconfigmemstatus">SQLITE_CONFIG_MEMSTATUS</a> is disabled, then the other +methods must be threadsafe or else make their own arrangements for +serialization.</p> + +<p>SQLite will never invoke xInit() more than once without an intervening +call to xShutdown(). +</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> + |