1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
|
<!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>The SQLite OS Interface or "VFS"</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>
<div class=fancy>
<div class=nosearch>
<div class="fancy_title">
The SQLite OS Interface or "VFS"
</div>
<div class="fancy_toc">
<a onclick="toggle_toc()">
<span class="fancy_toc_mark" id="toc_mk">►</span>
Table Of Contents
</a>
<div id="toc_sub"><div class="fancy-toc1"><a href="#introduction">1. Introduction</a></div>
<div class="fancy-toc1"><a href="#the_vfs_in_relation_to_the_rest_of_sqlite">2. The VFS In Relation To The Rest Of SQLite</a></div>
<div class="fancy-toc1"><a href="#multiple_vfses">3. Multiple VFSes</a></div>
<div class="fancy-toc2"><a href="#standard_unix_vfses">3.1. Standard Unix VFSes</a></div>
<div class="fancy-toc2"><a href="#standard_windows_vfses">3.2. Standard Windows VFSes</a></div>
<div class="fancy-toc2"><a href="#specifying_which_vfs_to_use">3.3. Specifying Which VFS To Use</a></div>
<div class="fancy-toc2"><a href="#vfs_shims">3.4. VFS Shims</a></div>
<div class="fancy-toc2"><a href="#other_example_vfses">3.5. Other Example VFSes</a></div>
<div class="fancy-toc1"><a href="#vfs_implementations">4. VFS Implementations</a></div>
</div>
</div>
<script>
function toggle_toc(){
var sub = document.getElementById("toc_sub")
var mk = document.getElementById("toc_mk")
if( sub.style.display!="block" ){
sub.style.display = "block";
mk.innerHTML = "▼";
} else {
sub.style.display = "none";
mk.innerHTML = "►";
}
}
</script>
</div>
<h1 id="introduction"><span>1. </span>Introduction</h1>
<p>
This article describes the SQLite OS portability layer or "VFS" - the
module at the bottom of the SQLite implementation stack
that provides portability across operating systems.
</p>
<h1 id="the_vfs_in_relation_to_the_rest_of_sqlite"><span>2. </span>The VFS In Relation To The Rest Of SQLite</h1>
<div>
<img src="images/vfs1.gif" align="right" hspace="10">
</div>
<p>
The internal organization of the SQLite library can be viewed as the
stack of modules shown to the right.
The Tokenizer, Parser, and Code Generator components are used to
process SQL statements and convert them into executable programs
in a virtual machine language or byte code.
Roughly speaking, these top three layers implement
<a href="c3ref/prepare.html">sqlite3_prepare_v2()</a>. The byte code generated by the top three
layers is a <a href="c3ref/stmt.html">prepared statement</a>.
The Virtual Machine module is responsible for running the SQL statement
byte code. The B-Tree module organizes a database file into multiple
key/value stores with ordered keys and logarithmic performance.
The Pager module is responsible for loading pages of the database
file into memory, for implementing and controlling transactions, and
for creating and maintaining the journal files that prevent database
corruption following a crash or power failure.
The OS Interface is a thin abstraction that provides a common set of
routines for adapting SQLite to run on different operating systems.
Roughly speaking, the bottom four layers implement
<a href="c3ref/step.html">sqlite3_step()</a>.
</p>
<p>
This article is about the bottom layer.
</p>
<p>The OS Interface - also called the "VFS" - is what makes SQLite
portable across operating systems. Whenever any of the other modules
in SQLite needs to communicate with the operating
system, they invoke methods in the VFS. The VFS then invokes the
operating-specific code needed to satisfy the request.
Hence, porting SQLite to a new
operating system is simply a matter of writing a new OS interface layer
or "VFS".</p>
<h1 id="multiple_vfses"><span>3. </span>Multiple VFSes</h1>
<p>
The standard SQLite source tree contains built-in VFSes for unix
and windows. Alternative VFSes can be
added at start-time or run-time using the
<a href="c3ref/vfs_find.html">sqlite3_vfs_register()</a> interface.
</p>
<p>
Multiple VFSes can be registered at the same time.
Each VFS has a unique names.
Separate <a href="c3ref/sqlite3.html">database connections</a> within the same process can be using
different VFSes at the same time. For that matter, if a single
database connection has multiple database files open using
the <a href="lang_attach.html">ATTACH</a> command, then each attached database might be using a
different VFS.
</p>
<h2 id="standard_unix_vfses"><span>3.1. </span>Standard Unix VFSes</h2>
<p>
Unix builds come with multiple built-in VFSes. The default VFS
for unix is called "unix" and is used in most applications.
Other VFSes that might be found in unix (depending on compile-time
options) include:
</p>
<ol>
<li><p><b>unix-dotfile</b> - uses dot-file locking rather than
POSIX advisory locks.
</p></li><li><p><b>unix-excl</b> - obtains and holds an exclusive lock on
database files, preventing other processes from accessing the
database. Also keeps the <a href="walformat.html#shm">wal-index</a> in heap rather than in
shared memory.
</p></li><li><p><b>unix-none</b> - all file locking operations are no-ops.
</p></li><li><p><b>unix-namedsem</b> - uses named semaphores for file locking.
VXWorks only.
</p></li></ol>
<p>
The various unix VFSes differ only in the way they handle file locking -
they share most of their implementation in common with one another and
are all located in the same SQLite source file:
<a href="https://www.sqlite.org/src/doc/trunk/src/os_unix.c">os_unix.c</a>.
Note that except for "unix" and "unix-excl", the various unix VFSes all
use incompatible locking implementations. If two processes are accessing
the same SQLite database using different unix VFSes, they may
not see each others locks and may end up interfering with one another,
resulting in database corruption. The "unix-none" VFS in particular
does no locking at all and will easily result in database corruption if
used by two or more database connections at the same time.
Programmers are encouraged to use only "unix" or "unix-excl" unless
there is a compelling reason to do otherwise.
</p>
<h2 id="standard_windows_vfses"><span>3.2. </span>Standard Windows VFSes</h2>
<p>
Windows builds also come with multiple built-in VFSes. The default
Windows VFS is called "win32" and is used in most applications.
Other VFSes that might be found on windows builds include:
</p>
<ol>
<li><p><b>win32-longpath</b> - like "win32" except that pathnames can
be up to 65534 bytes in length, whereas pathnames max out at
1040 bytes in "win32".
</p></li><li><p><b>win32-none</b> - all file locking operations are no-ops.
</p></li><li><p><b>win32-longpath-none</b> - combination of "win32-longpath"
and "win32-none" - long pathnames are supported and all lock
operations are no-ops.
</p></li></ol>
<p>As with unix, most of the code for the various Windows VFSes is shared.
</p><h2 id="specifying_which_vfs_to_use"><span>3.3. </span>Specifying Which VFS To Use</h2>
<p>
There is always one VFS which is the default VFS. On unix systems,
the "unix" VFS comes up as the default and on windows it is "win32".
If no other actions are taken, new database connections will make use
of the default VFS.
</p>
<p>
The default VFS can be changed by registering or re-registering the
VFS using the <a href="c3ref/vfs_find.html">sqlite3_vfs_register()</a> interface with a second parameter
of 1. Hence, if a (unix) process wants to always use the "unix-nolock" VFS
in place of "unix", the following code would work:
</p>
<blockquote><pre>
sqlite3_vfs_register(sqlite3_vfs_find("unix-nolock"), 1);
</pre></blockquote>
<p>
An alternate VFS can also be specified as the 4th parameter to the
<a href="c3ref/open.html">sqlite3_open_v2()</a> function. For example:
</p>
<blockquote><pre>
int rc = sqlite3_open_v2("demo.db", &db, SQLITE_OPEN_READWRITE, "unix-nolock");
</pre></blockquote>
<p>
Finally, if <a href="uri.html">URI filenames</a> have been enabled, then the alternative
VFS can be specified using the "vfs=" parameter on the URI. This technique
works with <a href="c3ref/open.html">sqlite3_open()</a>, <a href="c3ref/open.html">sqlite3_open16()</a>, <a href="c3ref/open.html">sqlite3_open_v2()</a>, and
when a new database is <a href="lang_attach.html">ATTACH</a>-ed to an existing database connection.
For example:
</p>
<blockquote><pre>
ATTACH 'file:demo2.db?vfs=unix-none' AS demo2;
</pre></blockquote>
<p>
The VFS specified by a URI has the highest priority. After that comes
a VFS specified as the fourth argument to <a href="c3ref/open.html">sqlite3_open_v2()</a>. The
default VFS is used if no VFS is specified otherwise.
</p>
<a name="shim"></a>
<h2 id="vfs_shims"><span>3.4. </span>VFS Shims</h2>
<p>
From the point of view of the uppers layers of the SQLite stack, each
open database file uses exactly one VFS.
But in practice, a particular VFS might
just be a thin wrapper around another VFS that does the real work.
We call a wrapper VFS a "shim".
</p>
<p>
A simple example of a shim is the "vfstrace" VFS. This is a VFS
(implemented in the
<a href="https://www.sqlite.org/src/doc/trunk/src/test_vfstrace.c">test_vfstrace.c</a>
source file) that writes a message associated with each VFS method call
into a log file, then passes control off to another VFS to do the actual
work.
</p>
<h2 id="other_example_vfses"><span>3.5. </span>Other Example VFSes</h2>
<p>
The following are other VFS implementations available in the public
SQLite source tree:
</p>
<ul>
<li><p>
<a href="https://www.sqlite.org/src/file/ext/misc/appendvfs.c">appendvfs.c</a> -
This VFS allows an SQLite database to be appended to the end of some
other file. This can be used, for example, to append an SQLite database
onto the end of an executable such that, when run, it can easily
locate the appended database. The <a href="cli.html">command-line shell</a> will use this
VFS if launched with the --append option, and its .archive command
will use it given the --append flag.
</p></li><li><p>
<a href="https://www.sqlite.org/src/doc/trunk/src/test_demovfs.c">test_demovfs.c</a> -
This file implements a very simple VFS named "demo" that uses POSIX
functions such as
open(), read(), write(), fsync(), close(), fsync(), sleep(), time(),
and so forth. This VFS only works on unix systems. But it is not
intended as a replacement for the standard "unix" VFS used by default
on unix platforms. The "demo" VFS is deliberately kept very simple
so that it can be used as a learning aid or as template for building
other VFSes or for porting SQLite to new operating systems.
</p></li><li><p>
<a href="https://www.sqlite.org/src/doc/trunk/src/test_quota.c">test_quota.c</a> -
This file implements a shim called "quota" that enforces cumulative
file size limits on a collection of database files. An auxiliary
interface is used to define "quota groups". A quota group is a
set of files (database files, journals, and temporary files) whose
names all match a <a href="lang_expr.html#glob">GLOB</a> pattern. The sum of the sizes of all files
in each quota group is tracked, and if that sum exceeds a threshold
defined for the quota group, a callback function is invoked. That
callback can either increase the threshold or cause the operation
that would have exceeded the quota to fail with an
<a href="rescode.html#full">SQLITE_FULL</a> error. One of the uses of this shim is used to enforce
resource limits on application databases in Firefox.
</p></li><li><p>
<a href="https://www.sqlite.org/src/doc/trunk/src/test_multiplex.c">test_multiplex.c</a> -
This file implements a shim that allows database files to exceed the
maximum file size of the underlying filesystem. This shim presents
an interface to the upper six layers of SQLite that makes it look like
very large files are being used, when in reality each such large file
is split up into many smaller files on the underlying system.
This shim has been used, for example, to allow databases to grow
larger than 2 gibibytes on FAT16 filesystems.
</p></li><li><p>
<a href="https://www.sqlite.org/src/doc/trunk/src/test_onefile.c">test_onefile.c</a> -
This file implements a demonstration VFS named "fs" that shows how SQLite
can be used on an embedded device that lacks a filesystem. Content is
written directly to the underlying media. A VFS derived from this
demonstration code could be used by a gadget with a limited amount of
flash memory to make SQLite behave as the filesystem for the flash memory
on the device.
</p></li><li><p>
<a href="https://www.sqlite.org/src/doc/trunk/src/test_journal.c">test_journal.c</a> -
This file implements a shim used during SQLite testing that verifies that
the database and rollback journal are written in the correct order and
are "synced" at appropriate times in order to guarantee that the database
can recover from a power lose are hard reset at any time. The shim
checks several invariants on the operation of databases and rollback
journals and raises exceptions if any of those invariants are violated.
These invariants, in turn, assure that the database is always recoverable.
Running a large suite of test cases using this shim provides added
assurance that SQLite databases will not be damaged by unexpected
power failures or device resets.
</p></li><li><p>
<a href="https://www.sqlite.org/src/doc/trunk/src/test_vfs.c">test_vfs.c</a> -
This file implements a shim that can be used to simulate filesystem faults.
This shim is used during testing to verify that SQLite responses sanely
to hardware malfunctions or to other error conditions such as running out
of filesystem space that are difficult to test on a real system.
</p></li></ul>
<p>
There are other VFS implementations both in the core SQLite source code
library and in available extensions. The list above is not meant to be
exhaustive but merely representative of the kinds of features that can
be realized using the VFS interface.
</p>
<h1 id="vfs_implementations"><span>4. </span>VFS Implementations</h1>
<p>
A new VFS is implemented by subclassing three objects:
</p>
<ul>
<li><a href="c3ref/vfs.html">sqlite3_vfs</a>
</li><li><a href="c3ref/io_methods.html">sqlite3_io_methods</a>
</li><li><a href="c3ref/file.html">sqlite3_file</a>
</li></ul>
<p>
An <a href="c3ref/vfs.html">sqlite3_vfs</a> object defines the name of the VFS and the core
methods that implement the interface to the operating system, such
as checking for existence of files, deleting files, creating files
and opening and for reading and/or writing, converting filenames
into their canonical form. The <a href="c3ref/vfs.html">sqlite3_vfs</a> object also contains
methods for obtaining randomness from the operating system, for
suspending a process (sleeping) and for finding the current date and
time.
</p>
<p>
The <a href="c3ref/file.html">sqlite3_file</a> object represents an open file.
The xOpen method of <a href="c3ref/vfs.html">sqlite3_vfs</a> constructs an <a href="c3ref/file.html">sqlite3_file</a>
object when the file is opened. The <a href="c3ref/file.html">sqlite3_file</a> keeps track
of the state of the file while it is opened.
</p>
<p>
The <a href="c3ref/io_methods.html">sqlite3_io_methods</a> object holds the methods used to interact
with an open file. Each <a href="c3ref/file.html">sqlite3_file</a> contains a pointer to
an <a href="c3ref/io_methods.html">sqlite3_io_methods</a> object that is appropriate for the file it
represents. The <a href="c3ref/io_methods.html">sqlite3_io_methods</a> object contains methods to do
things such as read and write from the file, to truncate the file,
to flush any changes to persistent storage, to find the size of the
file, to lock and unlock the file, and to close file and destroy
the <a href="c3ref/file.html">sqlite3_file</a> object.
</p>
<p>
Writing the code for a new VFS involves constructing a subclass for
the <a href="c3ref/vfs.html">sqlite3_vfs</a> object and then registering that VFS object using
a call to <a href="c3ref/vfs_find.html">sqlite3_vfs_register()</a>. The VFS implementation also
provides subclasses for <a href="c3ref/file.html">sqlite3_file</a> and <a href="c3ref/io_methods.html">sqlite3_io_methods</a> but
those objects are not registered directly with SQLite. Instead, the
<a href="c3ref/file.html">sqlite3_file</a> object is returned from the xOpen method of
<a href="c3ref/vfs.html">sqlite3_vfs</a> and the <a href="c3ref/file.html">sqlite3_file</a> object points to an instance
of the <a href="c3ref/io_methods.html">sqlite3_io_methods</a> object.
</p>
<p align="center"><small><i>This page last modified on <a href="https://sqlite.org/docsrc/honeypot" id="mtimelink" data-href="https://sqlite.org/docsrc/finfo/pages/vfs.in?m=90886f1151">2024-02-22 15:53:45</a> UTC </small></i></p>
|
