4 ** The author disclaims copyright to this source code. In place of
5 ** a legal notice, here is a blessing:
7 ** May you do good and not evil.
8 ** May you find forgiveness for yourself and forgive others.
9 ** May you share freely, never taking more than you give.
11 *************************************************************************
13 ** An example of a simple VFS implementation that omits complex features
14 ** often not required or not possible on embedded platforms. Also includes
15 ** code to buffer writes to the journal file, which can be a significant
16 ** performance improvement on some embedded platforms.
23 ** The code in this file implements a minimal SQLite VFS that can be
24 ** used on Linux and other posix-like operating systems. The following
25 ** system calls are used:
27 ** File-system: access(), unlink(), getcwd()
28 ** File IO: open(), read(), write(), fsync(), close(), fstat()
29 ** Other: sleep(), usleep(), time()
31 ** The following VFS features are omitted:
33 ** 1. File locking. The user must ensure that there is at most one
34 ** connection to each database when using this VFS. Multiple
35 ** connections to a single shared-cache count as a single connection
36 ** for the purposes of the previous statement.
38 ** 2. The loading of dynamic extensions (shared libraries).
40 ** 3. Temporary files. The user must configure SQLite to use in-memory
41 ** temp files when using this VFS. The easiest way to do this is to
44 ** -DSQLITE_TEMP_STORE=3
46 ** 4. File truncation. As of version 3.6.24, SQLite may run without
47 ** a working xTruncate() call, providing the user does not configure
48 ** SQLite to use "journal_mode=truncate", or use both
49 ** "journal_mode=persist" and ATTACHed databases.
51 ** It is assumed that the system uses UNIX-like path-names. Specifically,
52 ** that '/' characters are used to separate path components and that
53 ** a path-name is a relative path unless it begins with a '/'. And that
54 ** no UTF-8 encoded paths are greater than 512 bytes in length.
56 ** JOURNAL WRITE-BUFFERING
58 ** To commit a transaction to the database, SQLite first writes rollback
59 ** information into the journal file. This usually consists of 4 steps:
61 ** 1. The rollback information is sequentially written into the journal
62 ** file, starting at the start of the file.
63 ** 2. The journal file is synced to disk.
64 ** 3. A modification is made to the first few bytes of the journal file.
65 ** 4. The journal file is synced to disk again.
67 ** Most of the data is written in step 1 using a series of calls to the
68 ** VFS xWrite() method. The buffers passed to the xWrite() calls are of
69 ** various sizes. For example, as of version 3.6.24, when committing a
70 ** transaction that modifies 3 pages of a database file that uses 4096
71 ** byte pages residing on a media with 512 byte sectors, SQLite makes
72 ** eleven calls to the xWrite() method to create the rollback journal,
75 ** Write offset | Bytes written
76 ** ----------------------------
87 ** ++++++++++++SYNC+++++++++++
89 ** ++++++++++++SYNC+++++++++++
91 ** On many operating systems, this is an efficient way to write to a file.
92 ** However, on some embedded systems that do not cache writes in OS
93 ** buffers it is much more efficient to write data in blocks that are
94 ** an integer multiple of the sector-size in size and aligned at the
97 ** To work around this, the code in this file allocates a fixed size
98 ** buffer of SQLITE_DEMOVFS_BUFFERSZ using sqlite3_malloc() whenever a
99 ** journal file is opened. It uses the buffer to coalesce sequential
100 ** writes into aligned SQLITE_DEMOVFS_BUFFERSZ blocks. When SQLite
101 ** invokes the xSync() method to sync the contents of the file to disk,
102 ** all accumulated data is written out, even if it does not constitute
103 ** a complete block. This means the actual IO to create the rollback
104 ** journal for the example transaction above is this:
106 ** Write offset | Bytes written
107 ** ----------------------------
110 ** ++++++++++++SYNC+++++++++++
112 ** ++++++++++++SYNC+++++++++++
114 ** Much more efficient if the underlying OS is not caching write
118 #if !defined(SQLITE_TEST) || SQLITE_OS_UNIX
124 #include <sys/types.h>
125 #include <sys/stat.h>
126 #include <sys/file.h>
127 #include <sys/param.h>
133 ** Size of the write buffer used by journal files in bytes.
135 #ifndef SQLITE_DEMOVFS_BUFFERSZ
136 # define SQLITE_DEMOVFS_BUFFERSZ 8192
140 ** The maximum pathname length supported by this VFS.
142 #define MAXPATHNAME 512
145 ** When using this VFS, the sqlite3_file* handles that SQLite uses are
146 ** actually pointers to instances of type DemoFile.
148 typedef struct DemoFile DemoFile;
150 sqlite3_file base; /* Base class. Must be first. */
151 int fd; /* File descriptor */
153 char *aBuffer; /* Pointer to malloc'd buffer */
154 int nBuffer; /* Valid bytes of data in zBuffer */
155 sqlite3_int64 iBufferOfst; /* Offset in file of zBuffer[0] */
159 ** Write directly to the file passed as the first argument. Even if the
160 ** file has a write-buffer (DemoFile.aBuffer), ignore it.
162 static int demoDirectWrite(
163 DemoFile *p, /* File handle */
164 const void *zBuf, /* Buffer containing data to write */
165 int iAmt, /* Size of data to write in bytes */
166 sqlite_int64 iOfst /* File offset to write to */
168 off_t ofst; /* Return value from lseek() */
169 size_t nWrite; /* Return value from write() */
171 ofst = lseek(p->fd, iOfst, SEEK_SET);
173 return SQLITE_IOERR_WRITE;
176 nWrite = write(p->fd, zBuf, iAmt);
178 return SQLITE_IOERR_WRITE;
185 ** Flush the contents of the DemoFile.aBuffer buffer to disk. This is a
186 ** no-op if this particular file does not have a buffer (i.e. it is not
187 ** a journal file) or if the buffer is currently empty.
189 static int demoFlushBuffer(DemoFile *p){
192 rc = demoDirectWrite(p, p->aBuffer, p->nBuffer, p->iBufferOfst);
201 static int demoClose(sqlite3_file *pFile){
203 DemoFile *p = (DemoFile*)pFile;
204 rc = demoFlushBuffer(p);
205 sqlite3_free(p->aBuffer);
211 ** Read data from a file.
219 DemoFile *p = (DemoFile*)pFile;
220 off_t ofst; /* Return value from lseek() */
221 int nRead; /* Return value from read() */
222 int rc; /* Return code from demoFlushBuffer() */
224 /* Flush any data in the write buffer to disk in case this operation
225 ** is trying to read data the file-region currently cached in the buffer.
226 ** It would be possible to detect this case and possibly save an
227 ** unnecessary write here, but in practice SQLite will rarely read from
228 ** a journal file when there is data cached in the write-buffer.
230 rc = demoFlushBuffer(p);
235 ofst = lseek(p->fd, iOfst, SEEK_SET);
237 return SQLITE_IOERR_READ;
239 nRead = read(p->fd, zBuf, iAmt);
243 }else if( nRead>=0 ){
244 return SQLITE_IOERR_SHORT_READ;
247 return SQLITE_IOERR_READ;
251 ** Write data to a crash-file.
253 static int demoWrite(
259 DemoFile *p = (DemoFile*)pFile;
262 char *z = (char *)zBuf; /* Pointer to remaining data to write */
263 int n = iAmt; /* Number of bytes at z */
264 sqlite3_int64 i = iOfst; /* File offset to write to */
267 int nCopy; /* Number of bytes to copy into buffer */
269 /* If the buffer is full, or if this data is not being written directly
270 ** following the data already buffered, flush the buffer. Flushing
271 ** the buffer is a no-op if it is empty.
273 if( p->nBuffer==SQLITE_DEMOVFS_BUFFERSZ || p->iBufferOfst+p->nBuffer!=i ){
274 int rc = demoFlushBuffer(p);
279 assert( p->nBuffer==0 || p->iBufferOfst+p->nBuffer==i );
280 p->iBufferOfst = i - p->nBuffer;
282 /* Copy as much data as possible into the buffer. */
283 nCopy = SQLITE_DEMOVFS_BUFFERSZ - p->nBuffer;
287 memcpy(&p->aBuffer[p->nBuffer], z, nCopy);
295 return demoDirectWrite(p, zBuf, iAmt, iOfst);
302 ** Truncate a file. This is a no-op for this VFS (see header comments at
303 ** the top of the file).
305 static int demoTruncate(sqlite3_file *pFile, sqlite_int64 size){
307 if( ftruncate(((DemoFile *)pFile)->fd, size) ) return SQLITE_IOERR_TRUNCATE;
313 ** Sync the contents of the file to the persistent media.
315 static int demoSync(sqlite3_file *pFile, int flags){
316 DemoFile *p = (DemoFile*)pFile;
319 rc = demoFlushBuffer(p);
325 return (rc==0 ? SQLITE_OK : SQLITE_IOERR_FSYNC);
329 ** Write the size of the file in bytes to *pSize.
331 static int demoFileSize(sqlite3_file *pFile, sqlite_int64 *pSize){
332 DemoFile *p = (DemoFile*)pFile;
333 int rc; /* Return code from fstat() call */
334 struct stat sStat; /* Output of fstat() call */
336 /* Flush the contents of the buffer to disk. As with the flush in the
337 ** demoRead() method, it would be possible to avoid this and save a write
338 ** here and there. But in practice this comes up so infrequently it is
339 ** not worth the trouble.
341 rc = demoFlushBuffer(p);
346 rc = fstat(p->fd, &sStat);
347 if( rc!=0 ) return SQLITE_IOERR_FSTAT;
348 *pSize = sStat.st_size;
353 ** Locking functions. The xLock() and xUnlock() methods are both no-ops.
354 ** The xCheckReservedLock() always indicates that no other process holds
355 ** a reserved lock on the database file. This ensures that if a hot-journal
356 ** file is found in the file-system it is rolled back.
358 static int demoLock(sqlite3_file *pFile, int eLock){
361 static int demoUnlock(sqlite3_file *pFile, int eLock){
364 static int demoCheckReservedLock(sqlite3_file *pFile, int *pResOut){
370 ** No xFileControl() verbs are implemented by this VFS.
372 static int demoFileControl(sqlite3_file *pFile, int op, void *pArg){
377 ** The xSectorSize() and xDeviceCharacteristics() methods. These two
378 ** may return special values allowing SQLite to optimize file-system
379 ** access to some extent. But it is also safe to simply return 0.
381 static int demoSectorSize(sqlite3_file *pFile){
384 static int demoDeviceCharacteristics(sqlite3_file *pFile){
389 ** Open a file handle.
392 sqlite3_vfs *pVfs, /* VFS */
393 const char *zName, /* File to open, or 0 for a temp file */
394 sqlite3_file *pFile, /* Pointer to DemoFile struct to populate */
395 int flags, /* Input SQLITE_OPEN_XXX flags */
396 int *pOutFlags /* Output SQLITE_OPEN_XXX flags (or NULL) */
398 static const sqlite3_io_methods demoio = {
400 demoClose, /* xClose */
401 demoRead, /* xRead */
402 demoWrite, /* xWrite */
403 demoTruncate, /* xTruncate */
404 demoSync, /* xSync */
405 demoFileSize, /* xFileSize */
406 demoLock, /* xLock */
407 demoUnlock, /* xUnlock */
408 demoCheckReservedLock, /* xCheckReservedLock */
409 demoFileControl, /* xFileControl */
410 demoSectorSize, /* xSectorSize */
411 demoDeviceCharacteristics /* xDeviceCharacteristics */
414 DemoFile *p = (DemoFile*)pFile; /* Populate this structure */
415 int oflags = 0; /* flags to pass to open() call */
422 if( flags&SQLITE_OPEN_MAIN_JOURNAL ){
423 aBuf = (char *)sqlite3_malloc(SQLITE_DEMOVFS_BUFFERSZ);
429 if( flags&SQLITE_OPEN_EXCLUSIVE ) oflags |= O_EXCL;
430 if( flags&SQLITE_OPEN_CREATE ) oflags |= O_CREAT;
431 if( flags&SQLITE_OPEN_READONLY ) oflags |= O_RDONLY;
432 if( flags&SQLITE_OPEN_READWRITE ) oflags |= O_RDWR;
434 memset(p, 0, sizeof(DemoFile));
435 p->fd = open(zName, oflags, 0600);
438 return SQLITE_CANTOPEN;
445 p->base.pMethods = &demoio;
450 ** Delete the file identified by argument zPath. If the dirSync parameter
451 ** is non-zero, then ensure the file-system modification to delete the
452 ** file has been synced to disk before returning.
454 static int demoDelete(sqlite3_vfs *pVfs, const char *zPath, int dirSync){
455 int rc; /* Return code */
458 if( rc!=0 && errno==ENOENT ) return SQLITE_OK;
460 if( rc==0 && dirSync ){
461 int dfd; /* File descriptor open on directory */
462 int i; /* Iterator variable */
463 char zDir[MAXPATHNAME+1]; /* Name of directory containing file zPath */
465 /* Figure out the directory name from the path of the file deleted. */
466 sqlite3_snprintf(MAXPATHNAME, zDir, "%s", zPath);
467 zDir[MAXPATHNAME] = '\0';
468 for(i=strlen(zDir); i>1 && zDir[i]!='/'; i++);
471 /* Open a file-descriptor on the directory. Sync. Close. */
472 dfd = open(zDir, O_RDONLY, 0);
480 return (rc==0 ? SQLITE_OK : SQLITE_IOERR_DELETE);
494 ** Query the file-system to see if the named file exists, is readable or
495 ** is both readable and writable.
497 static int demoAccess(
503 int rc; /* access() return code */
504 int eAccess = F_OK; /* Second argument to access() */
506 assert( flags==SQLITE_ACCESS_EXISTS /* access(zPath, F_OK) */
507 || flags==SQLITE_ACCESS_READ /* access(zPath, R_OK) */
508 || flags==SQLITE_ACCESS_READWRITE /* access(zPath, R_OK|W_OK) */
511 if( flags==SQLITE_ACCESS_READWRITE ) eAccess = R_OK|W_OK;
512 if( flags==SQLITE_ACCESS_READ ) eAccess = R_OK;
514 rc = access(zPath, eAccess);
520 ** Argument zPath points to a nul-terminated string containing a file path.
521 ** If zPath is an absolute path, then it is copied as is into the output
522 ** buffer. Otherwise, if it is a relative path, then the equivalent full
523 ** path is written to the output buffer.
525 ** This function assumes that paths are UNIX style. Specifically, that:
527 ** 1. Path components are separated by a '/'. and
528 ** 2. Full paths begin with a '/' character.
530 static int demoFullPathname(
531 sqlite3_vfs *pVfs, /* VFS */
532 const char *zPath, /* Input path (possibly a relative path) */
533 int nPathOut, /* Size of output buffer in bytes */
534 char *zPathOut /* Pointer to output buffer */
536 char zDir[MAXPATHNAME+1];
540 getcwd(zDir, sizeof(zDir));
542 zDir[MAXPATHNAME] = '\0';
544 sqlite3_snprintf(nPathOut, zPathOut, "%s/%s", zDir, zPath);
545 zPathOut[nPathOut-1] = '\0';
551 ** The following four VFS methods:
558 ** are supposed to implement the functionality needed by SQLite to load
559 ** extensions compiled as shared objects. This simple VFS does not support
560 ** this functionality, so the following functions are no-ops.
562 static void *demoDlOpen(sqlite3_vfs *pVfs, const char *zPath){
565 static void demoDlError(sqlite3_vfs *pVfs, int nByte, char *zErrMsg){
566 sqlite3_snprintf(nByte, zErrMsg, "Loadable extensions are not supported");
567 zErrMsg[nByte-1] = '\0';
569 static void (*demoDlSym(sqlite3_vfs *pVfs, void *pH, const char *z))(void){
572 static void demoDlClose(sqlite3_vfs *pVfs, void *pHandle){
577 ** Parameter zByte points to a buffer nByte bytes in size. Populate this
578 ** buffer with pseudo-random data.
580 static int demoRandomness(sqlite3_vfs *pVfs, int nByte, char *zByte){
585 ** Sleep for at least nMicro microseconds. Return the (approximate) number
586 ** of microseconds slept for.
588 static int demoSleep(sqlite3_vfs *pVfs, int nMicro){
589 sleep(nMicro / 1000000);
590 usleep(nMicro % 1000000);
595 ** Set *pTime to the current UTC time expressed as a Julian day. Return
596 ** SQLITE_OK if successful, or an error code otherwise.
598 ** http://en.wikipedia.org/wiki/Julian_day
600 ** This implementation is not very good. The current time is rounded to
601 ** an integer number of seconds. Also, assuming time_t is a signed 32-bit
602 ** value, it will stop working some time in the year 2038 AD (the so-called
603 ** "year 2038" problem that afflicts systems that store time this way).
605 static int demoCurrentTime(sqlite3_vfs *pVfs, double *pTime){
607 *pTime = t/86400.0 + 2440587.5;
612 ** This function returns a pointer to the VFS implemented in this file.
613 ** To make the VFS available to SQLite:
615 ** sqlite3_vfs_register(sqlite3_demovfs(), 0);
617 sqlite3_vfs *sqlite3_demovfs(void){
618 static sqlite3_vfs demovfs = {
620 sizeof(DemoFile), /* szOsFile */
621 MAXPATHNAME, /* mxPathname */
625 demoOpen, /* xOpen */
626 demoDelete, /* xDelete */
627 demoAccess, /* xAccess */
628 demoFullPathname, /* xFullPathname */
629 demoDlOpen, /* xDlOpen */
630 demoDlError, /* xDlError */
631 demoDlSym, /* xDlSym */
632 demoDlClose, /* xDlClose */
633 demoRandomness, /* xRandomness */
634 demoSleep, /* xSleep */
635 demoCurrentTime, /* xCurrentTime */
640 #endif /* !defined(SQLITE_TEST) || SQLITE_OS_UNIX */
648 static int register_demovfs(
649 ClientData clientData, /* Pointer to sqlite3_enable_XXX function */
650 Tcl_Interp *interp, /* The TCL interpreter that invoked this command */
651 int objc, /* Number of arguments */
652 Tcl_Obj *CONST objv[] /* Command arguments */
654 sqlite3_vfs_register(sqlite3_demovfs(), 1);
657 static int unregister_demovfs(
658 ClientData clientData, /* Pointer to sqlite3_enable_XXX function */
659 Tcl_Interp *interp, /* The TCL interpreter that invoked this command */
660 int objc, /* Number of arguments */
661 Tcl_Obj *CONST objv[] /* Command arguments */
663 sqlite3_vfs_unregister(sqlite3_demovfs());
668 ** Register commands with the TCL interpreter.
670 int Sqlitetest_demovfs_Init(Tcl_Interp *interp){
671 Tcl_CreateObjCommand(interp, "register_demovfs", register_demovfs, 0, 0);
672 Tcl_CreateObjCommand(interp, "unregister_demovfs", unregister_demovfs, 0, 0);
677 int Sqlitetest_demovfs_Init(Tcl_Interp *interp){ return TCL_OK; }
680 #endif /* SQLITE_TEST */