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Clarify some comments describing the WAL index file. No changes to code.

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FossilOrigin-Name: 3be3aad9ecbe33060cfa9c6059b9206ed221d1fd72a69c355a9387f9f4e075e7
This commit is contained in:
drh
2017-10-30 20:44:36 +00:00
parent eba21f9eb2
commit 07dae088b9
3 changed files with 21 additions and 9 deletions
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16
src/wal.c
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@@ -132,6 +132,10 @@
** on a network filesystem. All users of the database must be able to
** share memory.
**
** In the default unix and windows implementation, the wal-index is a mmapped
** file whose name is the database name with a "-shm" suffix added. For that
** reason, the wal-index is sometimes called the "shm" file.
**
** The wal-index is transient. After a crash, the wal-index can (and should
** be) reconstructed from the original WAL file. In fact, the VFS is required
** to either truncate or zero the header of the wal-index when the last
@@ -271,9 +275,18 @@ int sqlite3WalTrace = 0;
#define WALINDEX_MAX_VERSION 3007000
/*
** Indices of various locking bytes. WAL_NREADER is the number
** Index numbers for various locking bytes. WAL_NREADER is the number
** of available reader locks and should be at least 3. The default
** is SQLITE_SHM_NLOCK==8 and WAL_NREADER==5.
**
** Technically, the various VFSes are free to implement these locks however
** they see fit. However, compatibility is encouraged so that VFSes can
** interoperate. The standard implemention used on both unix and windows
** is for the index number to indicate a byte offset into the
** WalCkptInfo.aLock[] array in the wal-index header. In other words, all
** locks are on the shm file. The WALINDEX_LOCK_OFFSET constant (which
** should be 120) is the location in the shm file for the first locking
** byte.
*/
#define WAL_WRITE_LOCK 0
#define WAL_ALL_BUT_WRITE 1
@@ -397,7 +410,6 @@ struct WalCkptInfo {
#define WAL_FRAME_HDRSIZE 24
/* Size of write ahead log header, including checksum. */
/* #define WAL_HDRSIZE 24 */
#define WAL_HDRSIZE 32
/* WAL magic value. Either this value, or the same value with the least