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Overview
| Comment: | Clarify that using an database file with a link count different from 1 results in undefined behavior. |
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| Timelines: | family | ancestors | descendants | both | branch-3.12 |
| Files: | files | file ages | folders |
| SHA1: |
7c96132d652c8752f567ab48ecc253b1 |
| User & Date: | drh 2016-05-05 13:28:23 |
Context
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2016-05-05
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| 13:33 | Merge changes from the 3.12 branch. check-in: d545903147 user: drh tags: trunk | |
| 13:28 | Clarify that using an database file with a link count different from 1 results in undefined behavior. Leaf check-in: 7c96132d65 user: drh tags: branch-3.12 | |
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2016-04-25
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| 21:58 | Clarify restrictions on the use of PRAGMA page_size. check-in: 44656180a0 user: drh tags: branch-3.12 | |
Changes
Changes to pages/howtocorrupt.in.
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then the two processes will be talking to different database files with the same name. (Note that this is only possible on Posix and Posix-like systems that permit a file to be unlinked while it is still open for reading and writing. Windows does not allow this to occur.) Since rollback journals and WAL files are based on the name of the database file, the two different database files will share the same rollback journal or WAL file. A rollback or recovery for one of the databases might use content from the other database, resulting in corruption.</p> <p>A similar problem occurs if a database file is renamed while it is opened and a new file is created with the old name.</p> <p>Beginning with SQLite [version 3.7.17], the unix OS interface will send SQLITE_WARNING messages to the [error log] if a database file is unlinked while it is still in use.</p> <tcl>hd_fragment alias {database filename aliasing}</tcl> <h3>2.5 Multiple links to the same file</h3> ................................................................................ then that is just another way of saying that the file has multiple names. If two or more processes open the database using different names, then they will use different rollback journals and WAL files. That means that if one process crashes, the other process will be unable to recover the transaction in progress because it will be looking in the wrong place for the appropriate journal.</p> <p>Beginning with SQLite [version 3.7.17], the unix OS interface will send SQLITE_WARNING messages to the [error log] if a database file has multiple hard links. As of this writing, SQLite still does not yet detect or warn about the use of database files through soft links.</p> <h2>3.0 Failure to sync</h2> <p>In order to guarantee that database files are always consistent, SQLite will occasionally ask the operating system to flush all pending writes to persistent storage then wait for that flush to complete. This is accomplished using the <tt>fsync()</tt> system call under unix and |
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then the two processes will be talking to different database files with the same name. (Note that this is only possible on Posix and Posix-like systems that permit a file to be unlinked while it is still open for reading and writing. Windows does not allow this to occur.) Since rollback journals and WAL files are based on the name of the database file, the two different database files will share the same rollback journal or WAL file. A rollback or recovery for one of the databases might use content from the other database, resulting in corruption. A similar problem occurs if a database file is renamed while it is opened and a new file is created with the old name.</p> <p>In other words, unlinking or renaming an open database file results in behavior that is undefined and probably undesirable.</p> <p>Beginning with SQLite [version 3.7.17], the unix OS interface will send SQLITE_WARNING messages to the [error log] if a database file is unlinked while it is still in use.</p> <tcl>hd_fragment alias {database filename aliasing}</tcl> <h3>2.5 Multiple links to the same file</h3> ................................................................................ then that is just another way of saying that the file has multiple names. If two or more processes open the database using different names, then they will use different rollback journals and WAL files. That means that if one process crashes, the other process will be unable to recover the transaction in progress because it will be looking in the wrong place for the appropriate journal.</p> <p>In other words, opening and using a database file that has two or more names results in behavior that is undefined and probably undesirable.</p> <p>Beginning with SQLite [version 3.7.17], the unix OS interface will send SQLITE_WARNING messages to the [error log] if a database file has multiple hard links.</p> <p>Beginning with SQLite [version 3.10.0], the unix OS interface will attempt to resolve symbolic links and open the database file by its canonical name. Prior to version 3.10.0, opening a database file through a symbolic link was similar to opening a database file that had multiple hard links and resulted in undefined behavior.</p> <h2>3.0 Failure to sync</h2> <p>In order to guarantee that database files are always consistent, SQLite will occasionally ask the operating system to flush all pending writes to persistent storage then wait for that flush to complete. This is accomplished using the <tt>fsync()</tt> system call under unix and |