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| author | Jörg Frings-Fürst <debian@jff-webhosting.net> | 2022-12-07 13:17:14 +0100 |
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| committer | Jörg Frings-Fürst <debian@jff-webhosting.net> | 2022-12-07 13:17:14 +0100 |
| commit | 4875a3dd9b183dcd2256e2abfc4ccf7484c233b4 (patch) | |
| tree | 0abbea881ded030851014ffdd60fbf71fead8f65 /html/xbc18.htm | |
| parent | daf17154bf13139d9375f48525d19d6aaba08155 (diff) | |
New upstream version 4.0.2upstream/4.0.2
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| -rwxr-xr-x | html/xbc18.htm | 261 |
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diff --git a/html/xbc18.htm b/html/xbc18.htm deleted file mode 100755 index 78d8de6..0000000 --- a/html/xbc18.htm +++ /dev/null @@ -1,261 +0,0 @@ -<!DOCTYPE HTML PUBLIC> -<HTML> -<TITLE>Xbase DBMS Chapter 18</TITLE> -<BODY BGCOLOR=#FFFFFF> -<H1><p align="center">Locking Methods</p></H1> -<p align="center">Chapter Updated 2/1/99</p><hr> - -The locking methods return either XB_LOCK_FAILED or XB_NO_ERROR. If they return -LOCK_FAILED the actual reason can be found in the global variable -<em>errno</em> or function <em>perror()</em> can be executed to view the -results. -<br><br> - -The errno field may contain one of the following values if the lock was not -successful.<br><br> -<TABLE BORDER> -<TR VALIGN="BASELINE"> -<TR><TH ALIGN="LEFT">Error Code<TD>Description -<TR><TH ALIGN="LEFT">EBADF<TD>Invalid file descriptor -<TR><TH ALIGN="LEFT">EINVAL<TD>Invalid lock information or file does not support locks -<TR><TH ALIGN="LEFT">EACCESS<BR>EAGAIN<TD>Lock can not be set because it is blocked by an existing lock on the file. -<TR><TH ALIGN="LEFT">ENOLCK<TD>The system is out of lock resources, too many file locks in place. -<TR><TH ALIGN="LEFT">EDEADLK<TD>Deadlock condition -<TR><TH ALIGN="LEFT">EINTR<TD>Process was interrupted by a signal while it was waiting -</TABLE> -<br><br> -<h3>Types of Locks</h3> - -<li><em>Write or Exclusive Locks</em> provide exclusive access to a -particular file location. No other process can lock the same location.<br><br> - -<li><em>Read or Shared Locks</em> prohibit any process from requesting a write -lock on a specified part of the file. Other processes can request -simultaneous read locks.<br><br><br> - -<h3>DBF File Locking Techniques</h3> - -Xbase DBMS uses the following protocol for DBF file and record locking: -<br><br> - -To lock a record - the first byte of the record is locked.<br> -To lock the file - the header bytes of the file are locked.<br><br> - -When a record is being appended to the file, the header bytes are locked.<br> -When a record is being updated, the header bytes and the specific record are -locked.<br><br> -This locking protocol is probably not compatable with other Xbase type products. -However, Xbase can be safely used for multi-user access when it is not -simultaneously updating DBF or NDX files while other products/programs are. -<br><br><br> - -<h3>NDX File Locking Techniques</h3> - -Xbase DBMS locks indexes by locking the first 512 bytes -of the index file. -The entire index is locked because any updates to the index potentially -can modify significant portions of the index tree. -<br><br><br> - -<h3>DBT File Locking Techniques</h3> - -Xbase DBMS locks memo files by locking the first 4 bytes -of the memo file. This effectively locks the entire file. The entire file -is locked because any updates to the free block chain can significantly -change the structure of the file. -<br><br><br> - - -<h3>AutoLocking Features</h3> - -If XB_LOCKING_ON is set in the <em>options.h</em> file, the locking methods -execute any appropriate locking logic. If XB_LOCKING_ON is not set in the -<em>options.h</em> file, all locking methods return NO_ERROR without -performing any actual record or file locking. This enables the application -program to always call locking routines regardless of the XB_LOCKING_ON switch -in the <em>options.h</em> file. -<br><br> -By leaving the autolocking features enabled, the application program does -not need to address record, file or index locking. All locking is handled -automatically by the Xbase routines. However, if access to the locking -routines is required, they are available to the applciation programmer. -<br><br> -When the files are automatically locked by the Xbase routines, the database -file is locked first, then it locks the indexes in alphabetical order. To -avoid deadlock conditions, files and record locks should always be done in -the same order. When the files are unlocked, then indexes are unlocked -first, then the database is unlocked. -<br><br> -Auto-locking works well in an on-line transaction based environment. -However, it does not function efficiently in batch mode. If you -will be writing programs which process files in a batch mode, disabling -auto-lock and locking the entire file at the beginning of the process -and unlocking the file at the end of the process will significantly -reduce process time. On a 586-200 class machine, a file with 45000 records -can be read thru in a few seconds with the file locked in batch mode. -In record-lock mode it takes about six minutes with the same processor. - -<br><br>For processing large files, locking the file instead of locking each -record is far more efficient. This is how you do it.<br><br> - -For reading the file in batch mode:<br> -xbDbf.AutoLockOff();<br> -xbDbf.LockDatabase( F_SETLKW, F_RDLCK, 0L );<br><br> -For updating the file in batch mode:<br> -xbDbf.AutoLockOff();<br> -xbDbf.LockDatabase( F_SETLKW, F_WRLCK, 0L );<br><br> -<br> -<hr><br> - -<h3>Method Table</h3> - -<TABLE BORDER> -<CAPTION ALIGN="TOP"><h3><Xbase Locking Method List</h3></CAPTION> -<TR VALIGN="BASELINE"> -<TR><TH ALIGN="LEFT">Method<TD>Description -<TR><TH ALIGN="LEFT">xbDbf::AutoLockOn<TD>Turns autolocking on -<TR><TH ALIGN="LEFT">xbDbf::AutoLockOff<TD>Turns autolocking off -<TR><TH ALIGN="LEFT">xbDbf::ExclusiveLock<TD>Lock file and indexes in exclusive mode -<TR><TH ALIGN="LEFT">xbDbf::ExclusiveUnlock<TD>Unlock files and indexes -<TR><TH ALIGN="LEFT">xbDbf::LockDatabase<TD>Locks or unlocks a DBF database -<TR><TH ALIGN="LEFT">xbNdx::LockIndex<TD>Locks or unlocks an xbNdx index -<TR><TH ALIGN="LEFT">xbNdx::LockMemoFile<TD>Locks or unlocks a DBT memo field file -</TABLE> -<BR><HR> - -<h4>Method Descriptions</h4> - -<h4>Method VOID xbDbf::AutoLockOn( VOID )</h4><br> - -This method turns automatic record locking on. Auto record locking is on -by default if XB_LOCKING_ON is set in the options.h file.<br><br> - -<h4>Example Program:</h4> - -See program <A HREF="/zips/loadzips.cpp">loadzips.cpp</A> for an example of -how to use this method. -<hr> - -<h4>Method VOID xbDbf::AutoLockOff( VOID )</h4><br> - -This method turns automatic record locking off. Auto record locking is on -by default if XB_LOCKING_ON is set in the options.h file. -<br><br> -Turning auto locking off will result in slightly better execution speeds -but should not be used in multi-user environments when multiple users can -update files simultanteously. If multiple users are accessing a file which -is read only then it is safe to turn off auto-locking for a particular file. -<br><br> -Turning autolocking off will disable any index file locking which is -particularly dangerous in a multi-user environment if updates on the files -are permitted. - - -<h4>Example Program:</h4> - -See program <A HREF="/zips/loadzips.cpp">loadzips.cpp</A> for an example of -how to use this method. - -<hr> -<h4>Method SHORT xbDbf::ExclusiveLock( SHORT WaitOption )</h4> -<h4>Method SHORT xbDbf::ExclusiveUnlock( VOID )</h4><br> - -ExclusiveLock and ExclusiveUnclock will lock the data file, memo file (if applicable) -and any associated indexes in an exclusive mode. They also turn auto-lock -on and off as appropriate.<br><br> - -WaitOption is either:<br><br> -<li>F_SETLK - returns immediately regardless if success or failure<br> -<li>F_SETLKW - waits until lock function executes<br><br> - -<h4>Example Program:</h4> - -See program <A HREF="/XbaseSamples/sample4.cpp">sample4.cpp</A> for an example of -how to use this method. - -<hr> -<h3>Method SHORT xbDbf::LockDatabase( SHORT WaitOption, SHORT LockType, LONG LRecNo ) -</h3><br> - -This method locks or unlocks an Xbase (.DBF) file which was previously opened.<br> -<br> -WaitOption is either:<br><br> -<li>F_SETLK - returns immediately regardless if success or failure<br> -<li>F_SETLKW - waits until lock function executes<br><br> - -LockType is one of:<br><br> -<li>F_RDLCK - Perform a Read or Shared Lock<br> -<li>F_WRLCK - Perform a Write or Exclusive Lock<br> -<li>F_UNLCK - Unlock it<br><br> - -LRecNo is:<br><br> -0 - Lock the header section of the file (use this to lock the file)<br> -1 through n - Lock a particular record<br><br> - -<TABLE BORDER> -<CAPTION ALIGN="TOP"<h4>Method Return Codes</h4></CAPTION> -<TR><TH ALIGN="LEFT">Return Code<TD>Description -<TR><TH ALIGN="LEFT">XB_INVALID_RECORD<TD>An invalid record given -<TR><TH ALIGN="LEFT">XB_LOCK_FAILED<TD>The lock action failed, see errno -<TR><TH ALIGN="LEFT">XB_NO_ERROR<TD>The lock was successful -</TABLE> - - -<h4>Example Program:</h4> - -See program <A HREF="/zips/loadzips.cpp">loadzips.cpp</A> for an example of -how to use this method. - -<hr> - -<h3>Method xbShort xbDbf::LockIndex( xbShort WaitOption, xbShort LockType ) -</h3><br> - -This method locks or unlocks an Index (.NDX) file which was previously opened.<br> -<br> -WaitOption is either:<br><br> -<li>F_SETLK - returns immediately regardless if success or failure<br> -<li>F_SETLKW - waits until lock function executes<br><br> - -LockType is one of:<br><br> -<li>F_RDLCK - Perform a Read or Shared Lock<br> -<li>F_WRLCK - Perform a Write or Exclusive Lock<br> -<li>F_UNLCK - Unlock it<br><br> - -<TABLE BORDER> -<CAPTION ALIGN="TOP"<h4>Method Return Codes</h4></CAPTION> -<TR><TH ALIGN="LEFT">Return Code<TD>Description -<TR><TH ALIGN="LEFT">XB_LOCK_FAILED<TD>The lock action failed, see errno -<TR><TH ALIGN="LEFT">XB_NO_ERROR<TD>The lock was successful -</TABLE> - -<h4>Example Program:</h4> -See program <A HREF="/zips/loadzips.cpp">loadzips.cpp</A> for an example of -how to use this method. -<hr> - -<h3>Method xbShort xbDbf::LockMemoFile( xbShort WaitOption, xbShort LockType ) -</h3><br> - -This method locks or unlocks a memo (.DBT) file which was previously opened. -It is not necessary for an application to call this method as locking is -handled automatically by other routines.<br><br> - -WaitOption is either:<br><br> -<li>F_SETLK - returns immediately regardless if success or failure<br> -<li>F_SETLKW - waits until lock function executes<br><br> - -LockType is one of:<br><br> -<li>F_RDLCK - Perform a Read or Shared Lock<br> -<li>F_WRLCK - Perform a Write or Exclusive Lock<br> -<li>F_UNLCK - Unlock it<br><br> -<TABLE BORDER> -<CAPTION ALIGN="TOP"<h4>Method Return Codes</h4></CAPTION> -<TR><TH ALIGN="LEFT">Return Code<TD>Description -<TR><TH ALIGN="LEFT">XB_LOCK_FAILED<TD>The lock action failed, see errno -<TR><TH ALIGN="LEFT">XB_NO_ERROR<TD>The lock was successful -</TABLE> -<hr> -<p><img src="xbase.jpg"><br><hr> -</BODY> -</HTML> |