<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"><title>Chapter 26. Extending MySQL</title><link rel="stylesheet" href="mysql-html.css" type="text/css"><meta name="generator" content="DocBook XSL Stylesheets V1.69.1"><link rel="start" href="index.html" title="MySQL 5.0 Reference Manual"><link rel="up" href="index.html" title="MySQL 5.0 Reference Manual"><link rel="prev" href="mysql-proxy.html" title="Chapter 25. MySQL Proxy"><link rel="next" href="faqs.html" title="Appendix A. MySQL 5.0 Frequently Asked Questions"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Chapter 26. Extending MySQL</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="mysql-proxy.html">Prev</a> </td><th width="60%" align="center"> </th><td width="20%" align="right"> <a accesskey="n" href="faqs.html">Next</a></td></tr></table><hr></div><div class="chapter" lang="en"><div class="titlepage"><div><div><h2 class="title"><a name="extending-mysql"></a>Chapter 26. Extending MySQL</h2></div></div></div><div class="toc"><p><b>Table of Contents</b></p><dl><dt><span class="section"><a href="extending-mysql.html#mysql-internals">26.1. MySQL Internals</a></span></dt><dd><dl><dt><span class="section"><a href="extending-mysql.html#mysql-threads">26.1.1. MySQL Threads</a></span></dt><dt><span class="section"><a href="extending-mysql.html#mysql-test-suite">26.1.2. MySQL Test Suite</a></span></dt></dl></dd><dt><span class="section"><a href="extending-mysql.html#adding-functions">26.2. Adding New Functions to MySQL</a></span></dt><dd><dl><dt><span class="section"><a href="extending-mysql.html#udf-features">26.2.1. Features of the User-Defined Function Interface</a></span></dt><dt><span class="section"><a href="extending-mysql.html#create-function">26.2.2. <code class="literal">CREATE FUNCTION</code> Syntax</a></span></dt><dt><span class="section"><a href="extending-mysql.html#drop-function">26.2.3. <code class="literal">DROP FUNCTION</code> Syntax</a></span></dt><dt><span class="section"><a href="extending-mysql.html#adding-udf">26.2.4. Adding a New User-Defined Function</a></span></dt><dt><span class="section"><a href="extending-mysql.html#adding-native-function">26.2.5. Adding a New Native Function</a></span></dt></dl></dd><dt><span class="section"><a href="extending-mysql.html#adding-procedures">26.3. Adding New Procedures to MySQL</a></span></dt><dd><dl><dt><span class="section"><a href="extending-mysql.html#procedure-analyse">26.3.1. Procedure Analyse</a></span></dt><dt><span class="section"><a href="extending-mysql.html#writing-a-procedure">26.3.2. Writing a Procedure</a></span></dt></dl></dd><dt><span class="section"><a href="extending-mysql.html#porting">26.4. Debugging and Porting MySQL</a></span></dt><dd><dl><dt><span class="section"><a href="extending-mysql.html#debugging-server">26.4.1. Debugging a MySQL Server</a></span></dt><dt><span class="section"><a href="extending-mysql.html#debugging-client">26.4.2. Debugging a MySQL Client</a></span></dt><dt><span class="section"><a href="extending-mysql.html#the-dbug-package">26.4.3. The DBUG Package</a></span></dt><dt><span class="section"><a href="extending-mysql.html#rts-threads">26.4.4. Comments about RTS Threads</a></span></dt><dt><span class="section"><a href="extending-mysql.html#thread-packages">26.4.5. Differences Between Thread Packages</a></span></dt></dl></dd></dl></div><div class="section" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="mysql-internals"></a>26.1. MySQL Internals</h2></div></div></div><div class="toc"><dl><dt><span class="section"><a href="extending-mysql.html#mysql-threads">26.1.1. MySQL Threads</a></span></dt><dt><span class="section"><a href="extending-mysql.html#mysql-test-suite">26.1.2. MySQL Test Suite</a></span></dt></dl></div><a class="indexterm" name="id3151835"></a><a class="indexterm" name="id3151844"></a><p>
This chapter describes a lot of things that you need to know when
working on the MySQL code. If you plan to contribute to MySQL
development, want to have access to the bleeding-edge versions of
the code, or just want to keep track of development, follow the
instructions in <a href="installing.html#installing-source-tree" title="2.4.15.3. Installing from the Development Source Tree">Section 2.4.15.3, “Installing from the Development Source Tree”</a>. If you
are interested in MySQL internals, you should also subscribe to
our <code class="literal">internals</code> mailing list. This list has
relatively low traffic. For details on how to subscribe, please
see <a href="introduction.html#mailing-lists" title="1.6.1. MySQL Mailing Lists">Section 1.6.1, “MySQL Mailing Lists”</a>. All developers at MySQL AB
are on the <code class="literal">internals</code> list and we help other
people who are working on the MySQL code. Feel free to use this
list both to ask questions about the code and to send patches that
you would like to contribute to the MySQL project!
</p><div class="section" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="mysql-threads"></a>26.1.1. MySQL Threads</h3></div></div></div><p>
The MySQL server creates the following threads:
</p><div class="itemizedlist"><ul type="disc"><li><p>
One thread manages TCP/IP file connection requests and
creates a new dedicated thread to handle the authentication
and SQL statement processing for each connection. (On Unix,
this thread also manages Unix socket file connection
requests.) On Windows, a similar thread manages
shared-memory connection requests, and a thread manages
named-pipe connection requests. Every client connection has
its own thread, although the manager threads try to avoid
creating threads by consulting the thread cache first to see
whether a cached thread can be used for a new connection.
</p></li><li><p>
On a master replication server, slave server connections are
like client connections: There is one thread per connected
slave.
</p></li><li><p>
On a slave replication server, an I/O thread is started to
connect to the master server and read updates from it. An
SQL thread is started to apply updates read from the master.
These two threads run independently and can be started and
stopped independently.
</p></li><li><p>
The signal thread handles all signals. This thread also
normally handles alarms and calls
<code class="literal">process_alarm()</code> to force timeouts on
connections that have been idle too long.
</p></li><li><p>
If <code class="literal">InnoDB</code> is used, there will be 4
additional threads by default. Those are file I/O threads,
controlled by the <code class="literal">innodb_file_io_threads</code>
parameter. See <a href="storage-engines.html#innodb-parameters" title="13.2.4. InnoDB Startup Options and System Variables">Section 13.2.4, “<code class="literal">InnoDB</code> Startup Options and System Variables”</a>.
</p></li><li><p>
If <span><strong class="command">mysqld</strong></span> is compiled with
<code class="option">-DUSE_ALARM_THREAD</code>, a dedicated thread that
handles alarms is created. This is only used on some systems
where there are problems with <code class="literal">sigwait()</code>
or if you want to use the <code class="literal">thr_alarm()</code>
code in your application without a dedicated signal handling
thread.
</p></li><li><p>
If the server is started with the
<code class="option">--flush_time=<em class="replaceable"><code>val</code></em></code>
option, a dedicated thread is created to flush all tables
every <em class="replaceable"><code>val</code></em> seconds.
</p></li><li><p>
Each table for which <code class="literal">INSERT DELAYED</code>
statements are issued gets its own thread.
</p></li></ul></div><p>
<span><strong class="command">mysqladmin processlist</strong></span> only shows the
connection, <code class="literal">INSERT DELAYED</code>, and replication
threads.
</p><p class="mnmas"><b>MySQL Enterprise</b>
For expert advice on thread management subscribe to the MySQL
Enterprise Monitor. For more information see,
<a href="http://www.mysql.com/products/enterprise/advisors.html" target="_top">http://www.mysql.com/products/enterprise/advisors.html</a>.
</p></div><div class="section" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="mysql-test-suite"></a>26.1.2. MySQL Test Suite</h3></div></div></div><a class="indexterm" name="id3152100"></a><a class="indexterm" name="id3152113"></a><p>
The test system that is included in Unix source and binary
distributions makes it possible for users and developers to
perform regression tests on the MySQL code. These tests can be
run on Unix.
</p><p>
The current set of test cases doesn't test everything in MySQL,
but it should catch most obvious bugs in the SQL processing
code, operating system or library issues, and is quite thorough
in testing replication. Our goal is to have the tests cover 100%
of the code. We welcome contributions to our test suite. You may
especially want to contribute tests that examine the
functionality critical to your system because this ensures that
all future MySQL releases work well with your applications.
</p><p>
The test system consists of a test language interpreter
(<span><strong class="command">mysqltest</strong></span>), a Perl script to run all tests
(<span><strong class="command">mysql-test-run.pl</strong></span>), the actual test cases
written in a special test language, and their expected results.
To run the test suite on your system after a build, type
<span><strong class="command">make test</strong></span> from the source root directory, or
change location to the <code class="filename">mysql-test</code> directory
and type <span><strong class="command">./mysql-test-run.pl</strong></span>. If you have
installed a binary distribution, change location to the
<code class="filename">mysql-test</code> directory under the installation
root directory (for example,
<code class="filename">/usr/local/mysql/mysql-test</code>), and run
<span><strong class="command">./mysql-test-run.pl</strong></span>. All tests should
succeed. If any do not, feel free to try to find out why and
report the problem if it indicates a bug in MySQL. See
<a href="introduction.html#bug-reports" title="1.7. How to Report Bugs or Problems">Section 1.7, “How to Report Bugs or Problems”</a>.
</p><p>
If one test fails, you should run
<span><strong class="command">mysql-test-run.pl</strong></span> with the
<code class="option">--force</code> option to check whether any other tests
fail.
</p><p>
If you have a copy of <span><strong class="command">mysqld</strong></span> running on the
machine where you want to run the test suite, you do not have to
stop it, as long as it is not using ports
<code class="literal">9306</code> or <code class="literal">9307</code>. If either of
those ports is taken, you should set the
<code class="literal">MTR_BUILD_THREAD</code> environment variable to an
appropriate value, and the test suite will use a different set
of ports for master, slave, NDB, and Instance Manager). For
example:
</p><pre class="programlisting">shell> export MTR_BUILD_THREAD=31
shell> ./mysql-test-run.pl [<em class="replaceable"><code>options</code></em>] [<em class="replaceable"><code>test_name</code></em>]
</pre><p>
In the <code class="filename">mysql-test</code> directory, you can run an
individual test case with <span><strong class="command">./mysql-test-run.pl
<em class="replaceable"><code>test_name</code></em></strong></span>.
</p><p>
You can use the <span><strong class="command">mysqltest</strong></span> language to write
your own test cases. This is documented in the MySQL Test
Framework manual, available at <a href="http://dev.mysql.com/doc/" target="_top">http://dev.mysql.com/doc/</a>.
</p><p>
If you have a question about the test suite, or have a test case
to contribute, send an email message to the MySQL
<code class="literal">internals</code> mailing list. See
<a href="introduction.html#mailing-lists" title="1.6.1. MySQL Mailing Lists">Section 1.6.1, “MySQL Mailing Lists”</a>. This list does not accept
attachments, so you should FTP all the relevant files to:
<a href="ftp://ftp.mysql.com/pub/mysql/upload/" target="_top">ftp://ftp.mysql.com/pub/mysql/upload/</a>
</p></div></div><div class="section" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="adding-functions"></a>26.2. Adding New Functions to MySQL</h2></div></div></div><div class="toc"><dl><dt><span class="section"><a href="extending-mysql.html#udf-features">26.2.1. Features of the User-Defined Function Interface</a></span></dt><dt><span class="section"><a href="extending-mysql.html#create-function">26.2.2. <code class="literal">CREATE FUNCTION</code> Syntax</a></span></dt><dt><span class="section"><a href="extending-mysql.html#drop-function">26.2.3. <code class="literal">DROP FUNCTION</code> Syntax</a></span></dt><dt><span class="section"><a href="extending-mysql.html#adding-udf">26.2.4. Adding a New User-Defined Function</a></span></dt><dt><span class="section"><a href="extending-mysql.html#adding-native-function">26.2.5. Adding a New Native Function</a></span></dt></dl></div><a class="indexterm" name="id3152347"></a><a class="indexterm" name="id3152359"></a><a class="indexterm" name="id3152371"></a><a class="indexterm" name="id3152384"></a><a class="indexterm" name="id3152396"></a><p>
There are three ways to add new functions to MySQL:
</p><div class="itemizedlist"><ul type="disc"><li><p>
You can add functions through the user-defined function (UDF)
interface. User-defined functions are compiled as object files
and then added to and removed from the server dynamically
using the <code class="literal">CREATE FUNCTION</code> and <code class="literal">DROP
FUNCTION</code> statements. See
<a href="extending-mysql.html#create-function" title="26.2.2. CREATE FUNCTION Syntax">Section 26.2.2, “<code class="literal">CREATE FUNCTION</code> Syntax”</a>.
</p></li><li><p>
You can add functions as native (built-in) MySQL functions.
Native functions are compiled into the
<span><strong class="command">mysqld</strong></span> server and become available on a
permanent basis.
</p></li><li><p>
Another way to add functions is by creating stored functions.
These are written using SQL statements rather than by
compiling object code. The syntax for writing stored functions
is not covered here. See <a href="stored-procedures.html" title="Chapter 18. Stored Procedures and Functions">Chapter 18, <i>Stored Procedures and Functions</i></a>.
</p></li></ul></div><p>
Each method of creating compiled functions has advantages and
disadvantages:
</p><div class="itemizedlist"><ul type="disc"><li><p>
If you write user-defined functions, you must install object
files in addition to the server itself. If you compile your
function into the server, you don't need to do that.
</p></li><li><p>
Native functions require you to modify a source distribution.
UDFs do not. You can add UDFs to a binary MySQL distribution.
No access to MySQL source is necessary.
</p></li><li><p>
If you upgrade your MySQL distribution, you can continue to
use your previously installed UDFs, unless you upgrade to a
newer version for which the UDF interface changes. For native
functions, you must repeat your modifications each time you
upgrade.
</p></li></ul></div><p>
Whichever method you use to add new functions, they can be invoked
in SQL statements just like native functions such as
<a href="functions.html#function_abs"><code class="literal">ABS()</code></a> or
<a href="functions.html#function_soundex"><code class="literal">SOUNDEX()</code></a>.
</p><p>
See <a href="language-structure.html#function-resolution" title="8.2.3. Function Name Parsing and Resolution">Section 8.2.3, “Function Name Parsing and Resolution”</a>, for the rules
describing how the server interprets references to different kinds
of functions.
</p><p>
The following sections describe features of the UDF interface,
provide instructions for writing UDFs, discuss security
precautions that MySQL takes to prevent UDF misuse, and describe
how to add native MySQL functions.
</p><p>
For example source code that illustrates how to write UDFs, take a
look at the <code class="filename">sql/udf_example.c</code> file that is
provided in MySQL source distributions.
</p><div class="section" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="udf-features"></a>26.2.1. Features of the User-Defined Function Interface</h3></div></div></div><p>
The MySQL interface for user-defined functions provides the
following features and capabilities:
</p><div class="itemizedlist"><ul type="disc"><li><p>
Functions can return string, integer, or real values and can
accept arguments of those same types.
</p></li><li><p>
You can define simple functions that operate on a single row
at a time, or aggregate functions that operate on groups of
rows.
</p></li><li><p>
Information is provided to functions that enables them to
check the number, types, and names of the arguments passed
to them.
</p></li><li><p>
You can tell MySQL to coerce arguments to a given type
before passing them to a function.
</p></li><li><p>
You can indicate that a function returns
<code class="literal">NULL</code> or that an error occurred.
</p></li></ul></div></div><div class="section" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="create-function"></a>26.2.2. <code class="literal">CREATE FUNCTION</code> Syntax</h3></div></div></div><a class="indexterm" name="id3152653"></a><a class="indexterm" name="id3152662"></a><a class="indexterm" name="id3152675"></a><a class="indexterm" name="id3152687"></a><a class="indexterm" name="id3152696"></a><a class="indexterm" name="id3152705"></a><pre class="programlisting">CREATE [AGGREGATE] FUNCTION <em class="replaceable"><code>function_name</code></em> RETURNS {STRING|INTEGER|REAL|DECIMAL}
SONAME <em class="replaceable"><code>shared_library_name</code></em>
</pre><p>
A user-defined function (UDF) is a way to extend MySQL with a
new function that works like a native (built-in) MySQL function
such as <a href="functions.html#function_abs"><code class="literal">ABS()</code></a> or
<a href="functions.html#function_concat"><code class="literal">CONCAT()</code></a>.
</p><p>
<em class="replaceable"><code>function_name</code></em> is the name that should
be used in SQL statements to invoke the function. The
<code class="literal">RETURNS</code> clause indicates the type of the
function's return value. As of MySQL 5.0.3,
<code class="literal">DECIMAL</code> is a legal value after
<code class="literal">RETURNS</code>, but currently
<code class="literal">DECIMAL</code> functions return string values and
should be written like <code class="literal">STRING</code> functions.
</p><p>
<em class="replaceable"><code>shared_library_name</code></em> is the basename
of the shared object file that contains the code that implements
the function. The file must be located in a directory that is
searched by your system's dynamic linker.
</p><p>
To create a function, you must have the
<code class="literal">INSERT</code> and privilege for the
<code class="literal">mysql</code> database. This is necessary because
<code class="literal">CREATE FUNCTION</code> adds a row to the
<code class="literal">mysql.func</code> system table that records the
function's name, type, and shared library name. If you do not
have this table, you should run the
<span><strong class="command">mysql_upgrade</strong></span> command to create it. See
<a href="server-administration.html#mysql-upgrade" title="5.5.8. mysql_upgrade — Check Tables for MySQL Upgrade">Section 5.5.8, “<span><strong class="command">mysql_upgrade</strong></span> — Check Tables for MySQL Upgrade”</a>.
</p><p>
An active function is one that has been loaded with
<code class="literal">CREATE FUNCTION</code> and not removed with
<code class="literal">DROP FUNCTION</code>. All active functions are
reloaded each time the server starts, unless you start
<span><strong class="command">mysqld</strong></span> with the
<code class="option">--skip-grant-tables</code> option. In this case, UDF
initialization is skipped and UDFs are unavailable.
</p><p>
For instructions on writing user-defined functions, see
<a href="extending-mysql.html#adding-udf" title="26.2.4. Adding a New User-Defined Function">Section 26.2.4, “Adding a New User-Defined Function”</a>. For the UDF mechanism to work,
functions must be written in C or C++ (or another language that
can use C calling conventions), your operating system must
support dynamic loading and you must have compiled
<span><strong class="command">mysqld</strong></span> dynamically (not statically).
</p><p>
An <code class="literal">AGGREGATE</code> function works exactly like a
native MySQL aggregate (summary) function such as
<code class="literal">SUM</code> or
<a href="functions.html#function_count"><code class="literal">COUNT()</code></a>. For
<code class="literal">AGGREGATE</code> to work, your
<code class="literal">mysql.func</code> table must contain a
<code class="literal">type</code> column. If your
<code class="literal">mysql.func</code> table does not have this column,
you should run the <span><strong class="command">mysql_upgrade</strong></span> program to
create it (see <a href="server-administration.html#mysql-upgrade" title="5.5.8. mysql_upgrade — Check Tables for MySQL Upgrade">Section 5.5.8, “<span><strong class="command">mysql_upgrade</strong></span> — Check Tables for MySQL Upgrade”</a>).
</p><div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Note</h3><p>
To upgrade the shared library associated with a UDF, issue a
<code class="literal">DROP FUNCTION</code> statement, upgrade the shared
library, and then issue a <code class="literal">CREATE FUNCTION</code>
statement. If you upgrade the shared library first and then
use <code class="literal">DROP FUNCTION</code>, the server may crash.
</p></div></div><div class="section" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="drop-function"></a>26.2.3. <code class="literal">DROP FUNCTION</code> Syntax</h3></div></div></div><a class="indexterm" name="id3153035"></a><a class="indexterm" name="id3153044"></a><a class="indexterm" name="id3153057"></a><a class="indexterm" name="id3153069"></a><a class="indexterm" name="id3153078"></a><a class="indexterm" name="id3153087"></a><pre class="programlisting">DROP FUNCTION <em class="replaceable"><code>function_name</code></em>
</pre><p>
This statement drops the user-defined function (UDF) named
<em class="replaceable"><code>function_name</code></em>.
</p><p>
To drop a function, you must have the <code class="literal">DELETE</code>
privilege for the <code class="literal">mysql</code> database. This is
because <code class="literal">DROP FUNCTION</code> removes a row from the
<code class="literal">mysql.func</code> system table that records the
function's name, type, and shared library name.
</p><div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Note</h3><p>
To upgrade the shared library associated with a UDF, issue a
<code class="literal">DROP FUNCTION</code> statement, upgrade the shared
library, and then issue a <code class="literal">CREATE FUNCTION</code>
statement. If you upgrade the shared library first and then
use <code class="literal">DROP FUNCTION</code>, the server may crash.
</p></div><p>
<code class="literal">DROP FUNCTION</code> is also used to drop stored
functions (see <a href="stored-procedures.html#drop-procedure" title="18.2.3. DROP PROCEDURE and DROP FUNCTION
Syntax">Section 18.2.3, “<code class="literal">DROP PROCEDURE</code> and <code class="literal">DROP FUNCTION</code>
Syntax”</a>).
</p></div><div class="section" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="adding-udf"></a>26.2.4. Adding a New User-Defined Function</h3></div></div></div><div class="toc"><dl><dt><span class="section"><a href="extending-mysql.html#udf-calling">26.2.4.1. UDF Calling Sequences for Simple Functions</a></span></dt><dt><span class="section"><a href="extending-mysql.html#udf-aggr-calling">26.2.4.2. UDF Calling Sequences for Aggregate Functions</a></span></dt><dt><span class="section"><a href="extending-mysql.html#udf-arguments">26.2.4.3. UDF Argument Processing</a></span></dt><dt><span class="section"><a href="extending-mysql.html#udf-return-values">26.2.4.4. UDF Return Values and Error Handling</a></span></dt><dt><span class="section"><a href="extending-mysql.html#udf-compiling">26.2.4.5. Compiling and Installing User-Defined Functions</a></span></dt><dt><span class="section"><a href="extending-mysql.html#udf-security">26.2.4.6. User-Defined Function Security Precautions</a></span></dt></dl></div><a class="indexterm" name="id3153233"></a><a class="indexterm" name="id3153246"></a><a class="indexterm" name="id3153259"></a><p>
For the UDF mechanism to work, functions must be written in C or
C++ (or another language that can use C calling conventions),
and your operating system must support dynamic loading. The
MySQL source distribution includes a file
<code class="filename">sql/udf_example.c</code> that defines 5 new
functions. Consult this file to see how UDF calling conventions
work. UDF-related symbols and data structures are defined in the
<code class="filename">include/mysql_com.h</code> header file. (You need
not include this header file directly because it is included by
<code class="filename">mysql.h</code>.)
</p><p>
A UDF contains code that becomes part of the running server, so
when you write a UDF, you are bound by any and all constraints
that otherwise apply to writing server code. For example, you
may have problems if you attempt to use functions from the
<code class="literal">libstdc++</code> library. Note that these
constraints may change in future versions of the server, so it
is possible that server upgrades will require revisions to UDFs
that were originally written for older servers. For information
about these constraints, see
<a href="installing.html#configure-options" title="2.4.15.2. Typical configure Options">Section 2.4.15.2, “Typical <span><strong class="command">configure</strong></span> Options”</a>, and
<a href="installing.html#compilation-problems" title="2.4.15.4. Dealing with Problems Compiling MySQL">Section 2.4.15.4, “Dealing with Problems Compiling MySQL”</a>.
</p><p>
To be able to use UDFs, you need to link
<span><strong class="command">mysqld</strong></span> dynamically. Don't configure MySQL
using <code class="option">--with-mysqld-ldflags=-all-static</code>. If you
want to use a UDF that needs to access symbols from
<span><strong class="command">mysqld</strong></span> (for example, the
<code class="literal">metaphone</code> function in
<code class="filename">sql/udf_example.c</code> that uses
<code class="literal">default_charset_info</code>), you must link the
program with <code class="option">-rdynamic</code> (see <code class="literal">man
dlopen</code>). If you plan to use UDFs, the rule of thumb is
to configure MySQL with
<code class="option">--with-mysqld-ldflags=-rdynamic</code> unless you have
a very good reason not to.
</p><p>
For each function that you want to use in SQL statements, you
should define corresponding C (or C++) functions. In the
following discussion, the name “<span class="quote">xxx</span>” is used for an
example function name. To distinguish between SQL and C/C++
usage, <code class="literal">XXX()</code> (uppercase) indicates an SQL
function call, and <code class="literal">xxx()</code> (lowercase)
indicates a C/C++ function call.
</p><p>
The C/C++ functions that you write to implement the interface
for <code class="literal">XXX()</code> are:
</p><div class="itemizedlist"><ul type="disc"><li><p>
<code class="literal">xxx()</code> (required)
</p><p>
The main function. This is where the function result is
computed. The correspondence between the SQL function data
type and the return type of your C/C++ function is shown
here:
</p><div class="informaltable"><table border="1"><colgroup><col><col></colgroup><tbody><tr><td><span class="bold"><strong>SQL Type</strong></span></td><td><span class="bold"><strong>C/C++ Type</strong></span></td></tr><tr><td><code class="literal">STRING</code></td><td><code class="literal">char *</code></td></tr><tr><td><code class="literal">INTEGER</code></td><td><code class="literal">long long</code></td></tr><tr><td><code class="literal">REAL</code></td><td><code class="literal">double</code></td></tr></tbody></table></div><p>
It is also possible to declare a <code class="literal">DECIMAL</code>
function, but currently the value is returned as a string,
so you should write the UDF as though it were a
<code class="literal">STRING</code> function. <code class="literal">ROW</code>
functions are not implemented.
</p></li><li><p>
<code class="literal">xxx_init()</code> (optional)
</p><p>
The initialization function for <code class="literal">xxx()</code>. It
can be used for the following purposes:
</p><div class="itemizedlist"><ul type="circle"><li><p>
To check the number of arguments to
<code class="literal">XXX()</code>.
</p></li><li><p>
To check that the arguments are of a required type or,
alternatively, to tell MySQL to coerce arguments to the
types you want when the main function is called.
</p></li><li><p>
To allocate any memory required by the main function.
</p></li><li><p>
To specify the maximum length of the result.
</p></li><li><p>
To specify (for <code class="literal">REAL</code> functions) the
maximum number of decimal places in the result.
</p></li><li><p>
To specify whether the result can be
<code class="literal">NULL</code>.
</p></li></ul></div></li><li><p>
<code class="literal">xxx_deinit()</code> (optional)
</p><p>
The deinitialization function for <code class="literal">xxx()</code>.
It should deallocate any memory allocated by the
initialization function.
</p></li></ul></div><p>
When an SQL statement invokes <code class="literal">XXX()</code>, MySQL
calls the initialization function <code class="literal">xxx_init()</code>
to let it perform any required setup, such as argument checking
or memory allocation. If <code class="literal">xxx_init()</code> returns
an error, MySQL aborts the SQL statement with an error message
and does not call the main or deinitialization functions.
Otherwise, MySQL calls the main function
<code class="literal">xxx()</code> once for each row. After all rows have
been processed, MySQL calls the deinitialization function
<code class="literal">xxx_deinit()</code> so that it can perform any
required cleanup.
</p><p>
For aggregate functions that work like
<a href="functions.html#function_sum"><code class="literal">SUM()</code></a>, you must also provide the
following functions:
</p><div class="itemizedlist"><ul type="disc"><li><p>
<code class="literal">xxx_clear()</code> (required in
5.0)
</p><p>
Reset the current aggregate value but do not insert the
argument as the initial aggregate value for a new group.
</p></li><li><p>
<code class="literal">xxx_add()</code> (required)
</p><p>
Add the argument to the current aggregate value.
</p></li></ul></div><p>
MySQL handles aggregate UDFs as follows:
</p><div class="orderedlist"><ol type="1"><li><p>
Call <code class="literal">xxx_init()</code> to let the aggregate
function allocate any memory it needs for storing results.
</p></li><li><p>
Sort the table according to the <code class="literal">GROUP BY</code>
expression.
</p></li><li><p>
Call <code class="literal">xxx_clear()</code> for the first row in
each new group.
</p></li><li><p>
Call <code class="literal">xxx_add()</code> for each new row that
belongs in the same group.
</p></li><li><p>
Call <code class="literal">xxx()</code> to get the result for the
aggregate when the group changes or after the last row has
been processed.
</p></li><li><p>
Repeat 3-5 until all rows has been processed
</p></li><li><p>
Call <code class="literal">xxx_deinit()</code> to let the UDF free any
memory it has allocated.
</p></li></ol></div><p>
All functions must be thread-safe. This includes not just the
main function, but the initialization and deinitialization
functions as well, and also the additional functions required by
aggregate functions. A consequence of this requirement is that
you are not allowed to allocate any global or static variables
that change! If you need memory, you should allocate it in
<code class="literal">xxx_init()</code> and free it in
<code class="literal">xxx_deinit()</code>.
</p><div class="section" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="udf-calling"></a>26.2.4.1. UDF Calling Sequences for Simple Functions</h4></div></div></div><a class="indexterm" name="id3153912"></a><p>
This section describes the different functions that you need
to define when you create a simple UDF.
<a href="extending-mysql.html#adding-udf" title="26.2.4. Adding a New User-Defined Function">Section 26.2.4, “Adding a New User-Defined Function”</a>, describes the order in which
MySQL calls these functions.
</p><p>
The main <code class="literal">xxx()</code> function should be declared
as shown in this section. Note that the return type and
parameters differ, depending on whether you declare the SQL
function <code class="literal">XXX()</code> to return
<code class="literal">STRING</code>, <code class="literal">INTEGER</code>, or
<code class="literal">REAL</code> in the <code class="literal">CREATE
FUNCTION</code> statement:
</p><p>
For <code class="literal">STRING</code> functions:
</p><pre class="programlisting">char *xxx(UDF_INIT *initid, UDF_ARGS *args,
char *result, unsigned long *length,
char *is_null, char *error);
</pre><p>
For <code class="literal">INTEGER</code> functions:
</p><pre class="programlisting">long long xxx(UDF_INIT *initid, UDF_ARGS *args,
char *is_null, char *error);
</pre><p>
For <code class="literal">REAL</code> functions:
</p><pre class="programlisting">double xxx(UDF_INIT *initid, UDF_ARGS *args,
char *is_null, char *error);
</pre><p>
<code class="literal">DECIMAL</code> functions return string values and
should be declared the same way as <code class="literal">STRING</code>
functions. <code class="literal">ROW</code> functions are not
implemented.
</p><p>
The initialization and deinitialization functions are declared
like this:
</p><pre class="programlisting">my_bool xxx_init(UDF_INIT *initid, UDF_ARGS *args, char *message);
void xxx_deinit(UDF_INIT *initid);
</pre><p>
The <code class="literal">initid</code> parameter is passed to all three
functions. It points to a <code class="literal">UDF_INIT</code>
structure that is used to communicate information between
functions. The <code class="literal">UDF_INIT</code> structure members
follow. The initialization function should fill in any members
that it wishes to change. (To use the default for a member,
leave it unchanged.)
</p><div class="itemizedlist"><ul type="disc"><li><p>
<code class="literal">my_bool maybe_null</code>
</p><p>
<code class="literal">xxx_init()</code> should set
<code class="literal">maybe_null</code> to <code class="literal">1</code> if
<code class="literal">xxx()</code> can return
<code class="literal">NULL</code>. The default value is
<code class="literal">1</code> if any of the arguments are declared
<code class="literal">maybe_null</code>.
</p></li><li><p>
<code class="literal">unsigned int decimals</code>
</p><p>
The number of decimal digits to the right of the decimal
point. The default value is the maximum number of decimal
digits in the arguments passed to the main function. (For
example, if the function is passed
<code class="literal">1.34</code>, <code class="literal">1.345</code>, and
<code class="literal">1.3</code>, the default would be 3, because
<code class="literal">1.345</code> has 3 decimal digits.
</p></li><li><p>
<code class="literal">unsigned int max_length</code>
</p><p>
The maximum length of the result. The default
<code class="literal">max_length</code> value differs depending on
the result type of the function. For string functions, the
default is the length of the longest argument. For integer
functions, the default is 21 digits. For real functions,
the default is 13 plus the number of decimal digits
indicated by <code class="literal">initid->decimals</code>. (For
numeric functions, the length includes any sign or decimal
point characters.)
</p><p>
If you want to return a blob value, you can set
<code class="literal">max_length</code> to 65KB or 16MB. This memory
is not allocated, but the value is used to decide which
data type to use if there is a need to temporarily store
the data.
</p></li><li><p>
<code class="literal">char *ptr</code>
</p><p>
A pointer that the function can use for its own purposes.
For example, functions can use
<code class="literal">initid->ptr</code> to communicate allocated
memory among themselves. <code class="literal">xxx_init()</code>
should allocate the memory and assign it to this pointer:
</p><pre class="programlisting">initid->ptr = allocated_memory;
</pre><p>
In <code class="literal">xxx()</code> and
<code class="literal">xxx_deinit()</code>, refer to
<code class="literal">initid->ptr</code> to use or deallocate the
memory.
</p></li><li><p>
<code class="literal">my_bool const_item</code>
</p><p>
<code class="literal">xxx_init()</code> should set
<code class="literal">const_item</code> to <code class="literal">1</code> if
<code class="literal">xxx()</code> always returns the same value and
to <code class="literal">0</code> otherwise.
</p></li></ul></div></div><div class="section" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="udf-aggr-calling"></a>26.2.4.2. UDF Calling Sequences for Aggregate Functions</h4></div></div></div><a class="indexterm" name="id3154371"></a><p>
This section describes the different functions that you need
to define when you create an aggregate UDF.
<a href="extending-mysql.html#adding-udf" title="26.2.4. Adding a New User-Defined Function">Section 26.2.4, “Adding a New User-Defined Function”</a>, describes the order in which
MySQL calls these functions.
</p><div class="itemizedlist"><ul type="disc"><li><p>
<code class="literal">xxx_reset()</code>
</p><p>
This function is called when MySQL finds the first row in
a new group. It should reset any internal summary
variables and then use the given
<code class="literal">UDF_ARGS</code> argument as the first value in
your internal summary value for the group. Declare
<code class="literal">xxx_reset()</code> as follows:
</p><pre class="programlisting">char *xxx_reset(UDF_INIT *initid, UDF_ARGS *args,
char *is_null, char *error);
</pre><p>
<code class="literal">xxx_reset()</code> is not needed or used in
MySQL 5.0, in which the UDF interface uses
<code class="literal">xxx_clear()</code> instead. However, you can
define both <code class="literal">xxx_reset()</code> and
<code class="literal">xxx_clear()</code> if you want to have your
UDF work with older versions of the server. (If you do
include both functions, the <code class="literal">xxx_reset()</code>
function in many cases can be implemented internally by
calling <code class="literal">xxx_clear()</code> to reset all
variables, and then calling <code class="literal">xxx_add()</code>
to add the <code class="literal">UDF_ARGS</code> argument as the
first value in the group.)
</p></li><li><p>
<code class="literal">xxx_clear()</code>
</p><p>
This function is called when MySQL needs to reset the
summary results. It is called at the beginning for each
new group but can also be called to reset the values for a
query where there were no matching rows. Declare
<code class="literal">xxx_clear()</code> as follows:
</p><pre class="programlisting">char *xxx_clear(UDF_INIT *initid, char *is_null, char *error);
</pre><p>
<code class="literal">is_null</code> is set to point to
<code class="literal">CHAR(0)</code> before calling
<code class="literal">xxx_clear()</code>.
</p><p>
If something went wrong, you can store a value in the
variable to which the <code class="literal">error</code> argument
points. <code class="literal">error</code> points to a single-byte
variable, not to a string buffer.
</p><p>
<code class="literal">xxx_clear()</code> is required by MySQL
5.0.
</p></li><li><p>
<code class="literal">xxx_add()</code>
</p><p>
This function is called for all rows that belong to the
same group, except for the first row. You should use it to
add the value in the <code class="literal">UDF_ARGS</code> argument
to your internal summary variable.
</p><pre class="programlisting">char *xxx_add(UDF_INIT *initid, UDF_ARGS *args,
char *is_null, char *error);
</pre></li></ul></div><p>
The <code class="literal">xxx()</code> function for an aggregate UDF
should be declared the same way as for a non-aggregate UDF.
See <a href="extending-mysql.html#udf-calling" title="26.2.4.1. UDF Calling Sequences for Simple Functions">Section 26.2.4.1, “UDF Calling Sequences for Simple Functions”</a>.
</p><p>
For an aggregate UDF, MySQL calls the <code class="literal">xxx()</code>
function after all rows in the group have been processed. You
should normally never access its <code class="literal">UDF_ARGS</code>
argument here but instead return a value based on your
internal summary variables.
</p><p>
Return value handling in <code class="literal">xxx()</code> should be
done the same way as for a non-aggregate UDF. See
<a href="extending-mysql.html#udf-return-values" title="26.2.4.4. UDF Return Values and Error Handling">Section 26.2.4.4, “UDF Return Values and Error Handling”</a>.
</p><p>
The <code class="literal">xxx_reset()</code> and
<code class="literal">xxx_add()</code> functions handle their
<code class="literal">UDF_ARGS</code> argument the same way as functions
for non-aggregate UDFs. See <a href="extending-mysql.html#udf-arguments" title="26.2.4.3. UDF Argument Processing">Section 26.2.4.3, “UDF Argument Processing”</a>.
</p><p>
The pointer arguments to <code class="literal">is_null</code> and
<code class="literal">error</code> are the same for all calls to
<code class="literal">xxx_reset()</code>,
<code class="literal">xxx_clear()</code>, <code class="literal">xxx_add()</code>
and <code class="literal">xxx()</code>. You can use this to remember
that you got an error or whether the <code class="literal">xxx()</code>
function should return <code class="literal">NULL</code>. You should not
store a string into <code class="literal">*error</code>!
<code class="literal">error</code> points to a single-byte variable, not
to a string buffer.
</p><p>
<code class="literal">*is_null</code> is reset for each group (before
calling <code class="literal">xxx_clear()</code>).
<code class="literal">*error</code> is never reset.
</p><p>
If <code class="literal">*is_null</code> or <code class="literal">*error</code>
are set when <code class="literal">xxx()</code> returns, MySQL returns
<code class="literal">NULL</code> as the result for the group function.
</p></div><div class="section" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="udf-arguments"></a>26.2.4.3. UDF Argument Processing</h4></div></div></div><a class="indexterm" name="id3154823"></a><a class="indexterm" name="id3154832"></a><p>
The <code class="literal">args</code> parameter points to a
<code class="literal">UDF_ARGS</code> structure that has the members
listed here:
</p><div class="itemizedlist"><ul type="disc"><li><p>
<code class="literal">unsigned int arg_count</code>
</p><p>
The number of arguments. Check this value in the
initialization function if you require your function to be
called with a particular number of arguments. For example:
</p><pre class="programlisting">if (args->arg_count != 2)
{
strcpy(message,"XXX() requires two arguments");
return 1;
}
</pre><p>
For other <code class="literal">UDF_ARGS</code> member values that
are arrays, array references are zero-based. That is,
refer to array members using index values from 0 to
<code class="literal">args->arg_count</code> – 1.
</p></li><li><p>
<code class="literal">enum Item_result *arg_type</code>
</p><p>
A pointer to an array containing the types for each
argument. The possible type values are
<code class="literal">STRING_RESULT</code>,
<code class="literal">INT_RESULT</code>,
<code class="literal">REAL_RESULT</code>, and
<code class="literal">DECIMAL_RESULT</code>.
</p><p>
To make sure that arguments are of a given type and return
an error if they are not, check the
<code class="literal">arg_type</code> array in the initialization
function. For example:
</p><pre class="programlisting">if (args->arg_type[0] != STRING_RESULT ||
args->arg_type[1] != INT_RESULT)
{
strcpy(message,"XXX() requires a string and an integer");
return 1;
}
</pre><p>
Arguments of type <code class="literal">DECIMAL_RESULT</code> are
passed as strings, so you should handle them the same way
as <code class="literal">STRING_RESULT</code> values.
</p><p>
As an alternative to requiring your function's arguments
to be of particular types, you can use the initialization
function to set the <code class="literal">arg_type</code> elements
to the types you want. This causes MySQL to coerce
arguments to those types for each call to
<code class="literal">xxx()</code>. For example, to specify that the
first two arguments should be coerced to string and
integer, respectively, do this in
<code class="literal">xxx_init()</code>:
</p><pre class="programlisting">args->arg_type[0] = STRING_RESULT;
args->arg_type[1] = INT_RESULT;
</pre><p>
Exact-value decimal arguments such as
<code class="literal">1.3</code> or <code class="literal">DECIMAL</code>
column values are passed with a type of
<code class="literal">DECIMAL_RESULT</code>. However, the values are
passed as strings. If you want to receive a number, use
the initialization function to specify that the argument
should be coerced to a <code class="literal">REAL_RESULT</code>
value:
</p><pre class="programlisting">args->arg_type[2] = REAL_RESULT;
</pre><div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Note</h3><p>
Prior to MySQL 5.0.3, decimal arguments were passed as
<code class="literal">REAL_RESULT</code> values. If you upgrade to
a newer version and find that your UDF now receives
string values, use the initialization function to coerce
the arguments to numbers as just described.
</p></div></li><li><p>
<code class="literal">char **args</code>
</p><p>
<code class="literal">args->args</code> communicates information
to the initialization function about the general nature of
the arguments passed to your function. For a constant
argument <code class="literal">i</code>,
<code class="literal">args->args[i]</code> points to the argument
value. (See below for instructions on how to access the
value properly.) For a non-constant argument,
<code class="literal">args->args[i]</code> is
<code class="literal">0</code>. A constant argument is an expression
that uses only constants, such as <code class="literal">3</code> or
<code class="literal">4*7-2</code> or <code class="literal">SIN(3.14)</code>.
A non-constant argument is an expression that refers to
values that may change from row to row, such as column
names or functions that are called with non-constant
arguments.
</p><p>
For each invocation of the main function,
<code class="literal">args->args</code> contains the actual
arguments that are passed for the row currently being
processed.
</p><p>
If argument <code class="literal">i</code> represents
<code class="literal">NULL</code>,
<code class="literal">args->args[i]</code> is a null pointer (0).
If the argument is not <code class="literal">NULL</code>, functions
can refer to it as follows:
</p><div class="itemizedlist"><ul type="circle"><li><p>
An argument of type <code class="literal">STRING_RESULT</code>
is given as a string pointer plus a length, to allow
handling of binary data or data of arbitrary length.
The string contents are available as
<code class="literal">args->args[i]</code> and the string
length is <code class="literal">args->lengths[i]</code>. Do
not assume that the string is null-terminated.
</p></li><li><p>
For an argument of type <code class="literal">INT_RESULT</code>,
you must cast <code class="literal">args->args[i]</code> to a
<code class="literal">long long</code> value:
</p><pre class="programlisting">long long int_val;
int_val = *((long long*) args->args[i]);
</pre></li><li><p>
For an argument of type
<code class="literal">REAL_RESULT</code>, you must cast
<code class="literal">args->args[i]</code> to a
<code class="literal">double</code> value:
</p><pre class="programlisting">double real_val;
real_val = *((double*) args->args[i]);
</pre></li><li><p>
For an argument of type
<code class="literal">DECIMAL_RESULT</code>, the value is passed
as a string and should be handled like a
<code class="literal">STRING_RESULT</code> value.
</p></li><li><p>
<code class="literal">ROW_RESULT</code> arguments are not
implemented.
</p></li></ul></div></li><li><p>
<code class="literal">unsigned long *lengths</code>
</p><p>
For the initialization function, the
<code class="literal">lengths</code> array indicates the maximum
string length for each argument. You should not change
these. For each invocation of the main function,
<code class="literal">lengths</code> contains the actual lengths of
any string arguments that are passed for the row currently
being processed. For arguments of types
<code class="literal">INT_RESULT</code> or
<code class="literal">REAL_RESULT</code>, <code class="literal">lengths</code>
still contains the maximum length of the argument (as for
the initialization function).
</p></li><li><p>
<code class="literal">char *maybe_null</code>
</p><p>
For the initialization function, the
<code class="literal">maybe_null</code> array indicates for each
argument whether the argument value might be null (0 if
no, 1 if yes).
</p></li><li><p>
<code class="literal">char **attributes</code>
</p><p>
<code class="literal">args->attributes</code> communicates
information information about the names of the UDF
arguments. For argument <code class="literal">i</code>, the
attribute name is available as a string in
<code class="literal">args->attributes[i]</code> and the
attribute length is
<code class="literal">args->attribute_lengths[i]</code>. Do not
assume that the string is null-terminated.
</p><p>
By default, the name of a UDF argument is the text of the
expression used to specify the argument. For UDFs, an
argument may also have an optional <code class="literal">[AS]
<em class="replaceable"><code>alias_name</code></em></code> clause, in
which case the argument name is
<em class="replaceable"><code>alias_name</code></em>. The
<code class="literal">attributes</code> value for each argument thus
depends on whether an alias was given.
</p><p>
Suppose that a UDF <code class="literal">my_udf()</code> is invoked
as follows:
</p><pre class="programlisting">SELECT my_udf(expr1, expr2 AS alias1, expr3 alias2);
</pre><p>
In this case, the <code class="literal">attributes</code> and
<code class="literal">attribute_lengths</code> arrays will have
these values:
</p><pre class="programlisting">args->attributes[0] = "expr1"
args->attribute_lengths[0] = 5
args->attributes[1] = "alias1"
args->attribute_lengths[1] = 6
args->attributes[2] = "alias2"
args->attribute_lengths[2] = 6
</pre></li><li><p>
<code class="literal">unsigned long *attribute_lengths</code>
</p><p>
The <code class="literal">attribute_lengths</code> array indicates
the length of each argument name.
</p></li></ul></div></div><div class="section" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="udf-return-values"></a>26.2.4.4. UDF Return Values and Error Handling</h4></div></div></div><a class="indexterm" name="id3155580"></a><a class="indexterm" name="id3155592"></a><a class="indexterm" name="id3155605"></a><a class="indexterm" name="id3155617"></a><p>
The initialization function should return <code class="literal">0</code>
if no error occurred and <code class="literal">1</code> otherwise. If an
error occurs, <code class="literal">xxx_init()</code> should store a
null-terminated error message in the
<code class="literal">message</code> parameter. The message is returned
to the client. The message buffer is
<code class="literal">MYSQL_ERRMSG_SIZE</code> characters long, but you
should try to keep the message to less than 80 characters so
that it fits the width of a standard terminal screen.
</p><p>
The return value of the main function <code class="literal">xxx()</code>
is the function value, for <code class="literal">long long</code> and
<code class="literal">double</code> functions. A string function should
return a pointer to the result and set
<code class="literal">*result</code> and <code class="literal">*length</code> to
the contents and length of the return value. For example:
</p><pre class="programlisting">memcpy(result, "result string", 13);
*length = 13;
</pre><p>
The <code class="literal">result</code> buffer that is passed to the
<code class="literal">xxx()</code> function is 255 bytes long. If your
result fits in this, you don't have to worry about memory
allocation for results.
</p><p>
If your string function needs to return a string longer than
255 bytes, you must allocate the space for it with
<code class="literal">malloc()</code> in your
<code class="literal">xxx_init()</code> function or your
<code class="literal">xxx()</code> function and free it in your
<code class="literal">xxx_deinit()</code> function. You can store the
allocated memory in the <code class="literal">ptr</code> slot in the
<code class="literal">UDF_INIT</code> structure for reuse by future
<code class="literal">xxx()</code> calls. See
<a href="extending-mysql.html#udf-calling" title="26.2.4.1. UDF Calling Sequences for Simple Functions">Section 26.2.4.1, “UDF Calling Sequences for Simple Functions”</a>.
</p><p>
To indicate a return value of <code class="literal">NULL</code> in the
main function, set <code class="literal">*is_null</code> to
<code class="literal">1</code>:
</p><pre class="programlisting">*is_null = 1;
</pre><p>
To indicate an error return in the main function, set
<code class="literal">*error</code> to <code class="literal">1</code>:
</p><pre class="programlisting">*error = 1;
</pre><p>
If <code class="literal">xxx()</code> sets <code class="literal">*error</code> to
<code class="literal">1</code> for any row, the function value is
<code class="literal">NULL</code> for the current row and for any
subsequent rows processed by the statement in which
<code class="literal">XXX()</code> was invoked.
(<code class="literal">xxx()</code> is not even called for subsequent
rows.)
</p></div><div class="section" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="udf-compiling"></a>26.2.4.5. Compiling and Installing User-Defined Functions</h4></div></div></div><a class="indexterm" name="id3155891"></a><a class="indexterm" name="id3155903"></a><a class="indexterm" name="id3155916"></a><p>
Files implementing UDFs must be compiled and installed on the
host where the server runs. This process is described below
for the example UDF file
<code class="filename">sql/udf_example.c</code> that is included in the
MySQL source distribution.
</p><p>
The immediately following instructions are for Unix.
Instructions for Windows are given later in this section.
</p><p>
The <code class="filename">udf_example.c</code> file contains the
following functions:
</p><div class="itemizedlist"><ul type="disc"><li><p>
<code class="literal">metaphon()</code> returns a metaphon string of
the string argument. This is something like a soundex
string, but it's more tuned for English.
</p></li><li><p>
<code class="literal">myfunc_double()</code> returns the sum of the
ASCII values of the characters in its arguments, divided
by the sum of the length of its arguments.
</p></li><li><p>
<code class="literal">myfunc_int()</code> returns the sum of the
length of its arguments.
</p></li><li><p>
<code class="literal">sequence([const int])</code> returns a
sequence starting from the given number or 1 if no number
has been given.
</p></li><li><p>
<code class="literal">lookup()</code> returns the IP number for a
hostname.
</p></li><li><p>
<code class="literal">reverse_lookup()</code> returns the hostname
for an IP number. The function may be called either with a
single string argument of the form
<code class="literal">'xxx.xxx.xxx.xxx'</code> or with four numbers.
</p></li></ul></div><p>
A dynamically loadable file should be compiled as a sharable
object file, using a command something like this:
</p><pre class="programlisting">shell> <strong class="userinput"><code>gcc -shared -o udf_example.so udf_example.c</code></strong>
</pre><p>
If you are using <span><strong class="command">gcc</strong></span> with
<span><strong class="command">configure</strong></span> and <span><strong class="command">libtool</strong></span>
(which is how MySQL is configured), you should be able to
create <code class="filename">udf_example.so</code> with a simpler
command:
</p><pre class="programlisting">shell> <strong class="userinput"><code>make udf_example.la</code></strong>
</pre><p>
After you compile a shared object containing UDFs, you must
install it and tell MySQL about it. Compiling a shared object
from <code class="filename">udf_example.c</code> using
<span><strong class="command">gcc</strong></span> directly produces a file named
<code class="filename">udf_example.so</code>. Compiling the shared
object using <span><strong class="command">make</strong></span> produces a file named
something like <code class="filename">udf_example.so.0.0.0</code> in
the <code class="filename">.libs</code> directory (the exact name may
vary from platform to platform). Copy the shared object to
some directory such as <code class="filename">/usr/lib</code> that is
searched by your system's dynamic (runtime) linker, or add the
directory in which you placed the shared object to the linker
configuration file (for example,
<code class="filename">/etc/ld.so.conf</code>).
</p><p>
The dynamic linker name is system-specific (for example,
<span><strong class="command">ld-elf.so.1</strong></span> on FreeBSD,
<span><strong class="command">ld.so</strong></span> on Linux, or <span><strong class="command">dyld</strong></span>
on Mac OS X). Consult your system documentation for
information about the linker name and how to configure it.
</p><p>
On many systems, you can also set the
<code class="literal">LD_LIBRARY</code> or
<code class="literal">LD_LIBRARY_PATH</code> environment variable to
point at the directory where you have the files for your UDF.
The <code class="literal">dlopen</code> manual page tells you which
variable you should use on your system. You should set this in
<span><strong class="command">mysql.server</strong></span> or
<span><strong class="command">mysqld_safe</strong></span> startup scripts and restart
<span><strong class="command">mysqld</strong></span>.
</p><p>
On some systems, the <span><strong class="command">ldconfig</strong></span> program that
configures the dynamic linker does not recognize a shared
object unless its name begins with <code class="literal">lib</code>. In
this case you should rename a file such as
<code class="filename">udf_example.so</code> to
<code class="filename">libudf_example.so</code>.
</p><p>
On Windows, you can compile user-defined functions by using
the following procedure:
</p><div class="orderedlist"><ol type="1"><li><p>
You need to obtain the BitKeeper source repository for
MySQL 5.0. See
<a href="installing.html#installing-source-tree" title="2.4.15.3. Installing from the Development Source Tree">Section 2.4.15.3, “Installing from the Development Source Tree”</a>.
</p></li><li><p>
You must obtain the CMake build utility from
<a href="http://www.cmake.org" target="_top">http://www.cmake.org</a>. (Version 2.4.2 or
later is required).
</p></li><li><p>
In the source repository, look in the
<code class="filename">sql</code> directory. There are files named
<code class="filename">udf_example.def</code>
<code class="filename">udf_example.c</code> there. Copy both files
from this directory to your working directory.
</p></li><li><p>
Create a CMake <code class="filename">makefile</code> with these
contents:
</p><pre class="programlisting">PROJECT(udf_example)
# Path for MySQL include directory
INCLUDE_DIRECTORIES("c:/mysql/include")
ADD_DEFINITIONS("-DHAVE_DLOPEN")
ADD_LIBRARY(udf_example MODULE udf_example.c udf_example.def)
TARGET_LINK_LIBRARIES(udf_example wsock32)
</pre></li><li><p>
Create the VC project and solution files:
</p><pre class="programlisting">cmake -G "<Generator>"
</pre><p>
Invoking <span><strong class="command">cmake --help</strong></span> shows you a list
of valid Generators.
</p></li><li><p>
Create <code class="filename">udf_example.dll</code>:
</p><pre class="programlisting">devenv udf_example.sln /build Release
</pre></li></ol></div><p>
After the shared object file has been installed, notify
<span><strong class="command">mysqld</strong></span> about the new functions with these
statements:
</p><pre class="programlisting">mysql> <strong class="userinput"><code>CREATE FUNCTION metaphon RETURNS STRING SONAME 'udf_example.so';</code></strong>
mysql> <strong class="userinput"><code>CREATE FUNCTION myfunc_double RETURNS REAL SONAME 'udf_example.so';</code></strong>
mysql> <strong class="userinput"><code>CREATE FUNCTION myfunc_int RETURNS INTEGER SONAME 'udf_example.so';</code></strong>
mysql> <strong class="userinput"><code>CREATE FUNCTION lookup RETURNS STRING SONAME 'udf_example.so';</code></strong>
mysql> <strong class="userinput"><code>CREATE FUNCTION reverse_lookup</code></strong>
-> <strong class="userinput"><code>RETURNS STRING SONAME 'udf_example.so';</code></strong>
mysql> <strong class="userinput"><code>CREATE AGGREGATE FUNCTION avgcost</code></strong>
-> <strong class="userinput"><code>RETURNS REAL SONAME 'udf_example.so';</code></strong>
</pre><p>
Functions can be deleted using <code class="literal">DROP
FUNCTION</code>:
</p><pre class="programlisting">mysql> <strong class="userinput"><code>DROP FUNCTION metaphon;</code></strong>
mysql> <strong class="userinput"><code>DROP FUNCTION myfunc_double;</code></strong>
mysql> <strong class="userinput"><code>DROP FUNCTION myfunc_int;</code></strong>
mysql> <strong class="userinput"><code>DROP FUNCTION lookup;</code></strong>
mysql> <strong class="userinput"><code>DROP FUNCTION reverse_lookup;</code></strong>
mysql> <strong class="userinput"><code>DROP FUNCTION avgcost;</code></strong>
</pre><p>
The <code class="literal">CREATE FUNCTION</code> and <code class="literal">DROP
FUNCTION</code> statements update the
<code class="literal">func</code> system table in the
<code class="literal">mysql</code> database. The function's name, type
and shared library name are saved in the table. You must have
the <code class="literal">INSERT</code> and <code class="literal">DELETE</code>
privileges for the <code class="literal">mysql</code> database to create
and drop functions.
</p><p>
You should not use <code class="literal">CREATE FUNCTION</code> to add a
function that has previously been created. If you need to
reinstall a function, you should remove it with <code class="literal">DROP
FUNCTION</code> and then reinstall it with <code class="literal">CREATE
FUNCTION</code>. You would need to do this, for example, if
you recompile a new version of your function, so that
<span><strong class="command">mysqld</strong></span> gets the new version. Otherwise, the
server continues to use the old version.
</p><p>
An active function is one that has been loaded with
<code class="literal">CREATE FUNCTION</code> and not removed with
<code class="literal">DROP FUNCTION</code>. All active functions are
reloaded each time the server starts, unless you start
<span><strong class="command">mysqld</strong></span> with the
<code class="option">--skip-grant-tables</code> option. In this case, UDF
initialization is skipped and UDFs are unavailable.
</p><p>
If the new function will be referred to in statements that
will be replicated to slave servers, you must ensure that
every slave server also has the function available. Otherwise,
replication will fail on the slaves when they attempt to
invoke the function.
</p></div><div class="section" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="udf-security"></a>26.2.4.6. User-Defined Function Security Precautions</h4></div></div></div><p>
MySQL takes the following measures to prevent misuse of
user-defined functions.
</p><p>
You must have the <code class="literal">INSERT</code> privilege to be
able to use <code class="literal">CREATE FUNCTION</code> and the
<code class="literal">DELETE</code> privilege to be able to use
<code class="literal">DROP FUNCTION</code>. This is necessary because
these statements add and delete rows from the
<code class="literal">mysql.func</code> table.
</p><p>
UDFs should have at least one symbol defined in addition to
the <code class="literal">xxx</code> symbol that corresponds to the main
<code class="literal">xxx()</code> function. These auxiliary symbols
correspond to the <code class="literal">xxx_init()</code>,
<code class="literal">xxx_deinit()</code>,
<code class="literal">xxx_reset()</code>,
<code class="literal">xxx_clear()</code>, and
<code class="literal">xxx_add()</code> functions. As of MySQL 5.0.3,
<span><strong class="command">mysqld</strong></span> supports an
<code class="option">--allow-suspicious-udfs</code> option that controls
whether UDFs that have only an <code class="literal">xxx</code> symbol
can be loaded. By default, the option is off, to prevent
attempts at loading functions from shared object files other
than those containing legitimate UDFs. If you have older UDFs
that contain only the <code class="literal">xxx</code> symbol and that
cannot be recompiled to include an auxiliary symbol, it may be
necessary to specify the
<code class="option">--allow-suspicious-udfs</code> option. Otherwise,
you should avoid enabling this capability.
</p><p>
UDF object files cannot be placed in arbitrary directories.
They must be located in some system directory that the dynamic
linker is configured to search. To enforce this restriction
and prevent attempts at specifying pathnames outside of
directories searched by the dynamic linker, MySQL checks the
shared object file name specified in <code class="literal">CREATE
FUNCTION</code> statements for pathname delimiter
characters. As of MySQL 5.0.3, MySQL also checks for pathname
delimiters in filenames stored in the
<code class="literal">mysql.func</code> table when it loads functions.
This prevents attempts at specifying illegitimate pathnames
through direct manipulation of the
<code class="literal">mysql.func</code> table. For information about
UDFs and the runtime linker, see
<a href="extending-mysql.html#udf-compiling" title="26.2.4.5. Compiling and Installing User-Defined Functions">Section 26.2.4.5, “Compiling and Installing User-Defined Functions”</a>.
</p></div></div><div class="section" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="adding-native-function"></a>26.2.5. Adding a New Native Function</h3></div></div></div><a class="indexterm" name="id3156848"></a><a class="indexterm" name="id3156861"></a><a class="indexterm" name="id3156873"></a><p>
The procedure for adding a new native function is described
here. Note that you cannot add native functions to a binary
distribution because the procedure involves modifying MySQL
source code. You must compile MySQL yourself from a source
distribution. Also note that if you migrate to another version
of MySQL (for example, when a new version is released), you need
to repeat the procedure with the new version.
</p><p>
To add a new native MySQL function, follow these steps:
</p><div class="orderedlist"><ol type="1"><li><p>
Add one line to <code class="filename">lex.h</code> that defines the
function name in the <code class="literal">sql_functions[]</code>
array.
</p></li><li><p>
If the function prototype is simple (just takes zero, one,
two or three arguments), you should in
<code class="filename">lex.h</code> specify
<code class="literal">SYM(FUNC_ARG<em class="replaceable"><code>N</code></em>)</code>
(where <em class="replaceable"><code>N</code></em> is the number of
arguments) as the second argument in the
<code class="literal">sql_functions[]</code> array and add a function
that creates a function object in
<code class="filename">item_create.cc</code>. Take a look at
<code class="literal">"ABS"</code> and
<code class="literal">create_funcs_abs()</code> for an example of
this.
</p><p>
If the function prototype is complicated (for example, if it
takes a variable number of arguments), you should add two
lines to <code class="filename">sql_yacc.yy</code>. One indicates the
preprocessor symbol that <span><strong class="command">yacc</strong></span> should
define (this should be added at the beginning of the file).
Then define the function parameters and add an
“<span class="quote">item</span>” with these parameters to the
<code class="literal">simple_expr</code> parsing rule. For an example,
check all occurrences of <code class="literal">ATAN</code> in
<code class="filename">sql_yacc.yy</code> to see how this is done.
</p></li><li><p>
In <code class="filename">item_func.h</code>, declare a class
inheriting from <code class="literal">Item_num_func</code> or
<code class="literal">Item_str_func</code>, depending on whether your
function returns a number or a string.
</p></li><li><p>
In <code class="filename">item_func.cc</code>, add one of the
following declarations, depending on whether you are
defining a numeric or string function:
</p><pre class="programlisting">double Item_func_newname::val()
longlong Item_func_newname::val_int()
String *Item_func_newname::Str(String *str)
</pre><p>
If you inherit your object from any of the standard items
(like <code class="literal">Item_num_func</code>), you probably only
have to define one of these functions and let the parent
object take care of the other functions. For example, the
<code class="literal">Item_str_func</code> class defines a
<code class="literal">val()</code> function that executes
<code class="literal">atof()</code> on the value returned by
<code class="literal">::str()</code>.
</p></li><li><p>
If the function is non-deterministic, you should include the
following statement in the item constructor to indicate that
function results should not be cached:
</p><pre class="programlisting">current_thd->lex->safe_to_cache_query=0;
</pre><p>
A function is non-deterministic if, given fixed values for
its arguments, it can return different results for different
invocations.
</p></li><li><p>
You should probably also define the following object
function:
</p><pre class="programlisting">void Item_func_newname::fix_length_and_dec()
</pre><p>
This function should at least calculate
<code class="literal">max_length</code> based on the given arguments.
<code class="literal">max_length</code> is the maximum number of
characters the function may return. This function should
also set <code class="literal">maybe_null = 0</code> if the main
function can't return a <code class="literal">NULL</code> value. The
function can check whether any of the function arguments can
return <code class="literal">NULL</code> by checking the arguments'
<code class="literal">maybe_null</code> variable. You can take a look
at <code class="literal">Item_func_mod::fix_length_and_dec</code> for
a typical example of how to do this.
</p></li></ol></div><p>
All functions must be thread-safe. In other words, don't use any
global or static variables in the functions without protecting
them with mutexes)
</p><p>
If you want to return <code class="literal">NULL</code>, from
<code class="literal">::val()</code>, <code class="literal">::val_int()</code> or
<code class="literal">::str()</code> you should set
<code class="literal">null_value</code> to 1 and return 0.
</p><p>
For <code class="literal">::str()</code> object functions, there are some
additional considerations to be aware of:
</p><div class="itemizedlist"><ul type="disc"><li><p>
The <code class="literal">String *str</code> argument provides a
string buffer that may be used to hold the result. (For more
information about the <code class="literal">String</code> type, take a
look at the <code class="filename">sql_string.h</code> file.)
</p></li><li><p>
The <code class="literal">::str()</code> function should return the
string that holds the result or <code class="literal">(char*) 0</code>
if the result is <code class="literal">NULL</code>.
</p></li><li><p>
All current string functions try to avoid allocating any
memory unless absolutely necessary!
</p></li></ul></div><p>
If the new native function will be referred to in statements
that will be replicated to slave servers, you must ensure that
every slave server also has the function available. Otherwise,
replication will fail on the slaves when they attempt to invoke
the function.
</p></div></div><div class="section" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="adding-procedures"></a>26.3. Adding New Procedures to MySQL</h2></div></div></div><div class="toc"><dl><dt><span class="section"><a href="extending-mysql.html#procedure-analyse">26.3.1. Procedure Analyse</a></span></dt><dt><span class="section"><a href="extending-mysql.html#writing-a-procedure">26.3.2. Writing a Procedure</a></span></dt></dl></div><a class="indexterm" name="id3157361"></a><a class="indexterm" name="id3157373"></a><a class="indexterm" name="id3157386"></a><p>
In MySQL, you can define a procedure in C++ that can access and
modify the data in a query before it is sent to the client. The
modification can be done on a row-by-row or <code class="literal">GROUP
BY</code> level.
</p><p>
We have created an example procedure to show you what can be done.
</p><p>
Additionally, we recommend that you take a look at
<code class="literal">mylua</code>. With this you can use the LUA language
to load a procedure at runtime into <span><strong class="command">mysqld</strong></span>.
</p><div class="section" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="procedure-analyse"></a>26.3.1. Procedure Analyse</h3></div></div></div><p>
<code class="literal">analyse([<em class="replaceable"><code>max_elements</code></em>[,<em class="replaceable"><code>max_memory</code></em>]])</code>
</p><p>
This procedure is defined in the
<code class="filename">sql/sql_analyse.cc</code> file. It examines the
result from a query and returns an analysis of the results that
suggests optimal data types for each column. To obtain this
analysis, append <code class="literal">PROCEDURE ANALYSE</code> to the end
of a <code class="literal">SELECT</code> statement:
</p><pre class="programlisting">SELECT ... FROM ... WHERE ... PROCEDURE ANALYSE([<em class="replaceable"><code>max_elements</code></em>,[<em class="replaceable"><code>max_memory</code></em>]])
</pre><p>
For example:
</p><pre class="programlisting">SELECT col1, col2 FROM table1 PROCEDURE ANALYSE(10, 2000);
</pre><p>
The results show some statistics for the values returned by the
query, and propose an optimal data type for the columns. This
can be helpful for checking your existing tables, or after
importing new data. You may need to try different settings for
the arguments so that <code class="literal">PROCEDURE ANALYSE()</code>
does not suggest the <code class="literal">ENUM</code> data type when it
is not appropriate.
</p><p>
The arguments are optional and are used as follows:
</p><div class="itemizedlist"><ul type="disc"><li><p>
<em class="replaceable"><code>max_elements</code></em> (default 256) is the
maximum number of distinct values that
<code class="literal">analyse</code> notices per column. This is used
by <code class="literal">analyse</code> to check whether the optimal
data type should be of type <code class="literal">ENUM</code>.
</p></li><li><p>
<em class="replaceable"><code>max_memory</code></em> (default 8192) is the
maximum amount of memory that <code class="literal">analyse</code>
should allocate per column while trying to find all distinct
values.
</p></li></ul></div></div><div class="section" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="writing-a-procedure"></a>26.3.2. Writing a Procedure</h3></div></div></div><p>
For the moment, the only documentation for this is the source.
</p><p>
You can find all information about procedures by examining the
following files:
</p><div class="itemizedlist"><ul type="disc"><li><p>
<code class="filename">sql/sql_analyse.cc</code>
</p></li><li><p>
<code class="filename">sql/procedure.h</code>
</p></li><li><p>
<code class="filename">sql/procedure.cc</code>
</p></li><li><p>
<code class="filename">sql/sql_select.cc</code>
</p></li></ul></div></div></div><div class="section" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="porting"></a>26.4. Debugging and Porting MySQL</h2></div></div></div><div class="toc"><dl><dt><span class="section"><a href="extending-mysql.html#debugging-server">26.4.1. Debugging a MySQL Server</a></span></dt><dt><span class="section"><a href="extending-mysql.html#debugging-client">26.4.2. Debugging a MySQL Client</a></span></dt><dt><span class="section"><a href="extending-mysql.html#the-dbug-package">26.4.3. The DBUG Package</a></span></dt><dt><span class="section"><a href="extending-mysql.html#rts-threads">26.4.4. Comments about RTS Threads</a></span></dt><dt><span class="section"><a href="extending-mysql.html#thread-packages">26.4.5. Differences Between Thread Packages</a></span></dt></dl></div><a class="indexterm" name="id3157700"></a><p>
This appendix helps you port MySQL to other operating systems. Do
check the list of currently supported operating systems first. See
<a href="installing.html#which-os" title="2.4.2. Operating Systems Supported by MySQL Community Server">Section 2.4.2, “Operating Systems Supported by MySQL Community Server”</a>. If you have created a new port of
MySQL, please let us know so that we can list it here and on our
Web site (<a href="http://www.mysql.com/" target="_top">http://www.mysql.com/</a>), recommending it
to other users.
</p><p>
Note: If you create a new port of MySQL, you are free to copy and
distribute it under the GPL license, but it does not make you a
copyright holder of MySQL.
</p><p>
A working POSIX thread library is needed for the server. On
Solaris 2.5 we use Sun PThreads (the native thread support in 2.4
and earlier versions is not good enough), on Linux we use
LinuxThreads by Xavier Leroy,
<code class="email"><<a href="mailto:Xavier.Leroy@inria.fr">Xavier.Leroy@inria.fr</a>></code>.
</p><p>
The hard part of porting to a new Unix variant without good native
thread support is probably to port MIT-pthreads. See
<code class="filename">mit-pthreads/README</code> and Programming POSIX
Threads (<a href="http://www.humanfactor.com/pthreads/" target="_top">http://www.humanfactor.com/pthreads/</a>).
</p><p>
Up to MySQL 4.0.2, the MySQL distribution included a patched
version of Chris Provenzano's Pthreads from MIT (see the MIT
Pthreads Web page at
<a href="http://www.mit.edu/afs/sipb/project/pthreads/" target="_top">http://www.mit.edu/afs/sipb/project/pthreads/</a> and a
programming introduction at
<a href="http://www.mit.edu:8001/people/proven/IAP_2000/" target="_top">http://www.mit.edu:8001/people/proven/IAP_2000/</a>).
These can be used for some operating systems that do not have
POSIX threads. See <a href="installing.html#mit-pthreads" title="2.4.15.5. MIT-pthreads Notes">Section 2.4.15.5, “MIT-pthreads Notes”</a>.
</p><p>
It is also possible to use another user level thread package named
FSU Pthreads (see
<a href="http://moss.csc.ncsu.edu/~mueller/pthreads/" target="_top">http://moss.csc.ncsu.edu/~mueller/pthreads/</a>). This
implementation is being used for the SCO port.
</p><p>
See the <code class="filename">thr_lock.c</code> and
<code class="filename">thr_alarm.c</code> programs in the
<code class="filename">mysys</code> directory for some tests/examples of
these problems.
</p><p>
Both the server and the client need a working C++ compiler. We use
<span><strong class="command">gcc</strong></span> on many platforms. Other compilers that are
known to work are SPARCworks, Sun Forte, Irix
<span><strong class="command">cc</strong></span>, HP-UX <span><strong class="command">aCC</strong></span>, IBM AIX
<span><strong class="command">xlC_r</strong></span>), Intel <span><strong class="command">ecc/icc</strong></span> and
Compaq <span><strong class="command">cxx</strong></span>).
</p><a class="indexterm" name="id3157865"></a><a class="indexterm" name="id3157878"></a><div class="important" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Important</h3><p>
If you are trying to build MySQL 5.1 with <span><strong class="command">icc</strong></span>
on the IA64 platform, and need support for MySQL Cluster, you
should first ensure that you are using <span><strong class="command">icc</strong></span>
version 9.1.043 or later. (For details, see <a href="http://bugs.mysql.com/21875" target="_top">Bug#21875</a>.)
</p></div><p>
To compile only the client use <span><strong class="command">./configure
--without-server</strong></span>.
</p><p>
There is currently no support for only compiling the server, nor
is it likely to be added unless someone has a good reason for it.
</p><p>
If you want/need to change any <code class="filename">Makefile</code> or
the configure script you also need GNU Automake and Autoconf. See
<a href="installing.html#installing-source-tree" title="2.4.15.3. Installing from the Development Source Tree">Section 2.4.15.3, “Installing from the Development Source Tree”</a>.
</p><p>
All steps needed to remake everything from the most basic files.
</p><pre class="programlisting">/bin/rm */.deps/*.P
/bin/rm -f config.cache
aclocal
autoheader
aclocal
automake
autoconf
./configure --with-debug=full --prefix='your installation directory'
# The makefiles generated above need GNU make 3.75 or newer.
# (called gmake below)
gmake clean all install init-db
</pre><p>
If you run into problems with a new port, you may have to do some
debugging of MySQL! See <a href="extending-mysql.html#debugging-server" title="26.4.1. Debugging a MySQL Server">Section 26.4.1, “Debugging a MySQL Server”</a>.
</p><div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Note</h3><p>
Before you start debugging <span><strong class="command">mysqld</strong></span>, first get
the test programs <code class="literal">mysys/thr_alarm</code> and
<code class="literal">mysys/thr_lock</code> to work. This ensures that
your thread installation has even a remote chance to work!
</p></div><div class="section" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="debugging-server"></a>26.4.1. Debugging a MySQL Server</h3></div></div></div><div class="toc"><dl><dt><span class="section"><a href="extending-mysql.html#compiling-for-debugging">26.4.1.1. Compiling MySQL for Debugging</a></span></dt><dt><span class="section"><a href="extending-mysql.html#making-trace-files">26.4.1.2. Creating Trace Files</a></span></dt><dt><span class="section"><a href="extending-mysql.html#making-windows-dumps">26.4.1.3. Using <code class="filename">pdb</code> to create a Windows crashdump</a></span></dt><dt><span class="section"><a href="extending-mysql.html#using-gdb-on-mysqld">26.4.1.4. Debugging <span><strong class="command">mysqld</strong></span> under <span><strong class="command">gdb</strong></span></a></span></dt><dt><span class="section"><a href="extending-mysql.html#using-stack-trace">26.4.1.5. Using a Stack Trace</a></span></dt><dt><span class="section"><a href="extending-mysql.html#using-log-files">26.4.1.6. Using Server Logs to Find Causes of Errors in <span><strong class="command">mysqld</strong></span></a></span></dt><dt><span class="section"><a href="extending-mysql.html#reproducible-test-case">26.4.1.7. Making a Test Case If You Experience Table Corruption</a></span></dt></dl></div><a class="indexterm" name="id3158012"></a><a class="indexterm" name="id3158024"></a><a class="indexterm" name="id3158036"></a><p>
If you are using some functionality that is very new in MySQL,
you can try to run <span><strong class="command">mysqld</strong></span> with the
<code class="option">--skip-new</code> (which disables all new, potentially
unsafe functionality) or with <code class="option">--safe-mode</code> which
disables a lot of optimization that may cause problems. See
<a href="error-handling.html#crashing" title="B.1.4.2. What to Do If MySQL Keeps Crashing">Section B.1.4.2, “What to Do If MySQL Keeps Crashing”</a>.
</p><p>
If <span><strong class="command">mysqld</strong></span> doesn't want to start, you should
verify that you don't have any <code class="filename">my.cnf</code> files
that interfere with your setup! You can check your
<code class="filename">my.cnf</code> arguments with <span><strong class="command">mysqld
--print-defaults</strong></span> and avoid using them by starting with
<span><strong class="command">mysqld --no-defaults ...</strong></span>.
</p><p>
If <span><strong class="command">mysqld</strong></span> starts to eat up CPU or memory or
if it “<span class="quote">hangs,</span>” you can use <span><strong class="command">mysqladmin
processlist status</strong></span> to find out if someone is executing
a query that takes a long time. It may be a good idea to run
<span><strong class="command">mysqladmin -i10 processlist status</strong></span> in some
window if you are experiencing performance problems or problems
when new clients can't connect.
</p><p>
The command <span><strong class="command">mysqladmin debug</strong></span> dumps some
information about locks in use, used memory and query usage to
the MySQL log file. This may help solve some problems. This
command also provides some useful information even if you
haven't compiled MySQL for debugging!
</p><p>
If the problem is that some tables are getting slower and slower
you should try to optimize the table with <code class="literal">OPTIMIZE
TABLE</code> or <span><strong class="command">myisamchk</strong></span>. See
<a href="server-administration.html" title="Chapter 5. MySQL Server Administration">Chapter 5, <i>MySQL Server Administration</i></a>. You should also check
the slow queries with <code class="literal">EXPLAIN</code>.
</p><p>
You should also read the OS-specific section in this manual for
problems that may be unique to your environment. See
<a href="installing.html#operating-system-specific-notes" title="2.4.19. Operating System-Specific Notes">Section 2.4.19, “Operating System-Specific Notes”</a>.
</p><div class="section" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="compiling-for-debugging"></a>26.4.1.1. Compiling MySQL for Debugging</h4></div></div></div><p>
If you have some very specific problem, you can always try to
debug MySQL. To do this you must configure MySQL with the
<code class="option">--with-debug</code> or the
<code class="option">--with-debug=full</code> option. You can check
whether MySQL was compiled with debugging by doing:
<span><strong class="command">mysqld --help</strong></span>. If the
<code class="option">--debug</code> flag is listed with the options then
you have debugging enabled. <span><strong class="command">mysqladmin ver</strong></span>
also lists the <span><strong class="command">mysqld</strong></span> version as
<span><strong class="command">mysql ... --debug</strong></span> in this case.
</p><p>
If you are using <span><strong class="command">gcc</strong></span>, the recommended
<span><strong class="command">configure</strong></span> line is:
</p><pre class="programlisting">CC=gcc CFLAGS="-O2" CXX=gcc CXXFLAGS="-O2 -felide-constructors \
-fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql \
--with-debug --with-extra-charsets=complex
</pre><p>
This avoids problems with the <code class="literal">libstdc++</code>
library and with C++ exceptions (many compilers have problems
with C++ exceptions in threaded code) and compile a MySQL
version with support for all character sets.
</p><p>
If you suspect a memory overrun error, you can configure MySQL
with <code class="option">--with-debug=full</code>, which installs a
memory allocation (<code class="literal">SAFEMALLOC</code>) checker.
However, running with <code class="literal">SAFEMALLOC</code> is quite
slow, so if you get performance problems you should start
<span><strong class="command">mysqld</strong></span> with the
<code class="option">--skip-safemalloc</code> option. This disables the
memory overrun checks for each call to
<code class="literal">malloc()</code> and <code class="literal">free()</code>.
</p><p>
If <span><strong class="command">mysqld</strong></span> stops crashing when you compile
it with <code class="option">--with-debug</code>, you probably have found
a compiler bug or a timing bug within MySQL. In this case, you
can try to add <code class="option">-g</code> to the
<code class="literal">CFLAGS</code> and <code class="literal">CXXFLAGS</code>
variables above and not use <code class="option">--with-debug</code>. If
<span><strong class="command">mysqld</strong></span> dies, you can at least attach to it
with <span><strong class="command">gdb</strong></span> or use <span><strong class="command">gdb</strong></span> on
the core file to find out what happened.
</p><p>
When you configure MySQL for debugging you automatically
enable a lot of extra safety check functions that monitor the
health of <span><strong class="command">mysqld</strong></span>. If they find something
“<span class="quote">unexpected,</span>” an entry is written to
<code class="literal">stderr</code>, which
<span><strong class="command">mysqld_safe</strong></span> directs to the error log! This
also means that if you are having some unexpected problems
with MySQL and are using a source distribution, the first
thing you should do is to configure MySQL for debugging! (The
second thing is to send mail to a MySQL mailing list and ask
for help. See <a href="introduction.html#mailing-lists" title="1.6.1. MySQL Mailing Lists">Section 1.6.1, “MySQL Mailing Lists”</a>. If you believe
that you have found a bug, please use the instructions at
<a href="introduction.html#bug-reports" title="1.7. How to Report Bugs or Problems">Section 1.7, “How to Report Bugs or Problems”</a>.
</p><p>
In the Windows MySQL distribution,
<code class="literal">mysqld.exe</code> is by default compiled with
support for trace files. See also
<a href="extending-mysql.html#making-trace-files" title="26.4.1.2. Creating Trace Files">Section 26.4.1.2, “Creating Trace Files”</a>.
</p></div><div class="section" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="making-trace-files"></a>26.4.1.2. Creating Trace Files</h4></div></div></div><p>
If the <span><strong class="command">mysqld</strong></span> server doesn't start or if
you can cause it to crash quickly, you can try to create a
trace file to find the problem.
</p><p>
To do this, you must have a <span><strong class="command">mysqld</strong></span> that has
been compiled with debugging support. You can check this by
executing <code class="literal">mysqld -V</code>. If the version number
ends with <code class="option">-debug</code>, it's compiled with support
for trace files. (On Windows, the debugging server is named
<span><strong class="command">mysqld-debug</strong></span> rather than
<span><strong class="command">mysqld</strong></span> as of MySQL 4.1.)
</p><p>
Start the <span><strong class="command">mysqld</strong></span> server with a trace log in
<code class="filename">/tmp/mysqld.trace</code> on Unix or
<code class="filename">C:\mysqld.trace</code> on Windows:
</p><pre class="programlisting">shell> <strong class="userinput"><code>mysqld --debug</code></strong>
</pre><p>
On Windows, you should also use the
<code class="option">--standalone</code> flag to not start
<span><strong class="command">mysqld</strong></span> as a service. In a console window,
use this command:
</p><pre class="programlisting">C:\> <strong class="userinput"><code>mysqld-debug --debug --standalone</code></strong>
</pre><p>
After this, you can use the <code class="literal">mysql.exe</code>
command-line tool in a second console window to reproduce the
problem. You can stop the <span><strong class="command">mysqld</strong></span> server
with <span><strong class="command">mysqladmin shutdown</strong></span>.
</p><p>
Note that the trace file become <span class="bold"><strong>very
big</strong></span>! If you want to generate a smaller trace file,
you can use debugging options something like this:
</p><p>
<span><strong class="command">mysqld
--debug=d,info,error,query,general,where:O,/tmp/mysqld.trace</strong></span>
</p><p>
This only prints information with the most interesting tags to
the trace file.
</p><p>
If you make a bug report about this, please only send the
lines from the trace file to the appropriate mailing list
where something seems to go wrong! If you can't locate the
wrong place, you can ftp the trace file, together with a full
bug report, to <a href="ftp://ftp.mysql.com/pub/mysql/upload/" target="_top">ftp://ftp.mysql.com/pub/mysql/upload/</a> so that a
MySQL developer can take a look at it.
</p><p>
The trace file is made with the
<span class="bold"><strong>DBUG</strong></span> package by Fred Fish.
See <a href="extending-mysql.html#the-dbug-package" title="26.4.3. The DBUG Package">Section 26.4.3, “The DBUG Package”</a>.
</p></div><div class="section" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="making-windows-dumps"></a>26.4.1.3. Using <code class="filename">pdb</code> to create a Windows crashdump</h4></div></div></div><p>
Starting with MySQL 5.0.24 the Program Database files
(extension <code class="filename">pdb</code>) are included in the
Noinstall distribution of MySQL. These files provide
information for debugging your MySQL installation in the event
of a problem.
</p><p>
The PDB file contains more detailed information about
<code class="literal">mysqld</code> and other tools that enables more
detailed trace and dump files to be created. You can use these
with Dr Watson, <span><strong class="command">WinDbg</strong></span> and Visual Studio to
debug <span><strong class="command">mysqld</strong></span>.
</p><p>
For more information on PDB files, see
<a href="http://support.microsoft.com/kb/121366/" target="_top">Microsoft
Knowledge Base Article 121366</a>. For more information on
the debugging options available, see
<a href="http://www.microsoft.com/whdc/devtools/debugging/default.mspx" target="_top">Debugging
Tools for Windows</a>.
</p><p>
Dr Watson is installed with all Windows distributions, but if
you have installed Windows development tools, Dr Watson may
have been replaced with WinDbg, the debugger included with
Visual Studio, or the debugging tools provided with Borland or
Delphi.
</p><p>
To generate a crash file using Dr Watson, follow these steps:
</p><div class="orderedlist"><ol type="1"><li><p>
Start Dr Watson by running <span><strong class="command">drwtsn32.exe</strong></span>
interactively using the <code class="option">-i</code> option:
</p><pre class="programlisting">C:\> drwtsn32 -i</pre></li><li><p>
Set the <span class="guilabel">Log File Path</span> to the
directory where you want to store trace files.
</p></li><li><p>
Make sure <span class="guilabel">Dump All Thread Contexts</span>
and <span class="guilabel">Append To Existing Log File</span>.
</p></li><li><p>
Uncheck <span class="guilabel">Dump Sumbol Table</span>,
<span class="guilabel">Visual Notification</span>, <span class="guilabel">Sound
Notification</span> and <span class="guilabel">Create Crash Dump
File</span>.
</p></li><li><p>
Set the <span class="guilabel">Number of Instructions</span> to a
suitable value to capture enough calls in the stacktrace.
A value of at 25 should be enough.
</p></li></ol></div><p>
Note that the file generated can be very large.
</p></div><div class="section" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="using-gdb-on-mysqld"></a>26.4.1.4. Debugging <span><strong class="command">mysqld</strong></span> under <span><strong class="command">gdb</strong></span></h4></div></div></div><a class="indexterm" name="id3158882"></a><p>
On most systems you can also start <span><strong class="command">mysqld</strong></span>
from <span><strong class="command">gdb</strong></span> to get more information if
<span><strong class="command">mysqld</strong></span> crashes.
</p><p>
With some older <span><strong class="command">gdb</strong></span> versions on Linux you
must use <code class="literal">run --one-thread</code> if you want to be
able to debug <span><strong class="command">mysqld</strong></span> threads. In this case,
you can only have one thread active at a time. We recommend
you to upgrade to gdb 5.1 ASAP as thread debugging works much
better with this version!
</p><p>
NPTL threads (the new thread library on Linux) may cause
problems while running <span><strong class="command">mysqld</strong></span> under
<span><strong class="command">gdb</strong></span>. Some symptoms are:
</p><div class="itemizedlist"><ul type="disc"><li><p>
<span><strong class="command">mysqld</strong></span> hangs during startup (before it
writes <code class="literal">ready for connections</code>).
</p></li><li><p>
<span><strong class="command">mysqld</strong></span> crashes during a
<code class="literal">pthread_mutex_lock()</code> or
<code class="literal">pthread_mutex_unlock()</code> call.
</p></li></ul></div><p>
In this case, you should set the following environment
variable in the shell before starting <span><strong class="command">gdb</strong></span>:
</p><pre class="programlisting">LD_ASSUME_KERNEL=2.4.1
export LD_ASSUME_KERNEL
</pre><p>
When running <span><strong class="command">mysqld</strong></span> under
<span><strong class="command">gdb</strong></span>, you should disable the stack trace
with <code class="option">--skip-stack-trace</code> to be able to catch
segfaults within <span><strong class="command">gdb</strong></span>.
</p><p>
In MySQL 4.0.14 and above you should use the
<code class="option">--gdb</code> option to <span><strong class="command">mysqld</strong></span>.
This installs an interrupt handler for
<code class="literal">SIGINT</code> (needed to stop
<span><strong class="command">mysqld</strong></span> with <code class="literal">^C</code> to set
breakpoints) and disable stack tracing and core file handling.
</p><p>
It's very hard to debug MySQL under <span><strong class="command">gdb</strong></span> if
you do a lot of new connections the whole time as
<span><strong class="command">gdb</strong></span> doesn't free the memory for old
threads. You can avoid this problem by starting
<span><strong class="command">mysqld</strong></span> with
<code class="option">--thread_cache_size='max_connections+1'</code>. In
most cases just using <code class="option">--thread_cache_size=5'</code>
helps a lot!
</p><p>
If you want to get a core dump on Linux if
<span><strong class="command">mysqld</strong></span> dies with a SIGSEGV signal, you can
start <span><strong class="command">mysqld</strong></span> with the
<code class="option">--core-file</code> option. This core file can be
used to make a backtrace that may help you find out why
<span><strong class="command">mysqld</strong></span> died:
</p><pre class="programlisting">shell> <strong class="userinput"><code>gdb mysqld core</code></strong>
gdb> backtrace full
gdb> quit
</pre><p>
See <a href="error-handling.html#crashing" title="B.1.4.2. What to Do If MySQL Keeps Crashing">Section B.1.4.2, “What to Do If MySQL Keeps Crashing”</a>.
</p><p>
If you are using <span><strong class="command">gdb</strong></span> 4.17.x or above on
Linux, you should install a <code class="filename">.gdb</code> file,
with the following information, in your current directory:
</p><pre class="programlisting">set print sevenbit off
handle SIGUSR1 nostop noprint
handle SIGUSR2 nostop noprint
handle SIGWAITING nostop noprint
handle SIGLWP nostop noprint
handle SIGPIPE nostop
handle SIGALRM nostop
handle SIGHUP nostop
handle SIGTERM nostop noprint
</pre><p>
If you have problems debugging threads with
<span><strong class="command">gdb</strong></span>, you should download gdb 5.x and try
this instead. The new <span><strong class="command">gdb</strong></span> version has very
improved thread handling!
</p><p>
Here is an example how to debug mysqld:
</p><pre class="programlisting">shell> <strong class="userinput"><code>gdb /usr/local/libexec/mysqld</code></strong>
gdb> run
...
backtrace full # Do this when mysqld crashes
</pre><p>
Include the above output in a bug report, which you can file
using the instructions in <a href="introduction.html#bug-reports" title="1.7. How to Report Bugs or Problems">Section 1.7, “How to Report Bugs or Problems”</a>.
</p><p>
If <span><strong class="command">mysqld</strong></span> hangs you can try to use some
system tools like <code class="literal">strace</code> or
<code class="literal">/usr/proc/bin/pstack</code> to examine where
<span><strong class="command">mysqld</strong></span> has hung.
</p><pre class="programlisting">strace /tmp/log libexec/mysqld
</pre><a class="indexterm" name="id3159280"></a><a class="indexterm" name="id3159289"></a><a class="indexterm" name="id3159298"></a><a class="indexterm" name="id3159307"></a><p>
If you are using the Perl <code class="literal">DBI</code> interface,
you can turn on debugging information by using the
<code class="literal">trace</code> method or by setting the
<code class="literal">DBI_TRACE</code> environment variable.
</p></div><div class="section" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="using-stack-trace"></a>26.4.1.5. Using a Stack Trace</h4></div></div></div><p>
On some operating systems, the error log contains a stack
trace if <span><strong class="command">mysqld</strong></span> dies unexpectedly. You can
use this to find out where (and maybe why)
<span><strong class="command">mysqld</strong></span> died. See
<a href="server-administration.html#error-log" title="5.10.1. The Error Log">Section 5.10.1, “The Error Log”</a>. To get a stack trace, you must
not compile <span><strong class="command">mysqld</strong></span> with the
<code class="option">-fomit-frame-pointer</code> option to gcc. See
<a href="extending-mysql.html#compiling-for-debugging" title="26.4.1.1. Compiling MySQL for Debugging">Section 26.4.1.1, “Compiling MySQL for Debugging”</a>.
</p><p>
If the error file contains something like the following:
</p><pre class="programlisting">mysqld got signal 11;
The manual section 'Debugging a MySQL server' tells you how to
use a stack trace and/or the core file to produce a readable
backtrace that may help in finding out why mysqld died
Attempting backtrace. You can use the following information
to find out where mysqld died. If you see no messages after
this, something went terribly wrong...
stack range sanity check, ok, backtrace follows
0x40077552
0x81281a0
0x8128f47
0x8127be0
0x8127995
0x8104947
0x80ff28f
0x810131b
0x80ee4bc
0x80c3c91
0x80c6b43
0x80c1fd9
0x80c1686
</pre><p>
you can find where <span><strong class="command">mysqld</strong></span> died by doing the
following:
</p><div class="orderedlist"><ol type="1"><li><p>
Copy the preceding numbers to a file, for example
<code class="filename">mysqld.stack</code>.
</p></li><li><p>
Make a symbol file for the <span><strong class="command">mysqld</strong></span>
server:
</p><pre class="programlisting">nm -n libexec/mysqld > /tmp/mysqld.sym
</pre><p>
Note that most MySQL binary distributions (except for the
"debug" packages, where this information is included
inside of the binaries themselves) ship with the above
file, named <code class="literal">mysqld.sym.gz</code>. In this
case, you can simply unpack it by doing:
</p><pre class="programlisting">gunzip < bin/mysqld.sym.gz > /tmp/mysqld.sym
</pre></li><li><p>
Execute <code class="literal">resolve_stack_dump -s /tmp/mysqld.sym -n
mysqld.stack</code>.
</p><p>
This prints out where <span><strong class="command">mysqld</strong></span> died. If
this doesn't help you find out why
<span><strong class="command">mysqld</strong></span> died, you should make a bug
report and include the output from the above command with
the bug report.
</p><p>
Note however that in most cases it does not help us to
just have a stack trace to find the reason for the
problem. To be able to locate the bug or provide a
workaround, we would in most cases need to know the query
that killed <span><strong class="command">mysqld</strong></span> and preferable a
test case so that we can repeat the problem! See
<a href="introduction.html#bug-reports" title="1.7. How to Report Bugs or Problems">Section 1.7, “How to Report Bugs or Problems”</a>.
</p></li></ol></div></div><div class="section" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="using-log-files"></a>26.4.1.6. Using Server Logs to Find Causes of Errors in <span><strong class="command">mysqld</strong></span></h4></div></div></div><p>
Note that before starting <span><strong class="command">mysqld</strong></span> with
<code class="option">--log</code> you should check all your tables with
<span><strong class="command">myisamchk</strong></span>. See
<a href="server-administration.html" title="Chapter 5. MySQL Server Administration">Chapter 5, <i>MySQL Server Administration</i></a>.
</p><p>
If <span><strong class="command">mysqld</strong></span> dies or hangs, you should start
<span><strong class="command">mysqld</strong></span> with <code class="option">--log</code>. When
<span><strong class="command">mysqld</strong></span> dies again, you can examine the end
of the log file for the query that killed
<span><strong class="command">mysqld</strong></span>.
</p><p>
If you are using <code class="option">--log</code> without a file name,
the log is stored in the database directory as
<code class="filename"><em class="replaceable"><code>host_name</code></em>.log</code>
In most cases it is the last query in the log file that killed
<span><strong class="command">mysqld</strong></span>, but if possible you should verify
this by restarting <span><strong class="command">mysqld</strong></span> and executing the
found query from the <span><strong class="command">mysql</strong></span> command-line
tools. If this works, you should also test all complicated
queries that didn't complete.
</p><p>
You can also try the command <code class="literal">EXPLAIN</code> on all
<code class="literal">SELECT</code> statements that takes a long time to
ensure that <span><strong class="command">mysqld</strong></span> is using indexes
properly. See <a href="optimization.html#explain" title="6.2.1. Optimizing Queries with EXPLAIN">Section 6.2.1, “Optimizing Queries with <code class="literal">EXPLAIN</code>”</a>.
</p><p>
You can find the queries that take a long time to execute by
starting <span><strong class="command">mysqld</strong></span> with
<code class="option">--log-slow-queries</code>. See
<a href="server-administration.html#slow-query-log" title="5.10.4. The Slow Query Log">Section 5.10.4, “The Slow Query Log”</a>.
</p><p>
If you find the text <code class="literal">mysqld restarted</code> in
the error log file (normally named
<code class="filename">hostname.err</code>) you probably have found a
query that causes <span><strong class="command">mysqld</strong></span> to fail. If this
happens, you should check all your tables with
<span><strong class="command">myisamchk</strong></span> (see
<a href="server-administration.html" title="Chapter 5. MySQL Server Administration">Chapter 5, <i>MySQL Server Administration</i></a>), and test the
queries in the MySQL log files to see whether one fails. If
you find such a query, try first upgrading to the newest MySQL
version. If this doesn't help and you can't find anything in
the <code class="literal">mysql</code> mail archive, you should report
the bug to a MySQL mailing list. The mailing lists are
described at <a href="http://lists.mysql.com/" target="_top">http://lists.mysql.com/</a>, which
also has links to online list archives.
</p><p>
If you have started <span><strong class="command">mysqld</strong></span> with
<code class="literal">myisam-recover</code>, MySQL automatically checks
and tries to repair <code class="literal">MyISAM</code> tables if they
are marked as 'not closed properly' or 'crashed'. If this
happens, MySQL writes an entry in the
<code class="literal">hostname.err</code> file <code class="literal">'Warning:
Checking table ...'</code> which is followed by
<code class="literal">Warning: Repairing table</code> if the table needs
to be repaired. If you get a lot of these errors, without
<span><strong class="command">mysqld</strong></span> having died unexpectedly just
before, then something is wrong and needs to be investigated
further. See <a href="server-administration.html#server-options" title="5.2.2. Command Options">Section 5.2.2, “Command Options”</a>.
</p><p>
It is not a good sign if <span><strong class="command">mysqld</strong></span> did die
unexpectedly, but in this case, you should not investigate the
<code class="literal">Checking table...</code> messages, but instead try
to find out why <span><strong class="command">mysqld</strong></span> died.
</p></div><div class="section" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="reproducible-test-case"></a>26.4.1.7. Making a Test Case If You Experience Table Corruption</h4></div></div></div><p>
If you get corrupted tables or if <span><strong class="command">mysqld</strong></span>
always fails after some update commands, you can test whether
this bug is reproducible by doing the following:
</p><div class="itemizedlist"><ul type="disc"><li><p>
Take down the MySQL daemon (with <span><strong class="command">mysqladmin
shutdown</strong></span>).
</p></li><li><p>
Make a backup of the tables (to guard against the very
unlikely case that the repair does something bad).
</p></li><li><p>
Check all tables with <span><strong class="command">myisamchk -s
database/*.MYI</strong></span>. Repair any wrong tables with
<span><strong class="command">myisamchk -r
database/<em class="replaceable"><code>table</code></em>.MYI</strong></span>.
</p></li><li><p>
Make a second backup of the tables.
</p></li><li><p>
Remove (or move away) any old log files from the MySQL
data directory if you need more space.
</p></li><li><p>
Start <span><strong class="command">mysqld</strong></span> with
<code class="option">--log-bin</code>. See
<a href="server-administration.html#binary-log" title="5.10.3. The Binary Log">Section 5.10.3, “The Binary Log”</a>. If you want to find a query
that crashes <span><strong class="command">mysqld</strong></span>, you should use
<code class="option">--log --log-bin</code>.
</p></li><li><p>
When you have gotten a crashed table, stop the
<code class="literal">mysqld server</code>.
</p></li><li><p>
Restore the backup.
</p></li><li><p>
Restart the <span><strong class="command">mysqld</strong></span> server
<span class="bold"><strong>without</strong></span>
<code class="option">--log-bin</code>
</p></li><li><p>
Re-execute the commands with <span><strong class="command">mysqlbinlog
binary-log-file | mysql</strong></span>. The binary log is saved
in the MySQL database directory with the name
<code class="literal">hostname-bin.#</code>.
</p></li><li><p>
If the tables are corrupted again or you can get
<span><strong class="command">mysqld</strong></span> to die with the above command,
you have found reproducible bug that should be easy to
fix! FTP the tables and the binary log to
<a href="ftp://ftp.mysql.com/pub/mysql/upload/" target="_top">ftp://ftp.mysql.com/pub/mysql/upload/</a> and report it in our
bugs database using the instructions given in
<a href="introduction.html#bug-reports" title="1.7. How to Report Bugs or Problems">Section 1.7, “How to Report Bugs or Problems”</a>. (Please note that the
<code class="filename">/pub/mysql/upload/</code> FTP directory is
not listable, so you'll not see what you've uploaded in
your FTP client.) If you are a support customer, you can
use the MySQL Customer Support Center
<a href="https://support.mysql.com/" target="_top">https://support.mysql.com/</a> to alert the
MySQL team about the problem and have it fixed as soon as
possible.
</p></li></ul></div><p>
You can also use the script <span><strong class="command">mysql_find_rows</strong></span>
to just execute some of the update statements if you want to
narrow down the problem.
</p><p>
The preceding discussion applies only to RHEL4. The patch is
unnecessary for RHEL5.
</p></div></div><div class="section" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="debugging-client"></a>26.4.2. Debugging a MySQL Client</h3></div></div></div><a class="indexterm" name="id3160100"></a><a class="indexterm" name="id3160112"></a><p>
To be able to debug a MySQL client with the integrated debug
package, you should configure MySQL with
<code class="option">--with-debug</code> or
<code class="option">--with-debug=full</code>. See
<a href="installing.html#configure-options" title="2.4.15.2. Typical configure Options">Section 2.4.15.2, “Typical <span><strong class="command">configure</strong></span> Options”</a>.
</p><a class="indexterm" name="id3160143"></a><a class="indexterm" name="id3160152"></a><p>
Before running a client, you should set the
<code class="literal">MYSQL_DEBUG</code> environment variable:
</p><pre class="programlisting">shell> <strong class="userinput"><code>MYSQL_DEBUG=d:t:O,/tmp/client.trace</code></strong>
shell> <strong class="userinput"><code>export MYSQL_DEBUG</code></strong>
</pre><p>
This causes clients to generate a trace file in
<code class="filename">/tmp/client.trace</code>.
</p><p>
If you have problems with your own client code, you should
attempt to connect to the server and run your query using a
client that is known to work. Do this by running
<span><strong class="command">mysql</strong></span> in debugging mode (assuming that you
have compiled MySQL with debugging on):
</p><pre class="programlisting">shell> <strong class="userinput"><code>mysql --debug=d:t:O,/tmp/client.trace</code></strong>
</pre><p>
This provides useful information in case you mail a bug report.
See <a href="introduction.html#bug-reports" title="1.7. How to Report Bugs or Problems">Section 1.7, “How to Report Bugs or Problems”</a>.
</p><p>
If your client crashes at some 'legal' looking code, you should
check that your <code class="filename">mysql.h</code> include file
matches your MySQL library file. A very common mistake is to use
an old <code class="filename">mysql.h</code> file from an old MySQL
installation with new MySQL library.
</p></div><div class="section" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="the-dbug-package"></a>26.4.3. The DBUG Package</h3></div></div></div><a class="indexterm" name="id3160269"></a><p>
The MySQL server and most MySQL clients are compiled with the
DBUG package originally created by Fred Fish. When you have
configured MySQL for debugging, this package makes it possible
to get a trace file of what the program is debugging. See
<a href="extending-mysql.html#making-trace-files" title="26.4.1.2. Creating Trace Files">Section 26.4.1.2, “Creating Trace Files”</a>.
</p><p>
This section summaries the argument values that you can specify
in debug options on the command line for MySQL programs that
have been built with debugging support. For more information
about programming with the DBUG package, see the DBUG manual in
the <code class="filename">dbug</code> directory of MySQL source
distributions. It's best to use a recent distribution to get the
most updated DBUG manual.
</p><p>
You use the debug package by invoking a program with the
<code class="option">--debug="..."</code> or the <code class="option">-#...</code>
option.
</p><p>
Most MySQL programs have a default debug string that is used if
you don't specify an option to <code class="option">--debug</code>. The
default trace file is usually
<code class="literal">/tmp/program_name.trace</code> on Unix and
<code class="literal">\program_name.trace</code> on Windows.
</p><p>
The debug control string is a sequence of colon-separated fields
as follows:
</p><pre class="programlisting"><field_1>:<field_2>:...:<field_N>
</pre><p>
Each field consists of a mandatory flag character followed by an
optional “<span class="quote"><code class="literal">,</code></span>” and comma-separated
list of modifiers:
</p><pre class="programlisting">flag[,modifier,modifier,...,modifier]
</pre><p>
The currently recognized flag characters are:
</p><div class="informaltable"><table border="1"><colgroup><col><col></colgroup><tbody><tr><td><span class="bold"><strong>Flag</strong></span></td><td><span class="bold"><strong>Description</strong></span></td></tr><tr><td><code class="literal">d</code></td><td>Enable output from DBUG_<N> macros for the current state. May be
followed by a list of keywords which selects output only
for the DBUG macros with that keyword. An empty list of
keywords implies output for all macros.</td></tr><tr><td><code class="literal">D</code></td><td>Delay after each debugger output line. The argument is the number of
tenths of seconds to delay, subject to machine
capabilities. For example, <code class="option">-#D,20</code>
specifies a delay of two seconds.</td></tr><tr><td><code class="literal">f</code></td><td>Limit debugging, tracing, and profiling to the list of named functions.
Note that a null list disables all functions. The
appropriate <code class="literal">d</code> or <code class="literal">t</code>
flags must still be given; this flag only limits their
actions if they are enabled.</td></tr><tr><td><code class="literal">F</code></td><td>Identify the source file name for each line of debug or trace output.</td></tr><tr><td><code class="literal">i</code></td><td>Identify the process with the PID or thread ID for each line of debug or
trace output.</td></tr><tr><td><code class="literal">g</code></td><td>Enable profiling. Create a file called <code class="filename">dbugmon.out</code>
containing information that can be used to profile the
program. May be followed by a list of keywords that
select profiling only for the functions in that list. A
null list implies that all functions are considered.</td></tr><tr><td><code class="literal">L</code></td><td>Identify the source file line number for each line of debug or trace
output.</td></tr><tr><td><code class="literal">n</code></td><td>Print the current function nesting depth for each line of debug or trace
output.</td></tr><tr><td><code class="literal">N</code></td><td>Number each line of debug output.</td></tr><tr><td><code class="literal">o</code></td><td>Redirect the debugger output stream to the specified file. The default
output is <code class="literal">stderr</code>.</td></tr><tr><td><code class="literal">O</code></td><td>Like <code class="literal">o</code>, but the file is really flushed between each
write. When needed, the file is closed and reopened
between each write.</td></tr><tr><td><code class="literal">p</code></td><td>Limit debugger actions to specified processes. A process must be
identified with the <code class="literal">DBUG_PROCESS</code>
macro and match one in the list for debugger actions to
occur.</td></tr><tr><td><code class="literal">P</code></td><td>Print the current process name for each line of debug or trace output.</td></tr><tr><td><code class="literal">r</code></td><td>When pushing a new state, do not inherit the previous state's function
nesting level. Useful when the output is to start at the
left margin.</td></tr><tr><td><code class="literal">S</code></td><td>Do function <code class="literal">_sanity(_file_,_line_)</code> at each debugged
function until <code class="literal">_sanity()</code> returns
something that differs from 0. (Mostly used with
<code class="literal">safemalloc</code> to find memory leaks)</td></tr><tr><td><code class="literal">t</code></td><td>Enable function call/exit trace lines. May be followed by a list
(containing only one modifier) giving a numeric maximum
trace level, beyond which no output occurs for either
debugging or tracing macros. The default is a compile
time option.</td></tr></tbody></table></div><p>
Some examples of debug control strings that might appear on a
shell command line (the <code class="option">-#</code> is typically used to
introduce a control string to an application program) are:
</p><pre class="programlisting">-#d:t
-#d:f,main,subr1:F:L:t,20
-#d,input,output,files:n
-#d:t:i:O,\\mysqld.trace
</pre><p>
In MySQL, common tags to print (with the <code class="literal">d</code>
option) are <code class="literal">enter</code>, <code class="literal">exit</code>,
<code class="literal">error</code>, <code class="literal">warning</code>,
<code class="literal">info</code>, and <code class="literal">loop</code>.
</p></div><div class="section" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="rts-threads"></a>26.4.4. Comments about RTS Threads</h3></div></div></div><a class="indexterm" name="id3160801"></a><a class="indexterm" name="id3160810"></a><p>
I have tried to use the RTS thread packages with MySQL but
stumbled on the following problems:
</p><p>
They use old versions of many POSIX calls and it is very tedious
to make wrappers for all functions. I am inclined to think that
it would be easier to change the thread libraries to the newest
POSIX specification.
</p><p>
Some wrappers are currently written. See
<code class="filename">mysys/my_pthread.c</code> for more info.
</p><p>
At least the following should be changed:
</p><p>
<code class="literal">pthread_get_specific</code> should use one argument.
<code class="literal">sigwait</code> should take two arguments. A lot of
functions (at least <code class="literal">pthread_cond_wait</code>,
<code class="literal">pthread_cond_timedwait()</code>) should return the
error code on error. Now they return -1 and set
<code class="literal">errno</code>.
</p><p>
Another problem is that user-level threads use the
<code class="literal">ALRM</code> signal and this aborts a lot of
functions (<code class="literal">read</code>, <code class="literal">write</code>,
<code class="literal">open</code>...). MySQL should do a retry on
interrupt on all of these but it is not that easy to verify it.
</p><p>
The biggest unsolved problem is the following:
</p><p>
To get thread-level alarms I changed
<code class="filename">mysys/thr_alarm.c</code> to wait between alarms
with <code class="literal">pthread_cond_timedwait()</code>, but this
aborts with error <code class="literal">EINTR</code>. I tried to debug the
thread library as to why this happens, but couldn't find any
easy solution.
</p><p>
If someone wants to try MySQL with RTS threads I suggest the
following:
</p><div class="itemizedlist"><ul type="disc"><li><p>
Change functions MySQL uses from the thread library to
POSIX. This shouldn't take that long.
</p></li><li><p>
Compile all libraries with the
<code class="option">-DHAVE_rts_threads</code>.
</p></li><li><p>
Compile <code class="literal">thr_alarm</code>.
</p></li><li><p>
If there are some small differences in the implementation,
they may be fixed by changing
<code class="filename">my_pthread.h</code> and
<code class="filename">my_pthread.c</code>.
</p></li><li><p>
Run <code class="literal">thr_alarm</code>. If it runs without any
“<span class="quote">warning,</span>” “<span class="quote">error,</span>” or aborted
messages, you are on the right track. Here is a successful
run on Solaris:
</p><pre class="programlisting">Main thread: 1
Thread 0 (5) started
Thread: 5 Waiting
process_alarm
Thread 1 (6) started
Thread: 6 Waiting
process_alarm
process_alarm
thread_alarm
Thread: 6 Slept for 1 (1) sec
Thread: 6 Waiting
process_alarm
process_alarm
thread_alarm
Thread: 6 Slept for 2 (2) sec
Thread: 6 Simulation of no alarm needed
Thread: 6 Slept for 0 (3) sec
Thread: 6 Waiting
process_alarm
process_alarm
thread_alarm
Thread: 6 Slept for 4 (4) sec
Thread: 6 Waiting
process_alarm
thread_alarm
Thread: 5 Slept for 10 (10) sec
Thread: 5 Waiting
process_alarm
process_alarm
thread_alarm
Thread: 6 Slept for 5 (5) sec
Thread: 6 Waiting
process_alarm
process_alarm
...
thread_alarm
Thread: 5 Slept for 0 (1) sec
end
</pre></li></ul></div></div><div class="section" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="thread-packages"></a>26.4.5. Differences Between Thread Packages</h3></div></div></div><a class="indexterm" name="id3161059"></a><p>
MySQL is very dependent on the thread package used. So when
choosing a good platform for MySQL, the thread package is very
important.
</p><p>
There are at least three types of thread packages:
</p><div class="itemizedlist"><ul type="disc"><li><p>
User threads in a single process. Thread switching is
managed with alarms and the threads library manages all
non-thread-safe functions with locks. Read, write and select
operations are usually managed with a thread-specific select
that switches to another thread if the running threads have
to wait for data. If the user thread packages are integrated
in the standard libs (FreeBSD and BSDI threads) the thread
package requires less overhead than thread packages that
have to map all unsafe calls (MIT-pthreads, FSU Pthreads and
RTS threads). In some environments (for example, SCO), all
system calls are thread-safe so the mapping can be done very
easily (FSU Pthreads on SCO). Downside: All mapped calls
take a little time and it's quite tricky to be able to
handle all situations. There are usually also some system
calls that are not handled by the thread package (like
MIT-pthreads and sockets). Thread scheduling isn't always
optimal.
</p></li><li><p>
User threads in separate processes. Thread switching is done
by the kernel and all data are shared between threads. The
thread package manages the standard thread calls to allow
sharing data between threads. LinuxThreads is using this
method. Downside: Lots of processes. Thread creating is
slow. If one thread dies the rest are usually left hanging
and you must kill them all before restarting. Thread
switching is somewhat expensive.
</p></li><li><p>
Kernel threads. Thread switching is handled by the thread
library or the kernel and is very fast. Everything is done
in one process, but on some systems, <span><strong class="command">ps</strong></span>
may show the different threads. If one thread aborts, the
whole process aborts. Most system calls are thread-safe and
should require very little overhead. Solaris, HP-UX, AIX and
OSF/1 have kernel threads.
</p></li></ul></div><p>
In some systems kernel threads are managed by integrating user
level threads in the system libraries. In such cases, the thread
switching can only be done by the thread library and the kernel
isn't really “<span class="quote">thread aware.</span>”
</p></div></div></div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="mysql-proxy.html">Prev</a> </td><td width="20%" align="center"> </td><td width="40%" align="right"> <a accesskey="n" href="faqs.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Chapter 25. MySQL Proxy </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> Appendix A. MySQL 5.0 Frequently Asked Questions</td></tr></table></div></body></html>
distrib
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Mandriva
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2008.1
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x86_64
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media
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main-testing
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by-pkgid
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b1e2421f2416edfc24c5845fbc1c5a2e
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files
>
8
