+++ /dev/null
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- vlink="#000080" alink="#FF0000">
- <!--#include virtual="header.html" -->
-
- <h1>Thread Safety</h1>
- <p>When using any of the threaded mpms in Apache 2.0 it is important
- that every function called from Apache be thread safe. When linking in 3rd
- party extensions it can be difficult to determine whether the resulting
- server will be thread safe. Casual testing generally won't tell you this
- either as thread safety problems can lead to subtle race conditons that
- may only show up in certain conditions under heavy load.</p>
-
- <h2>Global and static variables</h2>
-
- <p>When writing your module or when trying to determine if a module or
- 3rd party library is thread safe there are some common things to keep in mind.
- First, you need to recognize that in a threaded model each individual thread
- has its own program counter, stack and registers. Local variables live on the
- stack, so those are fine. You need to watch out for any static or global
- variables. This doesn't mean that you are absolutely not allowed to use static
- or global variables. There are times when you actually want something to affect
- all threads, but generally you need to avoid using them if you want your code to
- be thread safe.</p>
-
- <p>In the case where you have a global variable that needs to be global and
- accessed by all threads, be very careful when you update it. If, for example,
- it is an incrementing counter, you need to atomically increment it to avoid
- race conditions with other threads. You do this using a mutex (mutual exclusion).
- Lock the mutex, read the current value, increment it and write it back and then unlock
- the mutex. Any other thread that wants to modify the value has to first check the mutex
- and block until it is cleared.</p>
-
- <p>If you are using APR, have a look at the apr_atomic_* functions and the apr_thread_mutex_*
- functions. [would probably be a good idea to add an example here]</p>
-
- <h2>errno</h2>
-
- <p>This is a common global variable that holds the error number of the last error that occurred.
- If one thread calls a low-level function that sets errno and then another thread checks it, we
- are bleeding error numbers from one thread into another. To solve this, make sure your module
- or library defines _REENTRANT or is compiled with -D_REENTRANT. This will make errno a per-thread
- variable and should hopefully be transparent to the code. It does this by doing something like this:</p>
-
-<pre>#define errno (*(__errno_location()))</pre>
-
- <p>which means that accessing errno will call __errno_location() which is provided by the libc. Setting
- _REENTRANT also forces redefinition of some other functions to their *_r equivalents and sometimes
- changes the common getc/putc macros into safer function calls. Check your libc documentation for
- specifics. Instead of, or in addition to _REENTRANT the symbols that may affect this are
- _POSIX_C_SOURCE, _THREAD_SAFE, _SVID_SOURCE, and _BSD_SOURCE.</p>
-
- <h2>Common standard troublesome functions</h2>
-
- <p>Not only do things have to be thread safe, but they also have to be reentrant.
- <b>strtok()</b> is an obvious one. You call it the first time with your delimiter which
- it then remembers and on each subsequent call it returns the next token. Obviously if
- multiple threads are calling it you will have a problem. Most systems have a reentrant version
- of of the function called <b>strtok_r()</b> where you pass in an extra argument which contains
- an allocated char * which the function will use instead of its own static storage for maintaining
- the tokenizing state. If you are using APR you can use <b>apr_strtok()</b>.</p>
-
- <p><b>crypt()</b> is another function that tends to not be reentrant, so if you run across calls
- to that function in a library, watch out. On some systems it is reentrant though, so it is not
- always a problem. If your system has <b>crypt_r()</b> chances are you should be using that, or
- if possible simply avoid the whole mess by using md5 instead. [I don't see an apr_crypt() function.]</p>
-
-
- <h1>Common 3rd Party Libraries</h1>
- <p>The following is a list of common libraries that are used by 3rd party
- Apache modules. You can check to see if your module is using a potentially
- unsafe library by using tools such as <tt>ldd</tt> and <tt>nm</tt>. For
- PHP, for example, try this:</p>
-<pre>% ldd libphp4.so
-libsablot.so.0 => /usr/local/lib/libsablot.so.0 (0x401f6000)
-libexpat.so.0 => /usr/lib/libexpat.so.0 (0x402da000)
-libsnmp.so.0 => /usr/lib/libsnmp.so.0 (0x402f9000)
-libpdf.so.1 => /usr/local/lib/libpdf.so.1 (0x40353000)
-libz.so.1 => /usr/lib/libz.so.1 (0x403e2000)
-libpng.so.2 => /usr/lib/libpng.so.2 (0x403f0000)
-libmysqlclient.so.11 => /usr/lib/libmysqlclient.so.11 (0x40411000)
-libming.so => /usr/lib/libming.so (0x40449000)
-libm.so.6 => /lib/libm.so.6 (0x40487000)
-libfreetype.so.6 => /usr/lib/libfreetype.so.6 (0x404a8000)
-libjpeg.so.62 => /usr/lib/libjpeg.so.62 (0x404e7000)
-libcrypt.so.1 => /lib/libcrypt.so.1 (0x40505000)
-libssl.so.2 => /lib/libssl.so.2 (0x40532000)
-libcrypto.so.2 => /lib/libcrypto.so.2 (0x40560000)
-libresolv.so.2 => /lib/libresolv.so.2 (0x40624000)
-libdl.so.2 => /lib/libdl.so.2 (0x40634000)
-libnsl.so.1 => /lib/libnsl.so.1 (0x40637000)
-libc.so.6 => /lib/libc.so.6 (0x4064b000)
-/lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x80000000)</pre>
-<p>In addition to these libraries you will need to have a look at any libraries
-linked statically into the module. You can use <tt>nm</tt> to look for
-individual symbols in the module.</p>
-
- <h2>Library List</h2>
- <p>Please drop a note to dev@httpd.apache.org if you have additions or
- corrections to this list.</p>
- <table>
- <tr>
- <th>Library</th>
- <th>Version</th>
- <th>Thread Safe?</th>
- <th>Notes</th>
- </tr>
- <tr>
- <td><a href="http://aspell.sourceforge.net/">ASpell/PSpell</a></td>
- <td></td>
- <td>?</td>
- <td></td>
- </tr>
- <tr>
- <td><a href="http://www.sleepycat.com/">Berkeley DB</a></td>
- <td>3.x, 4.x</td>
- <td>Yes</td>
- <td>Be careful about sharing a connection across threads.</td>
- </tr>
- <tr>
- <td><a href="http://sources.redhat.com/bzip2/index.html">bzip2</a></td>
- <td></td>
- <td>Yes</td>
- <td>Both low-level and high-level APIs are thread-safe. However, high-level API requires thread-safe access to errno.</td>
- </tr>
- <tr>
- <td><a href="http://cr.yp.to/cdb.html">cdb</a></td>
- <td> </td>
- <td>?</td>
- <td> </td>
- </tr>
- <tr>
- <td><a href="http://www.washington.edu/imap/">C-Client</a></td>
- <td> </td>
- <td>Perhaps</td>
- <td>c-client uses strtok() and gethostbyname() which are not thread-safe on most C library implementations. c-client's static data is meant to be shared across threads. If strtok() and gethostbyname() are thread-safe on your OS, c-client <i>may</i> be thread-safe.</td>
- </tr>
- <tr>
- <td><a href="http://www.fastio.com/">cpdflib</a></td>
- <td> </td>
- <td>?</td>
- <td> </td>
- </tr>
- <tr>
- <td><a href="http://www.ijg.org/files/">libcrypt</a></td>
- <td> </td>
- <td>?</td>
- <td> </td>
- </tr>
- <tr>
- <td><a href="http://expat.sourceforge.net/">Expat</a></td>
- <td></td>
- <td>Yes</td>
- <td>Need a separate parser instance per thread</td>
- </tr>
- <tr>
- <td><a href="http://www.freetds.org/">FreeTDS</a></td>
- <td> </td>
- <td>?</td>
- <td> </td>
- </tr>
- <tr>
- <td><a href="http://www.freetype.org/">FreeType</a></td>
- <td> </td>
- <td>?</td>
- <td> </td>
- </tr>
- <tr>
- <td><a href="http://www.boutell.com/gd/">GD 1.8.x</a></td>
- <td> </td>
- <td>?</td>
- <td> </td>
- </tr>
- <tr>
- <td><a href="http://www.boutell.com/gd/">GD 2.0.x</a></td>
- <td> </td>
- <td>?</td>
- <td> </td>
- </tr>
- <tr>
- <td><a href="http://www.gnu.org/software/gdbm/gdbm.html">gdbm</a></td>
- <td> </td>
- <td>No</td>
- <td>Errors returned via a static gdbm_error variable</td>
- </tr>
- <tr>
- <td><a href="http://www.imagemagick.org/">ImageMagick</a></td>
- <td>5.2.2</td>
- <td>Yes</td>
- <td>ImageMagick docs claim it is thread safe since version 5.2.2
- <a href="http://www.cise.ufl.edu/depot/www/ImageMagick/www/Changelog.html"
- >http://www.cise.ufl.edu/depot/www/ImageMagick/www/Changelog.html</a>.</td>
- </tr>
- <tr>
- <td><a href="http://www.enlightenment.org/pages/imlib2.html">Imlib2</a></td>
- <td> </td>
- <td>?</td>
- <td> </td>
- </tr>
- <tr>
- <td><a href="http://www.ijg.org/files/">libjpeg</a></td>
- <td>v6b</td>
- <td>?</td>
- <td> </td>
- </tr>
- <tr>
- <td><a href="http://mysql.com">libmysqlclient</a></td>
- <td> </td>
- <td>Yes</td>
- <td>Use mysqlclient_r library variant to ensure thread-safety. For
- more information, please read <a
- href="http://www.mysql.com/doc/en/Threaded_clients.html"
- >http://www.mysql.com/doc/en/Threaded_clients.html</a>.</td>
- </tr>
- <tr>
- <td><a href="http://www.opaque.net/ming/">Ming</a></td>
- <td>0.2a</td>
- <td>?</td>
- <td> </td>
- </tr>
- <tr>
- <td><a href="http://net-snmp.sourceforge.net/">Net-SNMP</a></td>
- <td>5.0.x</td>
- <td>?</td>
- <td> </td>
- </tr>
- <tr>
- <td><a href="http://www.openldap.org/">OpenLDAP</a></td>
- <td>2.1.x</td>
- <td>Yes</td>
- <td>Use ldap_r library variant to ensure thread-safety.</td>
- </tr>
- <tr>
- <td><a href="http://www.openssl.org/">OpenSSL</a></td>
- <td>0.96g</td>
- <td>Yes</td>
- <td>Requires proper usage of <i>CRYPTO_num_locks</i>, <i>CRYPTO_set_locking_callback</i>, <i>CRYPTO_set_id_callback</i></td>
- </tr>
- <tr>
- <td><a href="http://www.oracle.com/">liboci8 (Oracle 8+)</a></td>
- <td>8.x,9.x</td>
- <td>?</td>
- <td> </td>
- </tr>
- <tr>
- <td><a href="http://pdflib.com/">pdflib</a></td>
- <td>5.0.x</td>
- <td>Yes</td>
- <td>PDFLib docs claim it is thread safe; changes.txt indicates it
- has been partially thread-safe since V1.91: <a
- href="http://www.pdflib.com/products/pdflib/index.html"
- >http://www.pdflib.com/products/pdflib/index.html</a>.</td>
- </tr>
- <tr>
- <td><a href="http://www.libpng.org/pub/png/libpng.html">libpng</a></td>
- <td>1.0.x</td>
- <td>?</td>
- <td> </td>
- </tr>
- <tr>
- <td><a href="http://www.libpng.org/pub/png/libpng.html">libpng</a></td>
- <td>1.2.x</td>
- <td>?</td>
- <td> </td>
- </tr>
- <tr>
- <td><a href="http://www.postgresql.org/idocs/index.php?libpq-threading.html">libpq (PostgreSQL)</a></td>
- <td>7.x</td>
- <td>Yes</td>
- <td>Don't share connections across threads and watch out for crypt() calls</td>
- </tr>
- <tr>
- <td><a href="http://www.gingerall.com/charlie/ga/xml/p_sab.xml">Sablotron</a></td>
- <td>0.95</td>
- <td>?</td>
- <td></td>
- </tr>
- <tr>
- <td><a href="http://www.gzip.org/zlib/">zlib</a></td>
- <td>1.1.4</td>
- <td>Yes</td>
- <td>Relies upon thread-safe zalloc and zfree functions. Default is to use libc's calloc/free which are thread-safe.</td>
- </tr>
- </table>
- <!--#include virtual="footer.html" -->
- </body>
-</html>
--- /dev/null
+<?xml version="1.0" encoding="ISO-8859-1"?>
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+ XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
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+<title>Apache 2.0 Thread Safety Issues - Apache HTTP Server</title>
+<link href="../style/css/manual.css" rel="stylesheet" media="all" type="text/css" title="Main stylesheet" />
+<link href="../style/css/manual-loose-100pc.css" rel="alternate stylesheet" media="all" type="text/css" title="No Sidebar - Default font size" />
+<link href="../style/css/manual-print.css" rel="stylesheet" media="print" type="text/css" />
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+<body id="manual-page"><div id="page-header">
+<p class="menu"><a href="../mod/">Modules</a> | <a href="../mod/directives.html">Directives</a> | <a href="../faq/">FAQ</a> | <a href="../glossary.html">Glossary</a> | <a href="../sitemap.html">Sitemap</a></p>
+<p class="apache">Apache HTTP Server Version 2.1</p>
+<img alt="" src="../images/feather.gif" /></div>
+<div class="up"><a href="./"><img title="<-" alt="<-" src="../images/left.gif" /></a></div>
+<div id="path">
+<a href="http://www.apache.org/">Apache</a> > <a href="http://httpd.apache.org/">HTTP Server</a> > <a href="http://httpd.apache.org/docs-project/">Documentation</a> > <a href="../">Version 2.1</a></div><div id="page-content"><div id="preamble"><h1>Apache 2.0 Thread Safety Issues</h1>
+ <p>When using any of the threaded mpms in Apache 2.0 it is important
+ that every function called from Apache be thread safe. When linking in 3rd
+ party extensions it can be difficult to determine whether the resulting
+ server will be thread safe. Casual testing generally won't tell you this
+ either as thread safety problems can lead to subtle race conditons that
+ may only show up in certain conditions under heavy load.</p>
+</div>
+<div id="quickview"><ul id="toc"><li><img alt="" src="../images/down.gif" /> <a href="#variables">Global and static variables</a></li>
+<li><img alt="" src="../images/down.gif" /> <a href="#errno">errno</a></li>
+<li><img alt="" src="../images/down.gif" /> <a href="#functions">Common standard troublesome functions</a></li>
+<li><img alt="" src="../images/down.gif" /> <a href="#commonlibs">Common 3rd Party Libraries</a></li>
+<li><img alt="" src="../images/down.gif" /> <a href="#liblist">Library List</a></li>
+</ul></div>
+<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
+<div class="section">
+<h2><a name="variables" id="variables">Global and static variables</a></h2>
+ <p>When writing your module or when trying to determine if a module or
+ 3rd party library is thread safe there are some common things to keep in
+ mind.</p>
+
+ <p>First, you need to recognize that in a threaded model each individual
+ thread has its own program counter, stack and registers. Local variables
+ live on the stack, so those are fine. You need to watch out for any
+ static or global variables. This doesn't mean that you are absolutely not
+ allowed to use static or global variables. There are times when you
+ actually want something to affect all threads, but generally you need to
+ avoid using them if you want your code to be thread safe.</p>
+
+ <p>In the case where you have a global variable that needs to be global and
+ accessed by all threads, be very careful when you update it. If, for
+ example, it is an incrementing counter, you need to atomically increment
+ it to avoid race conditions with other threads. You do this using a mutex
+ (mutual exclusion). Lock the mutex, read the current value, increment it
+ and write it back and then unlock the mutex. Any other thread that wants
+ to modify the value has to first check the mutex and block until it is
+ cleared.</p>
+
+ <p>If you are using <a href="http://apr.apache.org/">APR</a>, have a look
+ at the <code>apr_atomic_<var>*</var></code> functions and the
+ <code>apr_thread_mutex_<var>*</var></code> functions.</p>
+
+</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
+<div class="section">
+<h2><a name="errno" id="errno">errno</a></h2>
+ <p>This is a common global variable that holds the error number of the
+ last error that occurred. If one thread calls a low-level function that
+ sets errno and then another thread checks it, we are bleeding error
+ numbers from one thread into another. To solve this, make sure your
+ module or library defines <code>_REENTRANT</code> or is compiled with
+ <code>-D_REENTRANT</code>. This will make errno a per-thread variable
+ and should hopefully be transparent to the code. It does this by doing
+ something like this:</p>
+
+ <div class="example"><p><code>
+ #define errno (*(__errno_location()))
+ </code></p></div>
+
+ <p>which means that accessing errno will call
+ <code>__errno_location()</code> which is provided by the libc. Setting
+ <code>_REENTRANT</code> also forces redefinition of some other functions
+ to their <code><var>*</var>_r</code> equivalents and sometimes changes
+ the common <code>getc</code>/<code>putc</code> macros into safer function
+ calls. Check your libc documentation for specifics. Instead of, or in
+ addition to <code>_REENTRANT</code> the symbols that may affect this are
+ <code>_POSIX_C_SOURCE</code>, <code>_THREAD_SAFE</code>,
+ <code>_SVID_SOURCE</code>, and <code>_BSD_SOURCE</code>.</p>
+</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
+<div class="section">
+<h2><a name="functions" id="functions">Common standard troublesome functions</a></h2>
+ <p>Not only do things have to be thread safe, but they also have to be
+ reentrant. <code>strtok()</code> is an obvious one. You call it the first
+ time with your delimiter which it then remembers and on each subsequent
+ call it returns the next token. Obviously if multiple threads are
+ calling it you will have a problem. Most systems have a reentrant version
+ of of the function called <code>strtok_r()</code> where you pass in an
+ extra argument which contains an allocated <code>char *</code> which the
+ function will use instead of its own static storage for maintaining
+ the tokenizing state. If you are using <a href="http://apr.apache.org/">APR</a> you can use <code>apr_strtok()</code>.</p>
+
+ <p><code>crypt()</code> is another function that tends to not be reentrant,
+ so if you run across calls to that function in a library, watch out. On
+ some systems it is reentrant though, so it is not always a problem. If
+ your system has <code>crypt_r()</code> chances are you should be using
+ that, or if possible simply avoid the whole mess by using md5 instead.</p>
+
+</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
+<div class="section">
+<h2><a name="commonlibs" id="commonlibs">Common 3rd Party Libraries</a></h2>
+ <p>The following is a list of common libraries that are used by 3rd party
+ Apache modules. You can check to see if your module is using a potentially
+ unsafe library by using tools such as <code>ldd(1)</code> and
+ <code>nm(1)</code>. For <a href="http://www.php.net/">PHP</a>, for example,
+ try this:</p>
+
+ <div class="example"><p><code>
+ % ldd libphp4.so<br />
+ libsablot.so.0 => /usr/local/lib/libsablot.so.0 (0x401f6000)<br />
+ libexpat.so.0 => /usr/lib/libexpat.so.0 (0x402da000)<br />
+ libsnmp.so.0 => /usr/lib/libsnmp.so.0 (0x402f9000)<br />
+ libpdf.so.1 => /usr/local/lib/libpdf.so.1 (0x40353000)<br />
+ libz.so.1 => /usr/lib/libz.so.1 (0x403e2000)<br />
+ libpng.so.2 => /usr/lib/libpng.so.2 (0x403f0000)<br />
+ libmysqlclient.so.11 => /usr/lib/libmysqlclient.so.11 (0x40411000)<br />
+ libming.so => /usr/lib/libming.so (0x40449000)<br />
+ libm.so.6 => /lib/libm.so.6 (0x40487000)<br />
+ libfreetype.so.6 => /usr/lib/libfreetype.so.6 (0x404a8000)<br />
+ libjpeg.so.62 => /usr/lib/libjpeg.so.62 (0x404e7000)<br />
+ libcrypt.so.1 => /lib/libcrypt.so.1 (0x40505000)<br />
+ libssl.so.2 => /lib/libssl.so.2 (0x40532000)<br />
+ libcrypto.so.2 => /lib/libcrypto.so.2 (0x40560000)<br />
+ libresolv.so.2 => /lib/libresolv.so.2 (0x40624000)<br />
+ libdl.so.2 => /lib/libdl.so.2 (0x40634000)<br />
+ libnsl.so.1 => /lib/libnsl.so.1 (0x40637000)<br />
+ libc.so.6 => /lib/libc.so.6 (0x4064b000)<br />
+ /lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x80000000)
+ </code></p></div>
+
+ <p>In addition to these libraries you will need to have a look at any
+ libraries linked statically into the module. You can use <code>nm(1)</code>
+ to look for individual symbols in the module.</p>
+</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
+<div class="section">
+<h2><a name="liblist" id="liblist">Library List</a></h2>
+ <p>Please drop a note to <a href="http://httpd.apache.org/lists.html#http-dev">dev@httpd.apache.org</a>
+ if you have additions or corrections to this list.</p>
+
+ <table class="bordered"><tr class="header"><th>Library</th><th>Version</th><th>Thread Safe?</th><th>Notes</th></tr>
+<tr><td><a href="http://aspell.sourceforge.net/">ASpell/PSpell</a></td>
+ <td> </td>
+ <td>?</td>
+ <td> </td></tr>
+<tr class="odd"><td><a href="http://www.sleepycat.com/">Berkeley DB</a></td>
+ <td>3.x, 4.x</td>
+ <td>Yes</td>
+ <td>Be careful about sharing a connection across threads.</td></tr>
+<tr><td><a href="http://sources.redhat.com/bzip2/index.html">bzip2</a></td>
+ <td> </td>
+ <td>Yes</td>
+ <td>Both low-level and high-level APIs are thread-safe. However,
+ high-level API requires thread-safe access to errno.</td></tr>
+<tr class="odd"><td><a href="http://cr.yp.to/cdb.html">cdb</a></td>
+ <td> </td>
+ <td>?</td>
+ <td> </td></tr>
+<tr><td><a href="http://www.washington.edu/imap/">C-Client</a></td>
+ <td> </td>
+ <td>Perhaps</td>
+ <td>c-client uses <code>strtok()</code> and
+ <code>gethostbyname()</code> which are not thread-safe on most C
+ library implementations. c-client's static data is meant to be shared
+ across threads. If <code>strtok()</code> and
+ <code>gethostbyname()</code> are thread-safe on your OS, c-client
+ <em>may</em> be thread-safe.</td></tr>
+<tr class="odd"><td><a href="http://www.fastio.com/">cpdflib</a></td>
+ <td> </td>
+ <td>?</td>
+ <td> </td></tr>
+<tr><td><a href="http://www.ijg.org/files/">libcrypt</a></td>
+ <td> </td>
+ <td>?</td>
+ <td> </td></tr>
+<tr class="odd"><td><a href="http://expat.sourceforge.net/">Expat</a></td>
+ <td> </td>
+ <td>Yes</td>
+ <td>Need a separate parser instance per thread</td></tr>
+<tr><td><a href="http://www.freetds.org/">FreeTDS</a></td>
+ <td> </td>
+ <td>?</td>
+ <td> </td></tr>
+<tr class="odd"><td><a href="http://www.freetype.org/">FreeType</a></td>
+ <td> </td>
+ <td>?</td>
+ <td> </td></tr>
+<tr><td><a href="http://www.boutell.com/gd/">GD 1.8.x</a></td>
+ <td> </td>
+ <td>?</td>
+ <td> </td></tr>
+<tr class="odd"><td><a href="http://www.boutell.com/gd/">GD 2.0.x</a></td>
+ <td> </td>
+ <td>?</td>
+ <td> </td></tr>
+<tr><td><a href="http://www.gnu.org/software/gdbm/gdbm.html">gdbm</a></td>
+ <td> </td>
+ <td>No</td>
+ <td>Errors returned via a static <code>gdbm_error</code>
+ variable</td></tr>
+<tr class="odd"><td><a href="http://www.imagemagick.org/">ImageMagick</a></td>
+ <td>5.2.2</td>
+ <td>Yes</td>
+ <td>ImageMagick docs claim it is thread safe since version 5.2.2 (see <a href="http://www.cise.ufl.edu/depot/www/ImageMagick/www/Changelog.html">Change log</a>).
+ </td></tr>
+<tr><td><a href="http://www.enlightenment.org/pages/imlib2.html">Imlib2</a></td>
+ <td> </td>
+ <td>?</td>
+ <td> </td></tr>
+<tr class="odd"><td><a href="http://www.ijg.org/files/">libjpeg</a></td>
+ <td>v6b</td>
+ <td>?</td>
+ <td> </td></tr>
+<tr><td><a href="http://mysql.com">libmysqlclient</a></td>
+ <td> </td>
+ <td>Yes</td>
+ <td>Use mysqlclient_r library variant to ensure thread-safety. For
+ more information, please read <a href="http://www.mysql.com/doc/en/Threaded_clients.html">http://www.mysql.com/doc/en/Threaded_clients.html</a>.</td></tr>
+<tr class="odd"><td><a href="http://www.opaque.net/ming/">Ming</a></td>
+ <td>0.2a</td>
+ <td>?</td>
+ <td> </td></tr>
+<tr><td><a href="http://net-snmp.sourceforge.net/">Net-SNMP</a></td>
+ <td>5.0.x</td>
+ <td>?</td>
+ <td> </td></tr>
+<tr class="odd"><td><a href="http://www.openldap.org/">OpenLDAP</a></td>
+ <td>2.1.x</td>
+ <td>Yes</td>
+ <td>Use <code>ldap_r</code> library variant to ensure
+ thread-safety.</td></tr>
+<tr><td><a href="http://www.openssl.org/">OpenSSL</a></td>
+ <td>0.9.6g</td>
+ <td>Yes</td>
+ <td>Requires proper usage of <code>CRYPTO_num_locks</code>,
+ <code>CRYPTO_set_locking_callback</code>,
+ <code>CRYPTO_set_id_callback</code></td></tr>
+<tr class="odd"><td><a href="http://www.oracle.com/">liboci8 (Oracle 8+)</a></td>
+ <td>8.x,9.x</td>
+ <td>?</td>
+ <td> </td></tr>
+<tr><td><a href="http://pdflib.com/">pdflib</a></td>
+ <td>5.0.x</td>
+ <td>Yes</td>
+ <td>PDFLib docs claim it is thread safe; changes.txt indicates it
+ has been partially thread-safe since V1.91: <a href="http://www.pdflib.com/products/pdflib/index.html">http://www.pdflib.com/products/pdflib/index.html</a>.</td></tr>
+<tr class="odd"><td><a href="http://www.libpng.org/pub/png/libpng.html">libpng</a></td>
+ <td>1.0.x</td>
+ <td>?</td>
+ <td> </td></tr>
+<tr><td><a href="http://www.libpng.org/pub/png/libpng.html">libpng</a></td>
+ <td>1.2.x</td>
+ <td>?</td>
+ <td> </td></tr>
+<tr class="odd"><td><a href="http://www.postgresql.org/idocs/index.php?libpq-threading.html">libpq (PostgreSQL)</a></td>
+ <td>7.x</td>
+ <td>Yes</td>
+ <td>Don't share connections across threads and watch out for
+ <code>crypt()</code> calls</td></tr>
+<tr><td><a href="http://www.gingerall.com/charlie/ga/xml/p_sab.xml">Sablotron</a></td>
+ <td>0.95</td>
+ <td>?</td>
+ <td /></tr>
+<tr class="odd"><td><a href="http://www.gzip.org/zlib/">zlib</a></td>
+ <td>1.1.4</td>
+ <td>Yes</td>
+ <td>Relies upon thread-safe zalloc and zfree functions Default is to
+ use libc's calloc/free which are thread-safe.</td></tr>
+</table>
+</div></div>
+<div id="footer">
+<p class="apache">Maintained by the <a href="http://httpd.apache.org/docs-project/">Apache HTTP Server Documentation Project</a></p>
+<p class="menu"><a href="../mod/">Modules</a> | <a href="../mod/directives.html">Directives</a> | <a href="../faq/">FAQ</a> | <a href="../glossary.html">Glossary</a> | <a href="../sitemap.html">Sitemap</a></p></div>
+</body></html>
\ No newline at end of file
--- /dev/null
+<?xml version="1.0" encoding="UTF-8" ?>
+<!DOCTYPE manualpage SYSTEM "../style/manualpage.dtd">
+<?xml-stylesheet type="text/xsl" href="../style/manual.en.xsl"?>
+
+<manualpage>
+<relativepath href=".."/>
+
+<title>Apache 2.0 Thread Safety Issues</title>
+
+<summary>
+ <p>When using any of the threaded mpms in Apache 2.0 it is important
+ that every function called from Apache be thread safe. When linking in 3rd
+ party extensions it can be difficult to determine whether the resulting
+ server will be thread safe. Casual testing generally won't tell you this
+ either as thread safety problems can lead to subtle race conditons that
+ may only show up in certain conditions under heavy load.</p>
+</summary>
+
+<section id="variables"><title>Global and static variables</title>
+ <p>When writing your module or when trying to determine if a module or
+ 3rd party library is thread safe there are some common things to keep in
+ mind.</p>
+
+ <p>First, you need to recognize that in a threaded model each individual
+ thread has its own program counter, stack and registers. Local variables
+ live on the stack, so those are fine. You need to watch out for any
+ static or global variables. This doesn't mean that you are absolutely not
+ allowed to use static or global variables. There are times when you
+ actually want something to affect all threads, but generally you need to
+ avoid using them if you want your code to be thread safe.</p>
+
+ <p>In the case where you have a global variable that needs to be global and
+ accessed by all threads, be very careful when you update it. If, for
+ example, it is an incrementing counter, you need to atomically increment
+ it to avoid race conditions with other threads. You do this using a mutex
+ (mutual exclusion). Lock the mutex, read the current value, increment it
+ and write it back and then unlock the mutex. Any other thread that wants
+ to modify the value has to first check the mutex and block until it is
+ cleared.</p>
+
+ <p>If you are using <a href="http://apr.apache.org/">APR</a>, have a look
+ at the <code>apr_atomic_<var>*</var></code> functions and the
+ <code>apr_thread_mutex_<var>*</var></code> functions.</p>
+ <!-- [would probably be a good idea to add an example here] -->
+</section>
+
+<section id="errno"><title>errno</title>
+ <p>This is a common global variable that holds the error number of the
+ last error that occurred. If one thread calls a low-level function that
+ sets errno and then another thread checks it, we are bleeding error
+ numbers from one thread into another. To solve this, make sure your
+ module or library defines <code>_REENTRANT</code> or is compiled with
+ <code>-D_REENTRANT</code>. This will make errno a per-thread variable
+ and should hopefully be transparent to the code. It does this by doing
+ something like this:</p>
+
+ <example>
+ #define errno (*(__errno_location()))
+ </example>
+
+ <p>which means that accessing errno will call
+ <code>__errno_location()</code> which is provided by the libc. Setting
+ <code>_REENTRANT</code> also forces redefinition of some other functions
+ to their <code><var>*</var>_r</code> equivalents and sometimes changes
+ the common <code>getc</code>/<code>putc</code> macros into safer function
+ calls. Check your libc documentation for specifics. Instead of, or in
+ addition to <code>_REENTRANT</code> the symbols that may affect this are
+ <code>_POSIX_C_SOURCE</code>, <code>_THREAD_SAFE</code>,
+ <code>_SVID_SOURCE</code>, and <code>_BSD_SOURCE</code>.</p>
+</section>
+
+<section id="functions"><title>Common standard troublesome functions</title>
+ <p>Not only do things have to be thread safe, but they also have to be
+ reentrant. <code>strtok()</code> is an obvious one. You call it the first
+ time with your delimiter which it then remembers and on each subsequent
+ call it returns the next token. Obviously if multiple threads are
+ calling it you will have a problem. Most systems have a reentrant version
+ of of the function called <code>strtok_r()</code> where you pass in an
+ extra argument which contains an allocated <code>char *</code> which the
+ function will use instead of its own static storage for maintaining
+ the tokenizing state. If you are using <a href="http://apr.apache.org/"
+ >APR</a> you can use <code>apr_strtok()</code>.</p>
+
+ <p><code>crypt()</code> is another function that tends to not be reentrant,
+ so if you run across calls to that function in a library, watch out. On
+ some systems it is reentrant though, so it is not always a problem. If
+ your system has <code>crypt_r()</code> chances are you should be using
+ that, or if possible simply avoid the whole mess by using md5 instead.</p>
+ <!-- [I don't see an apr_crypt() function.] -->
+</section>
+
+<section id="commonlibs"><title>Common 3rd Party Libraries</title>
+ <p>The following is a list of common libraries that are used by 3rd party
+ Apache modules. You can check to see if your module is using a potentially
+ unsafe library by using tools such as <code>ldd(1)</code> and
+ <code>nm(1)</code>. For <a href="http://www.php.net/">PHP</a>, for example,
+ try this:</p>
+
+ <example>
+ % ldd libphp4.so<br />
+ libsablot.so.0 => /usr/local/lib/libsablot.so.0 (0x401f6000)<br />
+ libexpat.so.0 => /usr/lib/libexpat.so.0 (0x402da000)<br />
+ libsnmp.so.0 => /usr/lib/libsnmp.so.0 (0x402f9000)<br />
+ libpdf.so.1 => /usr/local/lib/libpdf.so.1 (0x40353000)<br />
+ libz.so.1 => /usr/lib/libz.so.1 (0x403e2000)<br />
+ libpng.so.2 => /usr/lib/libpng.so.2 (0x403f0000)<br />
+ libmysqlclient.so.11 => /usr/lib/libmysqlclient.so.11 (0x40411000)<br />
+ libming.so => /usr/lib/libming.so (0x40449000)<br />
+ libm.so.6 => /lib/libm.so.6 (0x40487000)<br />
+ libfreetype.so.6 => /usr/lib/libfreetype.so.6 (0x404a8000)<br />
+ libjpeg.so.62 => /usr/lib/libjpeg.so.62 (0x404e7000)<br />
+ libcrypt.so.1 => /lib/libcrypt.so.1 (0x40505000)<br />
+ libssl.so.2 => /lib/libssl.so.2 (0x40532000)<br />
+ libcrypto.so.2 => /lib/libcrypto.so.2 (0x40560000)<br />
+ libresolv.so.2 => /lib/libresolv.so.2 (0x40624000)<br />
+ libdl.so.2 => /lib/libdl.so.2 (0x40634000)<br />
+ libnsl.so.1 => /lib/libnsl.so.1 (0x40637000)<br />
+ libc.so.6 => /lib/libc.so.6 (0x4064b000)<br />
+ /lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x80000000)
+ </example>
+
+ <p>In addition to these libraries you will need to have a look at any
+ libraries linked statically into the module. You can use <code>nm(1)</code>
+ to look for individual symbols in the module.</p>
+</section>
+
+<section id="liblist"><title>Library List</title>
+ <p>Please drop a note to <a
+ href="http://httpd.apache.org/lists.html#http-dev">dev@httpd.apache.org</a>
+ if you have additions or corrections to this list.</p>
+
+ <table style="zebra" border="1">
+ <tr><th>Library</th><th>Version</th><th>Thread Safe?</th><th>Notes</th></tr>
+ <tr><td><a href="http://aspell.sourceforge.net/">ASpell/PSpell</a></td>
+ <td> </td>
+ <td>?</td>
+ <td> </td></tr>
+ <tr><td><a href="http://www.sleepycat.com/">Berkeley DB</a></td>
+ <td>3.x, 4.x</td>
+ <td>Yes</td>
+ <td>Be careful about sharing a connection across threads.</td></tr>
+ <tr><td><a href="http://sources.redhat.com/bzip2/index.html">bzip2</a></td>
+ <td> </td>
+ <td>Yes</td>
+ <td>Both low-level and high-level APIs are thread-safe. However,
+ high-level API requires thread-safe access to errno.</td></tr>
+ <tr><td><a href="http://cr.yp.to/cdb.html">cdb</a></td>
+ <td> </td>
+ <td>?</td>
+ <td> </td></tr>
+ <tr><td><a href="http://www.washington.edu/imap/">C-Client</a></td>
+ <td> </td>
+ <td>Perhaps</td>
+ <td>c-client uses <code>strtok()</code> and
+ <code>gethostbyname()</code> which are not thread-safe on most C
+ library implementations. c-client's static data is meant to be shared
+ across threads. If <code>strtok()</code> and
+ <code>gethostbyname()</code> are thread-safe on your OS, c-client
+ <em>may</em> be thread-safe.</td></tr>
+ <tr><td><a href="http://www.fastio.com/">cpdflib</a></td>
+ <td> </td>
+ <td>?</td>
+ <td> </td></tr>
+ <tr><td><a href="http://www.ijg.org/files/">libcrypt</a></td>
+ <td> </td>
+ <td>?</td>
+ <td> </td></tr>
+ <tr><td><a href="http://expat.sourceforge.net/">Expat</a></td>
+ <td> </td>
+ <td>Yes</td>
+ <td>Need a separate parser instance per thread</td></tr>
+ <tr><td><a href="http://www.freetds.org/">FreeTDS</a></td>
+ <td> </td>
+ <td>?</td>
+ <td> </td></tr>
+ <tr><td><a href="http://www.freetype.org/">FreeType</a></td>
+ <td> </td>
+ <td>?</td>
+ <td> </td></tr>
+ <tr><td><a href="http://www.boutell.com/gd/">GD 1.8.x</a></td>
+ <td> </td>
+ <td>?</td>
+ <td> </td></tr>
+ <tr><td><a href="http://www.boutell.com/gd/">GD 2.0.x</a></td>
+ <td> </td>
+ <td>?</td>
+ <td> </td></tr>
+ <tr><td><a href="http://www.gnu.org/software/gdbm/gdbm.html">gdbm</a></td>
+ <td> </td>
+ <td>No</td>
+ <td>Errors returned via a static <code>gdbm_error</code>
+ variable</td></tr>
+ <tr><td><a href="http://www.imagemagick.org/">ImageMagick</a></td>
+ <td>5.2.2</td>
+ <td>Yes</td>
+ <td>ImageMagick docs claim it is thread safe since version 5.2.2 (see <a
+ href="http://www.cise.ufl.edu/depot/www/ImageMagick/www/Changelog.html"
+ >Change log</a>).
+ </td></tr>
+ <tr><td><a href="http://www.enlightenment.org/pages/imlib2.html"
+ >Imlib2</a></td>
+ <td> </td>
+ <td>?</td>
+ <td> </td></tr>
+ <tr><td><a href="http://www.ijg.org/files/">libjpeg</a></td>
+ <td>v6b</td>
+ <td>?</td>
+ <td> </td></tr>
+ <tr><td><a href="http://mysql.com">libmysqlclient</a></td>
+ <td> </td>
+ <td>Yes</td>
+ <td>Use mysqlclient_r library variant to ensure thread-safety. For
+ more information, please read <a
+ href="http://www.mysql.com/doc/en/Threaded_clients.html"
+ >http://www.mysql.com/doc/en/Threaded_clients.html</a>.</td></tr>
+ <tr><td><a href="http://www.opaque.net/ming/">Ming</a></td>
+ <td>0.2a</td>
+ <td>?</td>
+ <td> </td></tr>
+ <tr><td><a href="http://net-snmp.sourceforge.net/">Net-SNMP</a></td>
+ <td>5.0.x</td>
+ <td>?</td>
+ <td> </td></tr>
+ <tr><td><a href="http://www.openldap.org/">OpenLDAP</a></td>
+ <td>2.1.x</td>
+ <td>Yes</td>
+ <td>Use <code>ldap_r</code> library variant to ensure
+ thread-safety.</td></tr>
+ <tr><td><a href="http://www.openssl.org/">OpenSSL</a></td>
+ <td>0.9.6g</td>
+ <td>Yes</td>
+ <td>Requires proper usage of <code>CRYPTO_num_locks</code>,
+ <code>CRYPTO_set_locking_callback</code>,
+ <code>CRYPTO_set_id_callback</code></td></tr>
+ <tr><td><a href="http://www.oracle.com/">liboci8 (Oracle 8+)</a></td>
+ <td>8.x,9.x</td>
+ <td>?</td>
+ <td> </td></tr>
+ <tr><td><a href="http://pdflib.com/">pdflib</a></td>
+ <td>5.0.x</td>
+ <td>Yes</td>
+ <td>PDFLib docs claim it is thread safe; changes.txt indicates it
+ has been partially thread-safe since V1.91: <a
+ href="http://www.pdflib.com/products/pdflib/index.html"
+ >http://www.pdflib.com/products/pdflib/index.html</a>.</td></tr>
+ <tr><td><a href="http://www.libpng.org/pub/png/libpng.html">libpng</a></td>
+ <td>1.0.x</td>
+ <td>?</td>
+ <td> </td></tr>
+ <tr><td><a href="http://www.libpng.org/pub/png/libpng.html">libpng</a></td>
+ <td>1.2.x</td>
+ <td>?</td>
+ <td> </td></tr>
+ <tr><td><a
+ href="http://www.postgresql.org/idocs/index.php?libpq-threading.html"
+ >libpq (PostgreSQL)</a></td>
+ <td>7.x</td>
+ <td>Yes</td>
+ <td>Don't share connections across threads and watch out for
+ <code>crypt()</code> calls</td></tr>
+ <tr><td><a href="http://www.gingerall.com/charlie/ga/xml/p_sab.xml"
+ >Sablotron</a></td>
+ <td>0.95</td>
+ <td>?</td>
+ <td></td></tr>
+ <tr><td><a href="http://www.gzip.org/zlib/">zlib</a></td>
+ <td>1.1.4</td>
+ <td>Yes</td>
+ <td>Relies upon thread-safe zalloc and zfree functions Default is to
+ use libc's calloc/free which are thread-safe.</td></tr>
+ </table>
+</section>
+</manualpage>
+
projects / apache / commitdiff