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<title>Apache 2.0 Thread Safety Issues - Apache HTTP Server</title>
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<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>
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<p class="apache">Apache HTTP Server Version 2.2</p>
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<img alt="" src="../images/feather.gif" /></div>
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<div class="up"><a href="./"><img title="<-" alt="<-" src="../images/left.gif" /></a></div>
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<a href="http://www.apache.org/">Apache</a> > <a href="http://httpd.apache.org/">HTTP Server</a> > <a href="http://httpd.apache.org/docs/">Documentation</a> > <a href="../">Version 2.2</a> > <a href="./">Developer Documentation</a></div><div id="page-content"><div id="preamble"><h1>Apache 2.0 Thread Safety Issues</h1>
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<p><span>Available Languages: </span><a href="../en/developer/thread_safety.html" title="English"> en </a></p>
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<p>When using any of the threaded mpms in Apache 2.0 it is important
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that every function called from Apache be thread safe. When linking in 3rd
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party extensions it can be difficult to determine whether the resulting
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server will be thread safe. Casual testing generally won't tell you this
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either as thread safety problems can lead to subtle race conditons that
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may only show up in certain conditions under heavy load.</p>
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<div id="quickview"><ul id="toc"><li><img alt="" src="../images/down.gif" /> <a href="#variables">Global and static variables</a></li>
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<li><img alt="" src="../images/down.gif" /> <a href="#errno">errno</a></li>
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<li><img alt="" src="../images/down.gif" /> <a href="#functions">Common standard troublesome functions</a></li>
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<li><img alt="" src="../images/down.gif" /> <a href="#commonlibs">Common 3rd Party Libraries</a></li>
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<li><img alt="" src="../images/down.gif" /> <a href="#liblist">Library List</a></li>
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<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
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<h2><a name="variables" id="variables">Global and static variables</a></h2>
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<p>When writing your module or when trying to determine if a module or
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3rd party library is thread safe there are some common things to keep in
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<p>First, you need to recognize that in a threaded model each individual
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thread has its own program counter, stack and registers. Local variables
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live on the stack, so those are fine. You need to watch out for any
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static or global variables. This doesn't mean that you are absolutely not
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allowed to use static or global variables. There are times when you
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actually want something to affect all threads, but generally you need to
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avoid using them if you want your code to be thread safe.</p>
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<p>In the case where you have a global variable that needs to be global and
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accessed by all threads, be very careful when you update it. If, for
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example, it is an incrementing counter, you need to atomically increment
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it to avoid race conditions with other threads. You do this using a mutex
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(mutual exclusion). Lock the mutex, read the current value, increment it
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and write it back and then unlock the mutex. Any other thread that wants
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to modify the value has to first check the mutex and block until it is
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<p>If you are using <a href="http://apr.apache.org/">APR</a>, have a look
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at the <code>apr_atomic_<var>*</var></code> functions and the
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<code>apr_thread_mutex_<var>*</var></code> functions.</p>
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</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
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<h2><a name="errno" id="errno">errno</a></h2>
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<p>This is a common global variable that holds the error number of the
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last error that occurred. If one thread calls a low-level function that
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sets errno and then another thread checks it, we are bleeding error
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numbers from one thread into another. To solve this, make sure your
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module or library defines <code>_REENTRANT</code> or is compiled with
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<code>-D_REENTRANT</code>. This will make errno a per-thread variable
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and should hopefully be transparent to the code. It does this by doing
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something like this:</p>
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<div class="example"><p><code>
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#define errno (*(__errno_location()))
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<p>which means that accessing errno will call
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<code>__errno_location()</code> which is provided by the libc. Setting
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<code>_REENTRANT</code> also forces redefinition of some other functions
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to their <code><var>*</var>_r</code> equivalents and sometimes changes
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the common <code>getc</code>/<code>putc</code> macros into safer function
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calls. Check your libc documentation for specifics. Instead of, or in
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addition to <code>_REENTRANT</code> the symbols that may affect this are
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<code>_POSIX_C_SOURCE</code>, <code>_THREAD_SAFE</code>,
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<code>_SVID_SOURCE</code>, and <code>_BSD_SOURCE</code>.</p>
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</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
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<h2><a name="functions" id="functions">Common standard troublesome functions</a></h2>
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<p>Not only do things have to be thread safe, but they also have to be
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reentrant. <code>strtok()</code> is an obvious one. You call it the first
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time with your delimiter which it then remembers and on each subsequent
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call it returns the next token. Obviously if multiple threads are
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calling it you will have a problem. Most systems have a reentrant version
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of of the function called <code>strtok_r()</code> where you pass in an
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extra argument which contains an allocated <code>char *</code> which the
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function will use instead of its own static storage for maintaining
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the tokenizing state. If you are using <a href="http://apr.apache.org/">APR</a> you can use <code>apr_strtok()</code>.</p>
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<p><code>crypt()</code> is another function that tends to not be reentrant,
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so if you run across calls to that function in a library, watch out. On
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some systems it is reentrant though, so it is not always a problem. If
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your system has <code>crypt_r()</code> chances are you should be using
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that, or if possible simply avoid the whole mess by using md5 instead.</p>
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</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
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<div class="section">
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<h2><a name="commonlibs" id="commonlibs">Common 3rd Party Libraries</a></h2>
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<p>The following is a list of common libraries that are used by 3rd party
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Apache modules. You can check to see if your module is using a potentially
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unsafe library by using tools such as <code>ldd(1)</code> and
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<code>nm(1)</code>. For <a href="http://www.php.net/">PHP</a>, for example,
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<div class="example"><p><code>
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% ldd libphp4.so<br />
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libsablot.so.0 => /usr/local/lib/libsablot.so.0 (0x401f6000)<br />
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libexpat.so.0 => /usr/lib/libexpat.so.0 (0x402da000)<br />
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libsnmp.so.0 => /usr/lib/libsnmp.so.0 (0x402f9000)<br />
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libpdf.so.1 => /usr/local/lib/libpdf.so.1 (0x40353000)<br />
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libz.so.1 => /usr/lib/libz.so.1 (0x403e2000)<br />
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libpng.so.2 => /usr/lib/libpng.so.2 (0x403f0000)<br />
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libmysqlclient.so.11 => /usr/lib/libmysqlclient.so.11 (0x40411000)<br />
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libming.so => /usr/lib/libming.so (0x40449000)<br />
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libm.so.6 => /lib/libm.so.6 (0x40487000)<br />
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libfreetype.so.6 => /usr/lib/libfreetype.so.6 (0x404a8000)<br />
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libjpeg.so.62 => /usr/lib/libjpeg.so.62 (0x404e7000)<br />
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libcrypt.so.1 => /lib/libcrypt.so.1 (0x40505000)<br />
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libssl.so.2 => /lib/libssl.so.2 (0x40532000)<br />
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libcrypto.so.2 => /lib/libcrypto.so.2 (0x40560000)<br />
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libresolv.so.2 => /lib/libresolv.so.2 (0x40624000)<br />
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libdl.so.2 => /lib/libdl.so.2 (0x40634000)<br />
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libnsl.so.1 => /lib/libnsl.so.1 (0x40637000)<br />
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libc.so.6 => /lib/libc.so.6 (0x4064b000)<br />
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/lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x80000000)
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<p>In addition to these libraries you will need to have a look at any
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libraries linked statically into the module. You can use <code>nm(1)</code>
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to look for individual symbols in the module.</p>
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</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
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<div class="section">
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<h2><a name="liblist" id="liblist">Library List</a></h2>
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<p>Please drop a note to <a href="http://httpd.apache.org/lists.html#http-dev">dev@httpd.apache.org</a>
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if you have additions or corrections to this list.</p>
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<table class="bordered"><tr class="header"><th>Library</th><th>Version</th><th>Thread Safe?</th><th>Notes</th></tr>
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<tr><td><a href="http://aspell.sourceforge.net/">ASpell/PSpell</a></td>
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<tr class="odd"><td><a href="http://www.sleepycat.com/">Berkeley DB</a></td>
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<td>Be careful about sharing a connection across threads.</td></tr>
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<tr><td><a href="http://sources.redhat.com/bzip2/index.html">bzip2</a></td>
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<td>Both low-level and high-level APIs are thread-safe. However,
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high-level API requires thread-safe access to errno.</td></tr>
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<tr class="odd"><td><a href="http://cr.yp.to/cdb.html">cdb</a></td>
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<tr><td><a href="http://www.washington.edu/imap/">C-Client</a></td>
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<td>c-client uses <code>strtok()</code> and
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<code>gethostbyname()</code> which are not thread-safe on most C
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library implementations. c-client's static data is meant to be shared
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across threads. If <code>strtok()</code> and
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<code>gethostbyname()</code> are thread-safe on your OS, c-client
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<em>may</em> be thread-safe.</td></tr>
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<tr class="odd"><td><a href="http://www.fastio.com/">cpdflib</a></td>
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<tr><td><a href="http://www.ijg.org/files/">libcrypt</a></td>
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<tr class="odd"><td><a href="http://expat.sourceforge.net/">Expat</a></td>
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<td>Need a separate parser instance per thread</td></tr>
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<tr><td><a href="http://www.freetds.org/">FreeTDS</a></td>
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<tr class="odd"><td><a href="http://www.freetype.org/">FreeType</a></td>
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<tr><td><a href="http://www.boutell.com/gd/">GD 1.8.x</a></td>
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<tr class="odd"><td><a href="http://www.boutell.com/gd/">GD 2.0.x</a></td>
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<tr><td><a href="http://www.gnu.org/software/gdbm/gdbm.html">gdbm</a></td>
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<td>Errors returned via a static <code>gdbm_error</code>
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<tr class="odd"><td><a href="http://www.imagemagick.org/">ImageMagick</a></td>
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<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>).
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<tr><td><a href="http://www.enlightenment.org/pages/imlib2.html">Imlib2</a></td>
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<tr class="odd"><td><a href="http://www.ijg.org/files/">libjpeg</a></td>
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<tr><td><a href="http://mysql.com">libmysqlclient</a></td>
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<td>Use mysqlclient_r library variant to ensure thread-safety. For
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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>
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<tr class="odd"><td><a href="http://www.opaque.net/ming/">Ming</a></td>
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<tr><td><a href="http://net-snmp.sourceforge.net/">Net-SNMP</a></td>
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<tr class="odd"><td><a href="http://www.openldap.org/">OpenLDAP</a></td>
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<td>Use <code>ldap_r</code> library variant to ensure
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thread-safety.</td></tr>
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<tr><td><a href="http://www.openssl.org/">OpenSSL</a></td>
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<td>Requires proper usage of <code>CRYPTO_num_locks</code>,
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<code>CRYPTO_set_locking_callback</code>,
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<code>CRYPTO_set_id_callback</code></td></tr>
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<tr class="odd"><td><a href="http://www.oracle.com/">liboci8 (Oracle 8+)</a></td>
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<tr><td><a href="http://pdflib.com/">pdflib</a></td>
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<td>PDFLib docs claim it is thread safe; changes.txt indicates it
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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>
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<tr class="odd"><td><a href="http://www.libpng.org/pub/png/libpng.html">libpng</a></td>
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<tr><td><a href="http://www.libpng.org/pub/png/libpng.html">libpng</a></td>
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<tr class="odd"><td><a href="http://www.postgresql.org/idocs/index.php?libpq-threading.html">libpq (PostgreSQL)</a></td>
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<td>Don't share connections across threads and watch out for
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<code>crypt()</code> calls</td></tr>
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<tr><td><a href="http://www.gingerall.com/charlie/ga/xml/p_sab.xml">Sablotron</a></td>
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<tr class="odd"><td><a href="http://www.gzip.org/zlib/">zlib</a></td>
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<td>Relies upon thread-safe zalloc and zfree functions Default is to
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use libc's calloc/free which are thread-safe.</td></tr>
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<div class="bottomlang">
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<p><span>Available Languages: </span><a href="../en/developer/thread_safety.html" title="English"> en </a></p>
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</div><div id="footer">
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<p class="apache">Copyright 2006 The Apache Software Foundation.<br />Licensed under the <a href="http://www.apache.org/licenses/LICENSE-2.0">Apache License, Version 2.0</a>.</p>
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