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<?xml version="1.0" encoding="ISO-8859-1"?> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> <html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en"><head> <meta content="text/html; charset=ISO-8859-1" http-equiv="Content-Type" /> <!-- XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX This file is generated from xml source: DO NOT EDIT XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX --> <title>Apache HTTP Server 2.x Thread Safety Issues - Apache HTTP Server Version 2.4</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" /><link rel="stylesheet" type="text/css" href="../style/css/prettify.css" /> <script src="../style/scripts/prettify.min.js" type="text/javascript"> </script> <link href="../images/favicon.ico" rel="shortcut icon" /></head> <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="http://wiki.apache.org/httpd/FAQ">FAQ</a> | <a href="../glossary.html">Glossary</a> | <a href="../sitemap.html">Sitemap</a></p> <p class="apache">Apache HTTP Server Version 2.4</p> <img alt="" src="../images/feather.png" /></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/">Documentation</a> > <a href="../">Version 2.4</a> > <a href="./">Developer Documentation</a></div><div id="page-content"><div id="preamble"><h1>Apache HTTP Server 2.x Thread Safety Issues</h1> <div class="toplang"> <p><span>Available Languages: </span><a href="../en/developer/thread_safety.html" title="English"> en </a></p> </div> <p>When using any of the threaded mpms in the Apache HTTP Server 2.x 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 conditions that may only show up in certain conditions under heavy load.</p> </div> <div id="quickview"><a href="https://www.apache.org/foundation/contributing.html" class="badge"><img src="https://www.apache.org/images/SupportApache-small.png" alt="Support Apache!" /></a><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><h3>See also</h3><ul class="seealso"><li><a href="#comments_section">Comments</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 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.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 class="odd"><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 class="odd"><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 class="odd"><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.imagemagick.com/www/changelog.html">Change log</a>). </td></tr> <tr class="odd"><td><a href="http://www.enlightenment.org/p.php?p=about/efl&l=en">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 class="odd"><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://dev.mysql.com/doc/mysql/en/Threaded_clients.html">http://dev.mysql.com/doc/mysql/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 class="odd"><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 class="odd"><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 class="odd"><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-family/pdflib/">http://www.pdflib.com/products/pdflib-family/pdflib/</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 class="odd"><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/docs/8.4/static/libpq-threading.html">libpq (PostgreSQL)</a></td> <td>8.x</td> <td>Yes</td> <td>Don't share connections across threads and watch out for <code>crypt()</code> calls</td></tr> <tr class="odd"><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><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 class="bottomlang"> <p><span>Available Languages: </span><a href="../en/developer/thread_safety.html" title="English"> en </a></p> </div><div class="top"><a href="#page-header"><img src="../images/up.gif" alt="top" /></a></div><div class="section"><h2><a id="comments_section" name="comments_section">Comments</a></h2><div class="warning"><strong>Notice:</strong><br />This is not a Q&A section. Comments placed here should be pointed towards suggestions on improving the documentation or server, and may be removed again by our moderators if they are either implemented or considered invalid/off-topic. Questions on how to manage the Apache HTTP Server should be directed at either our IRC channel, #httpd, on Freenode, or sent to our <a href="http://httpd.apache.org/lists.html">mailing lists</a>.</div> <script type="text/javascript"><!--//--><![CDATA[//><!-- var comments_shortname = 'httpd'; var comments_identifier = 'http://httpd.apache.org/docs/2.4/developer/thread_safety.html'; (function(w, d) { if (w.location.hostname.toLowerCase() == "httpd.apache.org") { d.write('<div id="comments_thread"><\/div>'); var s = d.createElement('script'); s.type = 'text/javascript'; s.async = true; s.src = 'https://comments.apache.org/show_comments.lua?site=' + comments_shortname + '&page=' + comments_identifier; (d.getElementsByTagName('head')[0] || d.getElementsByTagName('body')[0]).appendChild(s); } else { d.write('<div id="comments_thread">Comments are disabled for this page at the moment.<\/div>'); } })(window, document); //--><!]]></script></div><div id="footer"> <p class="apache">Copyright 2017 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> <p class="menu"><a href="../mod/">Modules</a> | <a href="../mod/directives.html">Directives</a> | <a href="http://wiki.apache.org/httpd/FAQ">FAQ</a> | <a href="../glossary.html">Glossary</a> | <a href="../sitemap.html">Sitemap</a></p></div><script type="text/javascript"><!--//--><![CDATA[//><!-- if (typeof(prettyPrint) !== 'undefined') { prettyPrint(); } //--><!]]></script> </body></html>