summaryrefslogtreecommitdiffstats
path: root/docs/manual/developer/thread_safety.html.en
diff options
context:
space:
mode:
authorDaniel Baumann <daniel.baumann@progress-linux.org>2024-04-07 15:01:30 +0000
committerDaniel Baumann <daniel.baumann@progress-linux.org>2024-04-07 15:01:30 +0000
commit6beeb1b708550be0d4a53b272283e17e5e35fe17 (patch)
tree1ce8673d4aaa948e5554000101f46536a1e4cc29 /docs/manual/developer/thread_safety.html.en
parentInitial commit. (diff)
downloadapache2-3d46059c28a1d66e6e1aeb209491f1324c913a25.tar.xz
apache2-3d46059c28a1d66e6e1aeb209491f1324c913a25.zip
Adding upstream version 2.4.57.upstream/2.4.57
Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
Diffstat (limited to 'docs/manual/developer/thread_safety.html.en')
-rw-r--r--docs/manual/developer/thread_safety.html.en307
1 files changed, 307 insertions, 0 deletions
diff --git a/docs/manual/developer/thread_safety.html.en b/docs/manual/developer/thread_safety.html.en
new file mode 100644
index 0000000..7e842d8
--- /dev/null
+++ b/docs/manual/developer/thread_safety.html.en
@@ -0,0 +1,307 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!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=UTF-8" 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="&lt;-" alt="&lt;-" src="../images/left.gif" /></a></div>
+<div id="path">
+<a href="http://www.apache.org/">Apache</a> &gt; <a href="http://httpd.apache.org/">HTTP Server</a> &gt; <a href="http://httpd.apache.org/docs/">Documentation</a> &gt; <a href="../">Version 2.4</a> &gt; <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">&nbsp;en&nbsp;</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 =&gt; /usr/local/lib/libsablot.so.0 (0x401f6000)<br />
+ libexpat.so.0 =&gt; /usr/lib/libexpat.so.0 (0x402da000)<br />
+ libsnmp.so.0 =&gt; /usr/lib/libsnmp.so.0 (0x402f9000)<br />
+ libpdf.so.1 =&gt; /usr/local/lib/libpdf.so.1 (0x40353000)<br />
+ libz.so.1 =&gt; /usr/lib/libz.so.1 (0x403e2000)<br />
+ libpng.so.2 =&gt; /usr/lib/libpng.so.2 (0x403f0000)<br />
+ libmysqlclient.so.11 =&gt; /usr/lib/libmysqlclient.so.11 (0x40411000)<br />
+ libming.so =&gt; /usr/lib/libming.so (0x40449000)<br />
+ libm.so.6 =&gt; /lib/libm.so.6 (0x40487000)<br />
+ libfreetype.so.6 =&gt; /usr/lib/libfreetype.so.6 (0x404a8000)<br />
+ libjpeg.so.62 =&gt; /usr/lib/libjpeg.so.62 (0x404e7000)<br />
+ libcrypt.so.1 =&gt; /lib/libcrypt.so.1 (0x40505000)<br />
+ libssl.so.2 =&gt; /lib/libssl.so.2 (0x40532000)<br />
+ libcrypto.so.2 =&gt; /lib/libcrypto.so.2 (0x40560000)<br />
+ libresolv.so.2 =&gt; /lib/libresolv.so.2 (0x40624000)<br />
+ libdl.so.2 =&gt; /lib/libdl.so.2 (0x40634000)<br />
+ libnsl.so.1 =&gt; /lib/libnsl.so.1 (0x40637000)<br />
+ libc.so.6 =&gt; /lib/libc.so.6 (0x4064b000)<br />
+ /lib/ld-linux.so.2 =&gt; /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&amp;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">&nbsp;en&nbsp;</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&amp;A section. Comments placed here should be pointed towards suggestions on improving the documentation or server, and may be removed 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 Libera.chat, or sent to our <a href="https://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 2023 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> \ No newline at end of file
generated by cgit v1.2.3 (git 2.39.1) at 2024-12-22 10:00:56 +0000