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| author | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-08 19:09:22 +0000 |
|---|---|---|
| committer | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-08 19:09:22 +0000 |
| commit | 2faa747e2303ee774a4b4aace961188e950e185a (patch) | |
| tree | 604e79c7481956ce48f458e3546eaf1090b3ffff /docs/manual/developer/thread_safety.html.en | |
| parent | Initial commit. (diff) | |
| download | apache2-2faa747e2303ee774a4b4aace961188e950e185a.tar.xz apache2-2faa747e2303ee774a4b4aace961188e950e185a.zip | |
Adding upstream version 2.4.58.upstream/2.4.58
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.en | 307 |
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="<-" 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 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>
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