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8 <title>Apache HTTP Server 2.x Thread Safety Issues - Apache HTTP Server</title>
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17 <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>
18 <p class="apache">Apache HTTP Server Version 2.5</p>
19 <img alt="" src="../images/feather.gif" /></div>
20 <div class="up"><a href="./"><img title="<-" alt="<-" src="../images/left.gif" /></a></div>
22 <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 2.5</a> > <a href="./">Developer Documentation</a></div><div id="page-content"><div id="preamble"><h1>Apache HTTP Server 2.x Thread Safety Issues</h1>
24 <p><span>Available Languages: </span><a href="../en/developer/thread_safety.html" title="English"> en </a></p>
27 <p>When using any of the threaded mpms in the Apache HTTP Server 2.x it is important
28 that every function called from Apache be thread safe. When linking in 3rd
29 party extensions it can be difficult to determine whether the resulting
30 server will be thread safe. Casual testing generally won't tell you this
31 either as thread safety problems can lead to subtle race conditions that
32 may only show up in certain conditions under heavy load.</p>
34 <div id="quickview"><ul id="toc"><li><img alt="" src="../images/down.gif" /> <a href="#variables">Global and static variables</a></li>
35 <li><img alt="" src="../images/down.gif" /> <a href="#errno">errno</a></li>
36 <li><img alt="" src="../images/down.gif" /> <a href="#functions">Common standard troublesome functions</a></li>
37 <li><img alt="" src="../images/down.gif" /> <a href="#commonlibs">Common 3rd Party Libraries</a></li>
38 <li><img alt="" src="../images/down.gif" /> <a href="#liblist">Library List</a></li>
40 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
42 <h2><a name="variables" id="variables">Global and static variables</a></h2>
43 <p>When writing your module or when trying to determine if a module or
44 3rd party library is thread safe there are some common things to keep in
47 <p>First, you need to recognize that in a threaded model each individual
48 thread has its own program counter, stack and registers. Local variables
49 live on the stack, so those are fine. You need to watch out for any
50 static or global variables. This doesn't mean that you are absolutely not
51 allowed to use static or global variables. There are times when you
52 actually want something to affect all threads, but generally you need to
53 avoid using them if you want your code to be thread safe.</p>
55 <p>In the case where you have a global variable that needs to be global and
56 accessed by all threads, be very careful when you update it. If, for
57 example, it is an incrementing counter, you need to atomically increment
58 it to avoid race conditions with other threads. You do this using a mutex
59 (mutual exclusion). Lock the mutex, read the current value, increment it
60 and write it back and then unlock the mutex. Any other thread that wants
61 to modify the value has to first check the mutex and block until it is
64 <p>If you are using <a href="http://apr.apache.org/">APR</a>, have a look
65 at the <code>apr_atomic_<var>*</var></code> functions and the
66 <code>apr_thread_mutex_<var>*</var></code> functions.</p>
68 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
70 <h2><a name="errno" id="errno">errno</a></h2>
71 <p>This is a common global variable that holds the error number of the
72 last error that occurred. If one thread calls a low-level function that
73 sets errno and then another thread checks it, we are bleeding error
74 numbers from one thread into another. To solve this, make sure your
75 module or library defines <code>_REENTRANT</code> or is compiled with
76 <code>-D_REENTRANT</code>. This will make errno a per-thread variable
77 and should hopefully be transparent to the code. It does this by doing
78 something like this:</p>
80 <div class="example"><p><code>
81 #define errno (*(__errno_location()))
84 <p>which means that accessing errno will call
85 <code>__errno_location()</code> which is provided by the libc. Setting
86 <code>_REENTRANT</code> also forces redefinition of some other functions
87 to their <code><var>*</var>_r</code> equivalents and sometimes changes
88 the common <code>getc</code>/<code>putc</code> macros into safer function
89 calls. Check your libc documentation for specifics. Instead of, or in
90 addition to <code>_REENTRANT</code> the symbols that may affect this are
91 <code>_POSIX_C_SOURCE</code>, <code>_THREAD_SAFE</code>,
92 <code>_SVID_SOURCE</code>, and <code>_BSD_SOURCE</code>.</p>
93 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
95 <h2><a name="functions" id="functions">Common standard troublesome functions</a></h2>
96 <p>Not only do things have to be thread safe, but they also have to be
97 reentrant. <code>strtok()</code> is an obvious one. You call it the first
98 time with your delimiter which it then remembers and on each subsequent
99 call it returns the next token. Obviously if multiple threads are
100 calling it you will have a problem. Most systems have a reentrant version
101 of of the function called <code>strtok_r()</code> where you pass in an
102 extra argument which contains an allocated <code>char *</code> which the
103 function will use instead of its own static storage for maintaining
104 the tokenizing state. If you are using <a href="http://apr.apache.org/">APR</a> you can use <code>apr_strtok()</code>.</p>
106 <p><code>crypt()</code> is another function that tends to not be reentrant,
107 so if you run across calls to that function in a library, watch out. On
108 some systems it is reentrant though, so it is not always a problem. If
109 your system has <code>crypt_r()</code> chances are you should be using
110 that, or if possible simply avoid the whole mess by using md5 instead.</p>
112 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
113 <div class="section">
114 <h2><a name="commonlibs" id="commonlibs">Common 3rd Party Libraries</a></h2>
115 <p>The following is a list of common libraries that are used by 3rd party
116 Apache modules. You can check to see if your module is using a potentially
117 unsafe library by using tools such as <code>ldd(1)</code> and
118 <code>nm(1)</code>. For <a href="http://www.php.net/">PHP</a>, for example,
121 <div class="example"><p><code>
122 % ldd libphp4.so<br />
123 libsablot.so.0 => /usr/local/lib/libsablot.so.0 (0x401f6000)<br />
124 libexpat.so.0 => /usr/lib/libexpat.so.0 (0x402da000)<br />
125 libsnmp.so.0 => /usr/lib/libsnmp.so.0 (0x402f9000)<br />
126 libpdf.so.1 => /usr/local/lib/libpdf.so.1 (0x40353000)<br />
127 libz.so.1 => /usr/lib/libz.so.1 (0x403e2000)<br />
128 libpng.so.2 => /usr/lib/libpng.so.2 (0x403f0000)<br />
129 libmysqlclient.so.11 => /usr/lib/libmysqlclient.so.11 (0x40411000)<br />
130 libming.so => /usr/lib/libming.so (0x40449000)<br />
131 libm.so.6 => /lib/libm.so.6 (0x40487000)<br />
132 libfreetype.so.6 => /usr/lib/libfreetype.so.6 (0x404a8000)<br />
133 libjpeg.so.62 => /usr/lib/libjpeg.so.62 (0x404e7000)<br />
134 libcrypt.so.1 => /lib/libcrypt.so.1 (0x40505000)<br />
135 libssl.so.2 => /lib/libssl.so.2 (0x40532000)<br />
136 libcrypto.so.2 => /lib/libcrypto.so.2 (0x40560000)<br />
137 libresolv.so.2 => /lib/libresolv.so.2 (0x40624000)<br />
138 libdl.so.2 => /lib/libdl.so.2 (0x40634000)<br />
139 libnsl.so.1 => /lib/libnsl.so.1 (0x40637000)<br />
140 libc.so.6 => /lib/libc.so.6 (0x4064b000)<br />
141 /lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x80000000)
144 <p>In addition to these libraries you will need to have a look at any
145 libraries linked statically into the module. You can use <code>nm(1)</code>
146 to look for individual symbols in the module.</p>
147 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
148 <div class="section">
149 <h2><a name="liblist" id="liblist">Library List</a></h2>
150 <p>Please drop a note to <a href="http://httpd.apache.org/lists.html#http-dev">dev@httpd.apache.org</a>
151 if you have additions or corrections to this list.</p>
153 <table class="bordered"><tr class="header"><th>Library</th><th>Version</th><th>Thread Safe?</th><th>Notes</th></tr>
154 <tr><td><a href="http://aspell.sourceforge.net/">ASpell/PSpell</a></td>
158 <tr class="odd"><td><a href="http://www.sleepycat.com/">Berkeley DB</a></td>
161 <td>Be careful about sharing a connection across threads.</td></tr>
162 <tr><td><a href="http://sources.redhat.com/bzip2/index.html">bzip2</a></td>
165 <td>Both low-level and high-level APIs are thread-safe. However,
166 high-level API requires thread-safe access to errno.</td></tr>
167 <tr class="odd"><td><a href="http://cr.yp.to/cdb.html">cdb</a></td>
171 <tr><td><a href="http://www.washington.edu/imap/">C-Client</a></td>
174 <td>c-client uses <code>strtok()</code> and
175 <code>gethostbyname()</code> which are not thread-safe on most C
176 library implementations. c-client's static data is meant to be shared
177 across threads. If <code>strtok()</code> and
178 <code>gethostbyname()</code> are thread-safe on your OS, c-client
179 <em>may</em> be thread-safe.</td></tr>
180 <tr class="odd"><td><a href="http://www.ijg.org/files/">libcrypt</a></td>
184 <tr><td><a href="http://expat.sourceforge.net/">Expat</a></td>
187 <td>Need a separate parser instance per thread</td></tr>
188 <tr class="odd"><td><a href="http://www.freetds.org/">FreeTDS</a></td>
192 <tr><td><a href="http://www.freetype.org/">FreeType</a></td>
196 <tr class="odd"><td><a href="http://www.boutell.com/gd/">GD 1.8.x</a></td>
200 <tr><td><a href="http://www.boutell.com/gd/">GD 2.0.x</a></td>
204 <tr class="odd"><td><a href="http://www.gnu.org/software/gdbm/gdbm.html">gdbm</a></td>
207 <td>Errors returned via a static <code>gdbm_error</code>
209 <tr><td><a href="http://www.imagemagick.org/">ImageMagick</a></td>
212 <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>).
214 <tr class="odd"><td><a href="http://www.enlightenment.org/p.php?p=about/efl&l=en">Imlib2</a></td>
218 <tr><td><a href="http://www.ijg.org/files/">libjpeg</a></td>
222 <tr class="odd"><td><a href="http://mysql.com">libmysqlclient</a></td>
225 <td>Use mysqlclient_r library variant to ensure thread-safety. For
226 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>
227 <tr><td><a href="http://www.opaque.net/ming/">Ming</a></td>
231 <tr class="odd"><td><a href="http://net-snmp.sourceforge.net/">Net-SNMP</a></td>
235 <tr><td><a href="http://www.openldap.org/">OpenLDAP</a></td>
238 <td>Use <code>ldap_r</code> library variant to ensure
239 thread-safety.</td></tr>
240 <tr class="odd"><td><a href="http://www.openssl.org/">OpenSSL</a></td>
243 <td>Requires proper usage of <code>CRYPTO_num_locks</code>,
244 <code>CRYPTO_set_locking_callback</code>,
245 <code>CRYPTO_set_id_callback</code></td></tr>
246 <tr><td><a href="http://www.oracle.com/">liboci8 (Oracle 8+)</a></td>
250 <tr class="odd"><td><a href="http://pdflib.com/">pdflib</a></td>
253 <td>PDFLib docs claim it is thread safe; changes.txt indicates it
254 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>
255 <tr><td><a href="http://www.libpng.org/pub/png/libpng.html">libpng</a></td>
259 <tr class="odd"><td><a href="http://www.libpng.org/pub/png/libpng.html">libpng</a></td>
263 <tr><td><a href="http://www.postgresql.org/docs/8.4/static/libpq-threading.html">libpq (PostgreSQL)</a></td>
266 <td>Don't share connections across threads and watch out for
267 <code>crypt()</code> calls</td></tr>
268 <tr class="odd"><td><a href="http://www.gingerall.com/charlie/ga/xml/p_sab.xml">Sablotron</a></td>
272 <tr><td><a href="http://www.gzip.org/zlib/">zlib</a></td>
275 <td>Relies upon thread-safe zalloc and zfree functions Default is to
276 use libc's calloc/free which are thread-safe.</td></tr>
279 <div class="bottomlang">
280 <p><span>Available Languages: </span><a href="../en/developer/thread_safety.html" title="English"> en </a></p>
281 </div><div id="footer">
282 <p class="apache">Copyright 2012 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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