1<?xml version="1.0" encoding="ISO-8859-1"?> 2<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> 3<html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en"><head><!-- 4 XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX 5 This file is generated from xml source: DO NOT EDIT 6 XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX 7 --> 8<title>Apache 2.0 Thread Safety Issues - Apache HTTP Server</title> 9<link href="/style/css/manual.css" rel="stylesheet" media="all" type="text/css" title="Main stylesheet" /> 10<link href="/style/css/manual-loose-100pc.css" rel="alternate stylesheet" media="all" type="text/css" title="No Sidebar - Default font size" /> 11<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" /> 12<script src="/style/scripts/prettify.js" type="text/javascript"> 13</script> 14 15<link href="/images/favicon.ico" rel="shortcut icon" /></head> 16<body id="manual-page"><div id="page-header"> 17<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> 18<p class="apache">Apache HTTP Server Version 2.2</p> 19<img alt="" src="/images/feather.gif" /></div> 20<div class="up"><a href="./"><img title="<-" alt="<-" src="/images/left.gif" /></a></div> 21<div id="path"> 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 2.2</a> > <a href="./">Developer Documentation</a></div><div id="page-content"><div id="preamble"><h1>Apache 2.0 Thread Safety Issues</h1> 23<div class="toplang"> 24<p><span>Available Languages: </span><a href="/en/developer/thread_safety.html" title="English"> en </a></p> 25</div> 26 27 <p>When using any of the threaded mpms in Apache 2.0 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 conditons that 32 may only show up in certain conditions under heavy load.</p> 33</div> 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> 39</ul><ul class="seealso"><li><a href="#comments_section">Comments</a></li></ul></div> 40<div class="top"><a href="#page-header"><img alt="top" src="/images/up.gif" /></a></div> 41<div class="section"> 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 45 mind.</p> 46 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> 54 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 62 cleared.</p> 63 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> 67 68</div><div class="top"><a href="#page-header"><img alt="top" src="/images/up.gif" /></a></div> 69<div class="section"> 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> 79 80 <div class="example"><p><code> 81 #define errno (*(__errno_location())) 82 </code></p></div> 83 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> 94<div class="section"> 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> 105 106 <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> 111 112</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, 119 try this:</p> 120 121 <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) 142 </code></p></div> 143 144 <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> 152 153 <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> 155 <td> </td> 156 <td>?</td> 157 <td> </td></tr> 158<tr class="odd"><td><a href="http://www.sleepycat.com/">Berkeley DB</a></td> 159 <td>3.x, 4.x</td> 160 <td>Yes</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> 163 <td> </td> 164 <td>Yes</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> 168 <td> </td> 169 <td>?</td> 170 <td> </td></tr> 171<tr><td><a href="http://www.washington.edu/imap/">C-Client</a></td> 172 <td> </td> 173 <td>Perhaps</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> 181 <td> </td> 182 <td>?</td> 183 <td> </td></tr> 184<tr><td><a href="http://expat.sourceforge.net/">Expat</a></td> 185 <td> </td> 186 <td>Yes</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> 189 <td> </td> 190 <td>?</td> 191 <td> </td></tr> 192<tr><td><a href="http://www.freetype.org/">FreeType</a></td> 193 <td> </td> 194 <td>?</td> 195 <td> </td></tr> 196<tr class="odd"><td><a href="http://www.boutell.com/gd/">GD 1.8.x</a></td> 197 <td> </td> 198 <td>?</td> 199 <td> </td></tr> 200<tr><td><a href="http://www.boutell.com/gd/">GD 2.0.x</a></td> 201 <td> </td> 202 <td>?</td> 203 <td> </td></tr> 204<tr class="odd"><td><a href="http://www.gnu.org/software/gdbm/gdbm.html">gdbm</a></td> 205 <td> </td> 206 <td>No</td> 207 <td>Errors returned via a static <code>gdbm_error</code> 208 variable</td></tr> 209<tr><td><a href="http://www.imagemagick.org/">ImageMagick</a></td> 210 <td>5.2.2</td> 211 <td>Yes</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>). 213 </td></tr> 214<tr class="odd"><td><a href="http://www.enlightenment.org/p.php?p=about/efl&l=en">Imlib2</a></td> 215 <td> </td> 216 <td>?</td> 217 <td> </td></tr> 218<tr><td><a href="http://www.ijg.org/files/">libjpeg</a></td> 219 <td>v6b</td> 220 <td>?</td> 221 <td> </td></tr> 222<tr class="odd"><td><a href="http://mysql.com">libmysqlclient</a></td> 223 <td> </td> 224 <td>Yes</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> 228 <td>0.2a</td> 229 <td>?</td> 230 <td> </td></tr> 231<tr class="odd"><td><a href="http://net-snmp.sourceforge.net/">Net-SNMP</a></td> 232 <td>5.0.x</td> 233 <td>?</td> 234 <td> </td></tr> 235<tr><td><a href="http://www.openldap.org/">OpenLDAP</a></td> 236 <td>2.1.x</td> 237 <td>Yes</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> 241 <td>0.9.6g</td> 242 <td>Yes</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> 247 <td>8.x,9.x</td> 248 <td>?</td> 249 <td> </td></tr> 250<tr class="odd"><td><a href="http://pdflib.com/">pdflib</a></td> 251 <td>5.0.x</td> 252 <td>Yes</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> 256 <td>1.0.x</td> 257 <td>?</td> 258 <td> </td></tr> 259<tr class="odd"><td><a href="http://www.libpng.org/pub/png/libpng.html">libpng</a></td> 260 <td>1.2.x</td> 261 <td>?</td> 262 <td> </td></tr> 263<tr><td><a href="http://www.postgresql.org/docs/8.4/static/libpq-threading.html">libpq (PostgreSQL)</a></td> 264 <td>8.x</td> 265 <td>Yes</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> 269 <td>0.95</td> 270 <td>?</td> 271 <td /></tr> 272<tr><td><a href="http://www.gzip.org/zlib/">zlib</a></td> 273 <td>1.1.4</td> 274 <td>Yes</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> 277</table> 278</div></div> 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 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. 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