github.com/krum110487/go-htaccess@v0.0.0-20240316004156-60641c8e7598/tests/data/apache_2_2_34/manual/developer/thread_safety.html.en (about) 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 <meta content="text/html; charset=ISO-8859-1" http-equiv="Content-Type" /> 5 <!-- 6 XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX 7 This file is generated from xml source: DO NOT EDIT 8 XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX 9 --> 10 <title>Apache 2.0 Thread Safety Issues - Apache HTTP Server Version 2.2</title> 11 <link href="../style/css/manual.css" rel="stylesheet" media="all" type="text/css" title="Main stylesheet" /> 12 <link href="../style/css/manual-loose-100pc.css" rel="alternate stylesheet" media="all" type="text/css" title="No Sidebar - Default font size" /> 13 <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" /> 14 <script src="../style/scripts/prettify.min.js" type="text/javascript"> 15 </script> 16 17 <link href="../images/favicon.ico" rel="shortcut icon" /><link href="http://httpd.apache.org/docs/current/developer/thread_safety.html" rel="canonical" /></head> 18 <body id="manual-page"><div id="page-header"> 19 <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> 20 <p class="apache">Apache HTTP Server Version 2.2</p> 21 <img alt="" src="../images/feather.gif" /></div> 22 <div class="up"><a href="./"><img title="<-" alt="<-" src="../images/left.gif" /></a></div> 23 <div id="path"> 24 <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 class="retired"><h4>Please note</h4> 25 <p> This document refers to a legacy release (<strong>2.2</strong>) of Apache httpd. The active release (<strong>2.4</strong>) is documented <a href="http://httpd.apache.org/docs/current">here</a>. If you have not already upgraded, please follow <a href="http://httpd.apache.org/docs/current/upgrading.html">this link</a> for more information.</p> 26 <p>You may follow <a href="http://httpd.apache.org/docs/current/developer/thread_safety.html">this link</a> to go to the current version of this document.</p></div><div id="preamble"><h1>Apache 2.0 Thread Safety Issues</h1> 27 <div class="toplang"> 28 <p><span>Available Languages: </span><a href="../en/developer/thread_safety.html" title="English"> en </a></p> 29 </div> 30 31 <p>When using any of the threaded mpms in Apache 2.0 it is important 32 that every function called from Apache be thread safe. When linking in 3rd 33 party extensions it can be difficult to determine whether the resulting 34 server will be thread safe. Casual testing generally won't tell you this 35 either as thread safety problems can lead to subtle race conditons that 36 may only show up in certain conditions under heavy load.</p> 37 </div> 38 <div id="quickview"><ul id="toc"><li><img alt="" src="../images/down.gif" /> <a href="#variables">Global and static variables</a></li> 39 <li><img alt="" src="../images/down.gif" /> <a href="#errno">errno</a></li> 40 <li><img alt="" src="../images/down.gif" /> <a href="#functions">Common standard troublesome functions</a></li> 41 <li><img alt="" src="../images/down.gif" /> <a href="#commonlibs">Common 3rd Party Libraries</a></li> 42 <li><img alt="" src="../images/down.gif" /> <a href="#liblist">Library List</a></li> 43 </ul><ul class="seealso"><li><a href="#comments_section">Comments</a></li></ul></div> 44 <div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div> 45 <div class="section"> 46 <h2><a name="variables" id="variables">Global and static variables</a></h2> 47 <p>When writing your module or when trying to determine if a module or 48 3rd party library is thread safe there are some common things to keep in 49 mind.</p> 50 51 <p>First, you need to recognize that in a threaded model each individual 52 thread has its own program counter, stack and registers. Local variables 53 live on the stack, so those are fine. You need to watch out for any 54 static or global variables. This doesn't mean that you are absolutely not 55 allowed to use static or global variables. There are times when you 56 actually want something to affect all threads, but generally you need to 57 avoid using them if you want your code to be thread safe.</p> 58 59 <p>In the case where you have a global variable that needs to be global and 60 accessed by all threads, be very careful when you update it. If, for 61 example, it is an incrementing counter, you need to atomically increment 62 it to avoid race conditions with other threads. You do this using a mutex 63 (mutual exclusion). Lock the mutex, read the current value, increment it 64 and write it back and then unlock the mutex. Any other thread that wants 65 to modify the value has to first check the mutex and block until it is 66 cleared.</p> 67 68 <p>If you are using <a href="http://apr.apache.org/">APR</a>, have a look 69 at the <code>apr_atomic_<var>*</var></code> functions and the 70 <code>apr_thread_mutex_<var>*</var></code> functions.</p> 71 72 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div> 73 <div class="section"> 74 <h2><a name="errno" id="errno">errno</a></h2> 75 <p>This is a common global variable that holds the error number of the 76 last error that occurred. If one thread calls a low-level function that 77 sets errno and then another thread checks it, we are bleeding error 78 numbers from one thread into another. To solve this, make sure your 79 module or library defines <code>_REENTRANT</code> or is compiled with 80 <code>-D_REENTRANT</code>. This will make errno a per-thread variable 81 and should hopefully be transparent to the code. It does this by doing 82 something like this:</p> 83 84 <div class="example"><p><code> 85 #define errno (*(__errno_location())) 86 </code></p></div> 87 88 <p>which means that accessing errno will call 89 <code>__errno_location()</code> which is provided by the libc. Setting 90 <code>_REENTRANT</code> also forces redefinition of some other functions 91 to their <code><var>*</var>_r</code> equivalents and sometimes changes 92 the common <code>getc</code>/<code>putc</code> macros into safer function 93 calls. Check your libc documentation for specifics. Instead of, or in 94 addition to <code>_REENTRANT</code> the symbols that may affect this are 95 <code>_POSIX_C_SOURCE</code>, <code>_THREAD_SAFE</code>, 96 <code>_SVID_SOURCE</code>, and <code>_BSD_SOURCE</code>.</p> 97 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div> 98 <div class="section"> 99 <h2><a name="functions" id="functions">Common standard troublesome functions</a></h2> 100 <p>Not only do things have to be thread safe, but they also have to be 101 reentrant. <code>strtok()</code> is an obvious one. You call it the first 102 time with your delimiter which it then remembers and on each subsequent 103 call it returns the next token. Obviously if multiple threads are 104 calling it you will have a problem. Most systems have a reentrant version 105 of of the function called <code>strtok_r()</code> where you pass in an 106 extra argument which contains an allocated <code>char *</code> which the 107 function will use instead of its own static storage for maintaining 108 the tokenizing state. If you are using <a href="http://apr.apache.org/">APR</a> you can use <code>apr_strtok()</code>.</p> 109 110 <p><code>crypt()</code> is another function that tends to not be reentrant, 111 so if you run across calls to that function in a library, watch out. On 112 some systems it is reentrant though, so it is not always a problem. If 113 your system has <code>crypt_r()</code> chances are you should be using 114 that, or if possible simply avoid the whole mess by using md5 instead.</p> 115 116 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div> 117 <div class="section"> 118 <h2><a name="commonlibs" id="commonlibs">Common 3rd Party Libraries</a></h2> 119 <p>The following is a list of common libraries that are used by 3rd party 120 Apache modules. You can check to see if your module is using a potentially 121 unsafe library by using tools such as <code>ldd(1)</code> and 122 <code>nm(1)</code>. For <a href="http://www.php.net/">PHP</a>, for example, 123 try this:</p> 124 125 <div class="example"><p><code> 126 % ldd libphp4.so<br /> 127 libsablot.so.0 => /usr/local/lib/libsablot.so.0 (0x401f6000)<br /> 128 libexpat.so.0 => /usr/lib/libexpat.so.0 (0x402da000)<br /> 129 libsnmp.so.0 => /usr/lib/libsnmp.so.0 (0x402f9000)<br /> 130 libpdf.so.1 => /usr/local/lib/libpdf.so.1 (0x40353000)<br /> 131 libz.so.1 => /usr/lib/libz.so.1 (0x403e2000)<br /> 132 libpng.so.2 => /usr/lib/libpng.so.2 (0x403f0000)<br /> 133 libmysqlclient.so.11 => /usr/lib/libmysqlclient.so.11 (0x40411000)<br /> 134 libming.so => /usr/lib/libming.so (0x40449000)<br /> 135 libm.so.6 => /lib/libm.so.6 (0x40487000)<br /> 136 libfreetype.so.6 => /usr/lib/libfreetype.so.6 (0x404a8000)<br /> 137 libjpeg.so.62 => /usr/lib/libjpeg.so.62 (0x404e7000)<br /> 138 libcrypt.so.1 => /lib/libcrypt.so.1 (0x40505000)<br /> 139 libssl.so.2 => /lib/libssl.so.2 (0x40532000)<br /> 140 libcrypto.so.2 => /lib/libcrypto.so.2 (0x40560000)<br /> 141 libresolv.so.2 => /lib/libresolv.so.2 (0x40624000)<br /> 142 libdl.so.2 => /lib/libdl.so.2 (0x40634000)<br /> 143 libnsl.so.1 => /lib/libnsl.so.1 (0x40637000)<br /> 144 libc.so.6 => /lib/libc.so.6 (0x4064b000)<br /> 145 /lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x80000000) 146 </code></p></div> 147 148 <p>In addition to these libraries you will need to have a look at any 149 libraries linked statically into the module. You can use <code>nm(1)</code> 150 to look for individual symbols in the module.</p> 151 </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div> 152 <div class="section"> 153 <h2><a name="liblist" id="liblist">Library List</a></h2> 154 <p>Please drop a note to <a href="http://httpd.apache.org/lists.html#http-dev">dev@httpd.apache.org</a> 155 if you have additions or corrections to this list.</p> 156 157 <table class="bordered"><tr class="header"><th>Library</th><th>Version</th><th>Thread Safe?</th><th>Notes</th></tr> 158 <tr><td><a href="http://aspell.sourceforge.net/">ASpell/PSpell</a></td> 159 <td> </td> 160 <td>?</td> 161 <td> </td></tr> 162 <tr class="odd"><td><a href="http://www.sleepycat.com/">Berkeley DB</a></td> 163 <td>3.x, 4.x</td> 164 <td>Yes</td> 165 <td>Be careful about sharing a connection across threads.</td></tr> 166 <tr><td><a href="http://sources.redhat.com/bzip2/index.html">bzip2</a></td> 167 <td> </td> 168 <td>Yes</td> 169 <td>Both low-level and high-level APIs are thread-safe. However, 170 high-level API requires thread-safe access to errno.</td></tr> 171 <tr class="odd"><td><a href="http://cr.yp.to/cdb.html">cdb</a></td> 172 <td> </td> 173 <td>?</td> 174 <td> </td></tr> 175 <tr><td><a href="http://www.washington.edu/imap/">C-Client</a></td> 176 <td> </td> 177 <td>Perhaps</td> 178 <td>c-client uses <code>strtok()</code> and 179 <code>gethostbyname()</code> which are not thread-safe on most C 180 library implementations. c-client's static data is meant to be shared 181 across threads. If <code>strtok()</code> and 182 <code>gethostbyname()</code> are thread-safe on your OS, c-client 183 <em>may</em> be thread-safe.</td></tr> 184 <tr class="odd"><td><a href="http://www.ijg.org/files/">libcrypt</a></td> 185 <td> </td> 186 <td>?</td> 187 <td> </td></tr> 188 <tr><td><a href="http://expat.sourceforge.net/">Expat</a></td> 189 <td> </td> 190 <td>Yes</td> 191 <td>Need a separate parser instance per thread</td></tr> 192 <tr class="odd"><td><a href="http://www.freetds.org/">FreeTDS</a></td> 193 <td> </td> 194 <td>?</td> 195 <td> </td></tr> 196 <tr><td><a href="http://www.freetype.org/">FreeType</a></td> 197 <td> </td> 198 <td>?</td> 199 <td> </td></tr> 200 <tr class="odd"><td><a href="http://www.boutell.com/gd/">GD 1.8.x</a></td> 201 <td> </td> 202 <td>?</td> 203 <td> </td></tr> 204 <tr><td><a href="http://www.boutell.com/gd/">GD 2.0.x</a></td> 205 <td> </td> 206 <td>?</td> 207 <td> </td></tr> 208 <tr class="odd"><td><a href="http://www.gnu.org/software/gdbm/gdbm.html">gdbm</a></td> 209 <td> </td> 210 <td>No</td> 211 <td>Errors returned via a static <code>gdbm_error</code> 212 variable</td></tr> 213 <tr><td><a href="http://www.imagemagick.org/">ImageMagick</a></td> 214 <td>5.2.2</td> 215 <td>Yes</td> 216 <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>). 217 </td></tr> 218 <tr class="odd"><td><a href="http://www.enlightenment.org/p.php?p=about/efl&l=en">Imlib2</a></td> 219 <td> </td> 220 <td>?</td> 221 <td> </td></tr> 222 <tr><td><a href="http://www.ijg.org/files/">libjpeg</a></td> 223 <td>v6b</td> 224 <td>?</td> 225 <td> </td></tr> 226 <tr class="odd"><td><a href="http://mysql.com">libmysqlclient</a></td> 227 <td> </td> 228 <td>Yes</td> 229 <td>Use mysqlclient_r library variant to ensure thread-safety. For 230 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> 231 <tr><td><a href="http://www.opaque.net/ming/">Ming</a></td> 232 <td>0.2a</td> 233 <td>?</td> 234 <td> </td></tr> 235 <tr class="odd"><td><a href="http://net-snmp.sourceforge.net/">Net-SNMP</a></td> 236 <td>5.0.x</td> 237 <td>?</td> 238 <td> </td></tr> 239 <tr><td><a href="http://www.openldap.org/">OpenLDAP</a></td> 240 <td>2.1.x</td> 241 <td>Yes</td> 242 <td>Use <code>ldap_r</code> library variant to ensure 243 thread-safety.</td></tr> 244 <tr class="odd"><td><a href="http://www.openssl.org/">OpenSSL</a></td> 245 <td>0.9.6g</td> 246 <td>Yes</td> 247 <td>Requires proper usage of <code>CRYPTO_num_locks</code>, 248 <code>CRYPTO_set_locking_callback</code>, 249 <code>CRYPTO_set_id_callback</code></td></tr> 250 <tr><td><a href="http://www.oracle.com/">liboci8 (Oracle 8+)</a></td> 251 <td>8.x,9.x</td> 252 <td>?</td> 253 <td> </td></tr> 254 <tr class="odd"><td><a href="http://pdflib.com/">pdflib</a></td> 255 <td>5.0.x</td> 256 <td>Yes</td> 257 <td>PDFLib docs claim it is thread safe; changes.txt indicates it 258 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> 259 <tr><td><a href="http://www.libpng.org/pub/png/libpng.html">libpng</a></td> 260 <td>1.0.x</td> 261 <td>?</td> 262 <td> </td></tr> 263 <tr class="odd"><td><a href="http://www.libpng.org/pub/png/libpng.html">libpng</a></td> 264 <td>1.2.x</td> 265 <td>?</td> 266 <td> </td></tr> 267 <tr><td><a href="http://www.postgresql.org/docs/8.4/static/libpq-threading.html">libpq (PostgreSQL)</a></td> 268 <td>8.x</td> 269 <td>Yes</td> 270 <td>Don't share connections across threads and watch out for 271 <code>crypt()</code> calls</td></tr> 272 <tr class="odd"><td><a href="http://www.gingerall.com/charlie/ga/xml/p_sab.xml">Sablotron</a></td> 273 <td>0.95</td> 274 <td>?</td> 275 <td /></tr> 276 <tr><td><a href="http://www.gzip.org/zlib/">zlib</a></td> 277 <td>1.1.4</td> 278 <td>Yes</td> 279 <td>Relies upon thread-safe zalloc and zfree functions Default is to 280 use libc's calloc/free which are thread-safe.</td></tr> 281 </table> 282 </div></div> 283 <div class="bottomlang"> 284 <p><span>Available Languages: </span><a href="../en/developer/thread_safety.html" title="English"> en </a></p> 285 </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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