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From n.@apache.org
Subject cvs commit: httpd-2.0/docs/manual/developer thread_safety.html.en thread_safety.xml thread_safety.html
Date Sat, 05 Apr 2003 13:23:34 GMT
nd          2003/04/05 05:23:34

  Added:       docs/manual/developer thread_safety.html.en
                        thread_safety.xml
  Removed:     docs/manual/developer thread_safety.html
  Log:
  new XML
  
  Revision  Changes    Path
  1.1                  httpd-2.0/docs/manual/developer/thread_safety.html.en
  
  Index: thread_safety.html.en
  ===================================================================
  <?xml version="1.0" encoding="ISO-8859-1"?>
  <!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><!--
          XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
                This file is generated from xml source: DO NOT EDIT
          XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
        -->
  <title>Apache 2.0 Thread Safety Issues - Apache HTTP Server</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 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="../faq/">FAQ</a> | <a href="../glossary.html">Glossary</a>
| <a href="../sitemap.html">Sitemap</a></p>
  <p class="apache">Apache HTTP Server Version 2.1</p>
  <img alt="" src="../images/feather.gif" /></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-project/">Documentation</a>
&gt; <a href="../">Version 2.1</a></div><div id="page-content"><div
id="preamble"><h1>Apache 2.0 Thread Safety Issues</h1>
      <p>When using any of the threaded mpms in Apache 2.0 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 conditons that
      may only show up in certain conditions under heavy load.</p>
  </div>
  <div id="quickview"><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></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 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.fastio.com/">cpdflib</a></td>
          <td> </td>
          <td>?</td>
          <td> </td></tr>
  <tr><td><a href="http://www.ijg.org/files/">libcrypt</a></td>
          <td> </td>
          <td>?</td>
          <td> </td></tr>
  <tr class="odd"><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><td><a href="http://www.freetds.org/">FreeTDS</a></td>
          <td> </td>
          <td>?</td>
          <td> </td></tr>
  <tr class="odd"><td><a href="http://www.freetype.org/">FreeType</a></td>
          <td> </td>
          <td>?</td>
          <td> </td></tr>
  <tr><td><a href="http://www.boutell.com/gd/">GD 1.8.x</a></td>
          <td> </td>
          <td>?</td>
          <td> </td></tr>
  <tr class="odd"><td><a href="http://www.boutell.com/gd/">GD 2.0.x</a></td>
          <td> </td>
          <td>?</td>
          <td> </td></tr>
  <tr><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 class="odd"><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.cise.ufl.edu/depot/www/ImageMagick/www/Changelog.html">Change log</a>).
          </td></tr>
  <tr><td><a href="http://www.enlightenment.org/pages/imlib2.html">Imlib2</a></td>
          <td> </td>
          <td>?</td>
          <td> </td></tr>
  <tr class="odd"><td><a href="http://www.ijg.org/files/">libjpeg</a></td>
          <td>v6b</td>
          <td>?</td>
          <td> </td></tr>
  <tr><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://www.mysql.com/doc/en/Threaded_clients.html">http://www.mysql.com/doc/en/Threaded_clients.html</a>.</td></tr>
  <tr class="odd"><td><a href="http://www.opaque.net/ming/">Ming</a></td>
          <td>0.2a</td>
          <td>?</td>
          <td> </td></tr>
  <tr><td><a href="http://net-snmp.sourceforge.net/">Net-SNMP</a></td>
          <td>5.0.x</td>
          <td>?</td>
          <td> </td></tr>
  <tr class="odd"><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><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 class="odd"><td><a href="http://www.oracle.com/">liboci8 (Oracle 8+)</a></td>
          <td>8.x,9.x</td>
          <td>?</td>
          <td> </td></tr>
  <tr><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/index.html">http://www.pdflib.com/products/pdflib/index.html</a>.</td></tr>
  <tr class="odd"><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><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 class="odd"><td><a href="http://www.postgresql.org/idocs/index.php?libpq-threading.html">libpq
(PostgreSQL)</a></td>
          <td>7.x</td>
          <td>Yes</td>
          <td>Don't share connections across threads and watch out for
          <code>crypt()</code> calls</td></tr>
  <tr><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 class="odd"><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 id="footer">
  <p class="apache">Maintained by the <a href="http://httpd.apache.org/docs-project/">Apache
HTTP Server Documentation Project</a></p>
  <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></div>
  </body></html>
  
  
  1.1                  httpd-2.0/docs/manual/developer/thread_safety.xml
  
  Index: thread_safety.xml
  ===================================================================
  <?xml version="1.0" encoding="UTF-8" ?>
  <!DOCTYPE manualpage SYSTEM "../style/manualpage.dtd">
  <?xml-stylesheet type="text/xsl" href="../style/manual.en.xsl"?>
  
  <manualpage>
  <relativepath href=".."/>
  
  <title>Apache 2.0 Thread Safety Issues</title>
  
  <summary>
      <p>When using any of the threaded mpms in Apache 2.0 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 conditons that
      may only show up in certain conditions under heavy load.</p>
  </summary>
  
  <section id="variables"><title>Global and static variables</title>
      <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>
      <!-- [would probably be a good idea to add an example here] -->
  </section>
  
  <section id="errno"><title>errno</title>
      <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>
  
      <example>
        #define errno (*(__errno_location()))
      </example>
  
      <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>
  </section>
  
  <section id="functions"><title>Common standard troublesome functions</title>
      <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 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>
      <!-- [I don't see an apr_crypt() function.] -->
  </section>
  
  <section id="commonlibs"><title>Common 3rd Party Libraries</title>
      <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>
  
      <example>
        % 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)
      </example>
  
      <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>
  </section>
  
  <section id="liblist"><title>Library List</title>
      <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 style="zebra" border="1">
      <tr><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><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><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><td><a href="http://www.fastio.com/">cpdflib</a></td>
          <td> </td>
          <td>?</td>
          <td> </td></tr>
      <tr><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><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><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><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.cise.ufl.edu/depot/www/ImageMagick/www/Changelog.html"
          >Change log</a>).
          </td></tr>
      <tr><td><a href="http://www.enlightenment.org/pages/imlib2.html"
          >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><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://www.mysql.com/doc/en/Threaded_clients.html"
              >http://www.mysql.com/doc/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><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><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><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/index.html"
              >http://www.pdflib.com/products/pdflib/index.html</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><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/idocs/index.php?libpq-threading.html"
          >libpq (PostgreSQL)</a></td>
          <td>7.x</td>
          <td>Yes</td>
          <td>Don't share connections across threads and watch out for
          <code>crypt()</code> calls</td></tr>
      <tr><td><a href="http://www.gingerall.com/charlie/ga/xml/p_sab.xml"
          >Sablotron</a></td>
          <td>0.95</td>
          <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>
  </section>
  </manualpage>
  
  
  
  

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