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| <manualpage metafile="thread_safety.xml.meta"> |
| <parentdocument href="./">Developer Documentation</parentdocument> |
| |
| <title>Apache HTTP Server 2.x Thread Safety Issues</title> |
| |
| <summary> |
| <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> |
| </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 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.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.imagemagick.com/www/changelog.html" |
| >Change log</a>). |
| </td></tr> |
| <tr><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><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><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-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><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><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> |