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| <h1>Thread Safety</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> |
| |
| <h2>Global and static variables</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. |
| 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 APR, have a look at the apr_atomic_* functions and the apr_thread_mutex_* |
| functions. [would probably be a good idea to add an example here]</p> |
| |
| <h2>errno</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 _REENTRANT or is compiled with -D_REENTRANT. 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> |
| |
| <pre>#define errno (*(__errno_location()))</pre> |
| |
| <p>which means that accessing errno will call __errno_location() which is provided by the libc. Setting |
| _REENTRANT also forces redefinition of some other functions to their *_r equivalents and sometimes |
| changes the common getc/putc macros into safer function calls. Check your libc documentation for |
| specifics. Instead of, or in addition to _REENTRANT the symbols that may affect this are |
| _POSIX_C_SOURCE, _THREAD_SAFE, _SVID_SOURCE, and _BSD_SOURCE.</p> |
| |
| <h2>Common standard troublesome functions</h2> |
| |
| <p>Not only do things have to be thread safe, but they also have to be reentrant. |
| <b>strtok()</b> 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 <b>strtok_r()</b> where you pass in an extra argument which contains |
| an allocated char * which the function will use instead of its own static storage for maintaining |
| the tokenizing state. If you are using APR you can use <b>apr_strtok()</b>.</p> |
| |
| <p><b>crypt()</b> 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 <b>crypt_r()</b> chances are you should be using that, or |
| if possible simply avoid the whole mess by using md5 instead. [I don't see an apr_crypt() function.]</p> |
| |
| |
| <h1>Common 3rd Party Libraries</h1> |
| <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 <tt>ldd</tt> and <tt>nm</tt>. For |
| PHP, for example, try this:</p> |
| <pre>% ldd libphp4.so |
| libsablot.so.0 => /usr/local/lib/libsablot.so.0 (0x401f6000) |
| libexpat.so.0 => /usr/lib/libexpat.so.0 (0x402da000) |
| libsnmp.so.0 => /usr/lib/libsnmp.so.0 (0x402f9000) |
| libpdf.so.1 => /usr/local/lib/libpdf.so.1 (0x40353000) |
| libz.so.1 => /usr/lib/libz.so.1 (0x403e2000) |
| libpng.so.2 => /usr/lib/libpng.so.2 (0x403f0000) |
| libmysqlclient.so.11 => /usr/lib/libmysqlclient.so.11 (0x40411000) |
| libming.so => /usr/lib/libming.so (0x40449000) |
| libm.so.6 => /lib/libm.so.6 (0x40487000) |
| libfreetype.so.6 => /usr/lib/libfreetype.so.6 (0x404a8000) |
| libjpeg.so.62 => /usr/lib/libjpeg.so.62 (0x404e7000) |
| libcrypt.so.1 => /lib/libcrypt.so.1 (0x40505000) |
| libssl.so.2 => /lib/libssl.so.2 (0x40532000) |
| libcrypto.so.2 => /lib/libcrypto.so.2 (0x40560000) |
| libresolv.so.2 => /lib/libresolv.so.2 (0x40624000) |
| libdl.so.2 => /lib/libdl.so.2 (0x40634000) |
| libnsl.so.1 => /lib/libnsl.so.1 (0x40637000) |
| libc.so.6 => /lib/libc.so.6 (0x4064b000) |
| /lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x80000000)</pre> |
| <p>In addition to these libraries you will need to have a look at any libraries |
| linked statically into the module. You can use <tt>nm</tt> to look for |
| individual symbols in the module.</p> |
| |
| <h2>Library List</h2> |
| <p>Please drop a note to dev@httpd.apache.org if you have additions or |
| corrections to this list.</p> |
| <table> |
| <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 strtok() and gethostbyname() which are not thread-safe on most C library implementations. c-client's static data is meant to be shared across threads. If strtok() and gethostbyname() are thread-safe on your OS, c-client <i>may</i> 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 gdbm_error variable</td> |
| </tr> |
| <tr> |
| <td><a href="http://www.imagemagick.org/">ImageMagick</a></td> |
| <td> </td> |
| <td>?</td> |
| <td> </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 ldap_r library variant to ensure thread-safety.</td> |
| </tr> |
| <tr> |
| <td><a href="http://www.openssl.org/">OpenSSL</a></td> |
| <td>0.96g</td> |
| <td>Yes</td> |
| <td>Requires proper usage of <i>CRYPTO_num_locks</i>, <i>CRYPTO_set_locking_callback</i>, <i>CRYPTO_set_id_callback</i></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>4.0.x</td> |
| <td>?</td> |
| <td> </td> |
| </tr> |
| <tr> |
| <td><a href="http://www.libpng.org/pub/png/libpng.html">libpng</a></td> |
| <td>1.0.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 crypt() 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> |
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