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 193   <div class="section" id="working-with-data-on-the-server-in-an-atomic-fashion">
 194 <h1>Working with data on the server in an atomic fashion<a class="headerlink" href="#working-with-data-on-the-server-in-an-atomic-fashion" title="Permalink to this headline">¶</a></h1>
 195 <div class="section" id="synopsis">
 196 <h2>SYNOPSIS<a class="headerlink" href="#synopsis" title="Permalink to this headline">¶</a></h2>
 197 <dl class="simple">
 198 <dt>#include &lt;libmemcached/memcached.h&gt;</dt><dd><p>Compile and link with -lmemcached</p>
 199 </dd>
 200 </dl>
 201 <dl class="function">
 202 <dt id="_CPPv413memcached_casP12memcached_stPKc6size_tPKc6size_t6time_t8uint32_t8uint64_t">
 203 <span id="_CPPv313memcached_casP12memcached_stPKc6size_tPKc6size_t6time_t8uint32_t8uint64_t"></span><span id="_CPPv213memcached_casP12memcached_stPKc6size_tPKc6size_t6time_t8uint32_t8uint64_t"></span><span id="memcached_cas__memcached_stP.cCP.s.cCP.s.time_t.uint32_t.uint64_t"></span><a class="reference internal" href="memcached_return_t.html#_CPPv418memcached_return_t" title="memcached_return_t">memcached_return_t</a> <code class="sig-name descname">memcached_cas</code><span class="sig-paren">(</span><a class="reference internal" href="memcached_create.html#_CPPv412memcached_st" title="memcached_st">memcached_st</a> *<em>ptr</em>, <em class="property">const</em> char *<em>key</em>, size_t <em>key_length</em>, <em class="property">const</em> char *<em>value</em>, size_t <em>value_length</em>, time_t <em>expiration</em>, uint32_t <em>flags</em>, uint64_t <em>cas</em><span class="sig-paren">)</span><a class="headerlink" href="#_CPPv413memcached_casP12memcached_stPKc6size_tPKc6size_t6time_t8uint32_t8uint64_t" title="Permalink to this definition">¶</a><br /></dt>
 204 <dd></dd></dl>
 205
 206 <dl class="function">
 207 <dt id="_CPPv420memcached_cas_by_keyP12memcached_stPKc6size_tPKc6size_tPKc6size_t6time_t8uint32_t8uint64_t">
 208 <span id="_CPPv320memcached_cas_by_keyP12memcached_stPKc6size_tPKc6size_tPKc6size_t6time_t8uint32_t8uint64_t"></span><span id="_CPPv220memcached_cas_by_keyP12memcached_stPKc6size_tPKc6size_tPKc6size_t6time_t8uint32_t8uint64_t"></span><span id="memcached_cas_by_key__memcached_stP.cCP.s.cCP.s.cCP.s.time_t.uint32_t.uint64_t"></span><a class="reference internal" href="memcached_return_t.html#_CPPv418memcached_return_t" title="memcached_return_t">memcached_return_t</a> <code class="sig-name descname">memcached_cas_by_key</code><span class="sig-paren">(</span><a class="reference internal" href="memcached_create.html#_CPPv412memcached_st" title="memcached_st">memcached_st</a> *<em>ptr</em>, <em class="property">const</em> char *<em>group_key</em>, size_t <em>group_key_length</em>, <em class="property">const</em> char *<em>key</em>, size_t <em>key_length</em>, <em class="property">const</em> char *<em>value</em>, size_t <em>value_length</em>, time_t <em>expiration</em>, uint32_t <em>flags</em>, uint64_t <em>cas</em><span class="sig-paren">)</span><a class="headerlink" href="#_CPPv420memcached_cas_by_keyP12memcached_stPKc6size_tPKc6size_tPKc6size_t6time_t8uint32_t8uint64_t" title="Permalink to this definition">¶</a><br /></dt>
 209 <dd></dd></dl>
 210
 211 </div>
 212 <div class="section" id="description">
 213 <h2>DESCRIPTION<a class="headerlink" href="#description" title="Permalink to this headline">¶</a></h2>
 214 <p><a class="reference internal" href="#_CPPv413memcached_casP12memcached_stPKc6size_tPKc6size_t6time_t8uint32_t8uint64_t" title="memcached_cas"><code class="xref cpp cpp-any docutils literal notranslate"><span class="pre">memcached_cas()</span></code></a> overwrites data in the server as long as the <code class="docutils literal notranslate"><span class="pre">cas</span></code> value is
 215 still the same in the server. You can get the <code class="docutils literal notranslate"><span class="pre">cas</span></code> value of a result by
 216 calling <a class="reference internal" href="memcached_result_st.html#_CPPv420memcached_result_casPK19memcached_result_st" title="memcached_result_cas"><code class="xref cpp cpp-any docutils literal notranslate"><span class="pre">memcached_result_cas()</span></code></a> on a <a class="reference internal" href="memcached_result_st.html#_CPPv419memcached_result_st" title="memcached_result_st"><code class="xref cpp cpp-any docutils literal notranslate"><span class="pre">memcached_result_st</span></code></a> structure.</p>
 217 <p>At the point that this note was written cas is still buggy in memcached. Turning
 218 on tests for it in <code class="xref cpp cpp-any docutils literal notranslate"><span class="pre">libmemcached</span></code> is optional. Please see <a class="reference internal" href="memcached_set.html#_CPPv413memcached_setP12memcached_stPKc6size_tPKc6size_t6time_t8uint32_t" title="memcached_set"><code class="xref cpp cpp-any docutils literal notranslate"><span class="pre">memcached_set()</span></code></a> for
 219 information on how to do this.</p>
 220 <p><a class="reference internal" href="#_CPPv420memcached_cas_by_keyP12memcached_stPKc6size_tPKc6size_tPKc6size_t6time_t8uint32_t8uint64_t" title="memcached_cas_by_key"><code class="xref cpp cpp-any docutils literal notranslate"><span class="pre">memcached_cas_by_key()</span></code></a> method behaves in a similar way as the non key methods.
 221 The difference is that it uses the <code class="docutils literal notranslate"><span class="pre">group_key</span></code> parameter to map objects to
 222 particular servers.</p>
 223 <p><a class="reference internal" href="#_CPPv413memcached_casP12memcached_stPKc6size_tPKc6size_t6time_t8uint32_t8uint64_t" title="memcached_cas"><code class="xref cpp cpp-any docutils literal notranslate"><span class="pre">memcached_cas()</span></code></a> is tested with the <code class="xref cpp cpp-any docutils literal notranslate"><span class="pre">MEMCACHED_BEHAVIOR_USE_UDP</span></code> behavior
 224 enabled. However, when using these operations with this behavior on, there are
 225 limits to the size of the payload being sent to the server.  The reason for
 226 these limits is that the Memcached Server does not allow multi-datagram requests
 227 and the current server implementation sets a datagram size to 1400 bytes. Due to
 228 protocol overhead, the actual limit of the user supplied data is less than 1400
 229 bytes and depends on the protocol in use as, well as the operation being
 230 executed. When running with the binary protocol,
 231 <code class="xref cpp cpp-any docutils literal notranslate"><span class="pre">MEMCACHED_BEHAVIOR_BINARY_PROTOCOL</span></code>, the size of the key,value, flags and
 232 expiry combined may not exceed 1368 bytes. When running with the ASCII protocol,
 233 the exact limit fluctuates depending on which function is being executed and
 234 whether the function is a cas operation or not. For non-cas ASCII set
 235 operations, there are at least 1335 bytes available to split among the key,
 236 key_prefix, and value; for cas ASCII operations there are at least 1318 bytes
 237 available to split among the key, key_prefix and value. If the total size of the
 238 command, including overhead, exceeds 1400 bytes, a <a class="reference internal" href="memcached_return_t.html#_CPPv4N18memcached_return_t23MEMCACHED_WRITE_FAILUREE" title="MEMCACHED_WRITE_FAILURE"><code class="xref cpp cpp-any docutils literal notranslate"><span class="pre">MEMCACHED_WRITE_FAILURE</span></code></a>
 239 will be returned.</p>
 240 </div>
 241 <div class="section" id="return-value">
 242 <h2>RETURN VALUE<a class="headerlink" href="#return-value" title="Permalink to this headline">¶</a></h2>
 243 <p>All methods return a value of type <a class="reference internal" href="memcached_return_t.html#_CPPv418memcached_return_t" title="memcached_return_t"><code class="xref cpp cpp-any docutils literal notranslate"><span class="pre">memcached_return_t</span></code></a>.
 244 On success the value will be <a class="reference internal" href="memcached_return_t.html#_CPPv4N18memcached_return_t17MEMCACHED_SUCCESSE" title="MEMCACHED_SUCCESS"><code class="xref cpp cpp-any docutils literal notranslate"><span class="pre">MEMCACHED_SUCCESS</span></code></a>.
 245 Use <a class="reference internal" href="memcached_strerror.html#_CPPv418memcached_strerrorP12memcached_st18memcached_return_t" title="memcached_strerror"><code class="xref cpp cpp-any docutils literal notranslate"><span class="pre">memcached_strerror()</span></code></a> to translate this value to a printable
 246 string.</p>
 247 </div>
 248 <div class="section" id="see-also">
 249 <h2>SEE ALSO<a class="headerlink" href="#see-also" title="Permalink to this headline">¶</a></h2>
 250 <ul class="simple">
 251 <li><p><em class="manpage"><a class="manpage reference external" href="http://man7.org/linux/man-pages/man1/memcached.1.html">memcached(1)</a></em></p></li>
 252 <li><p><a class="reference internal" href="../libmemcached.html"><span class="doc">C/C++ Client Library for memcached</span></a></p></li>
 253 <li><p><a class="reference internal" href="memcached_set.html"><span class="doc">Storing data on the server</span></a></p></li>
 254 <li><p><a class="reference internal" href="memcached_append.html"><span class="doc">Appending or Prepending Data</span></a></p></li>
 255 <li><p><a class="reference internal" href="memcached_strerror.html"><span class="doc">Converting Error Codes to Messages</span></a></p></li>
 256 </ul>
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