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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<title>Clingo C API: backend.c</title>
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<div id="projectname">Clingo C API
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<div id="projectbrief">C API for clingo providing high level functions to control grounding and solving.</div>
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<div class="title">backend.c</div> </div>
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<p>The example shows how to used the backend to extend a grounded program.</p><h2>Output</h2>
<div class="fragment"><div class="line">./backend 0</div><div class="line">Model: a b</div><div class="line">Model: a b c</div><div class="line">Model:</div><div class="line">Model: a</div><div class="line">Model: b</div></div><!-- fragment --><h2>Code</h2>
<div class="fragment"><div class="line"><span class="preprocessor">#include <<a class="code" href="clingo_8h.html">clingo.h</a>></span></div><div class="line"><span class="preprocessor">#include <stdlib.h></span></div><div class="line"><span class="preprocessor">#include <stdio.h></span></div><div class="line"><span class="preprocessor">#include <assert.h></span></div><div class="line"></div><div class="line"><span class="keywordtype">bool</span> on_model(<a class="code" href="group__Model.html#gaaf9a93819f023f3cb8aa80598c46556b">clingo_model_t</a> *model, <span class="keywordtype">void</span> *data, <span class="keywordtype">bool</span> *goon) {</div><div class="line"> (void)data;</div><div class="line"> <span class="keywordtype">bool</span> ret = <span class="keyword">true</span>;</div><div class="line"> <a class="code" href="group__Symbols.html#ga6c75c60fa57c3b97505265ff08f6f951">clingo_symbol_t</a> *atoms = NULL;</div><div class="line"> <span class="keywordtype">size_t</span> atoms_n;</div><div class="line"> <a class="code" href="group__Symbols.html#ga6c75c60fa57c3b97505265ff08f6f951">clingo_symbol_t</a> <span class="keyword">const</span> *it, *ie;</div><div class="line"> <span class="keywordtype">char</span> *str = NULL;</div><div class="line"> <span class="keywordtype">size_t</span> str_n = 0;</div><div class="line"></div><div class="line"> <span class="comment">// determine the number of (shown) symbols in the model</span></div><div class="line"> <span class="keywordflow">if</span> (!<a name="a0"></a><a class="code" href="group__Model.html#ga6fd012a9b6deb0d743701ceba35b65ec">clingo_model_symbols_size</a>(model, <a name="a1"></a><a class="code" href="group__Model.html#ggaa943fe81f0874ea63820b627bc19845ca82a698ef41cd919cb370d22fc0b44826">clingo_show_type_shown</a>, &atoms_n)) { <span class="keywordflow">goto</span> error; }</div><div class="line"></div><div class="line"> <span class="comment">// allocate required memory to hold all the symbols</span></div><div class="line"> <span class="keywordflow">if</span> (!(atoms = (<a class="code" href="group__Symbols.html#ga6c75c60fa57c3b97505265ff08f6f951">clingo_symbol_t</a>*)malloc(<span class="keyword">sizeof</span>(*atoms) * atoms_n))) {</div><div class="line"> <a name="a2"></a><a class="code" href="group__BasicTypes.html#ga861bc86ccee813d744920ac0de35ee11">clingo_set_error</a>(<a name="a3"></a><a class="code" href="group__BasicTypes.html#gga5adba1f5e366e7489ac4a3fb5266dba6aa14a0926eb3e653fcc13299b33d8d348">clingo_error_bad_alloc</a>, <span class="stringliteral">"could not allocate memory for atoms"</span>);</div><div class="line"> <span class="keywordflow">goto</span> error;</div><div class="line"> }</div><div class="line"></div><div class="line"> <span class="comment">// retrieve the symbols in the model</span></div><div class="line"> <span class="keywordflow">if</span> (!<a name="a4"></a><a class="code" href="group__Model.html#gaa32137ad8fa747f5fa5dae3345ddf982">clingo_model_symbols</a>(model, <a class="code" href="group__Model.html#ggaa943fe81f0874ea63820b627bc19845ca82a698ef41cd919cb370d22fc0b44826">clingo_show_type_shown</a>, atoms, atoms_n)) { <span class="keywordflow">goto</span> error; }</div><div class="line"></div><div class="line"> printf(<span class="stringliteral">"Model:"</span>);</div><div class="line"></div><div class="line"> <span class="keywordflow">for</span> (it = atoms, ie = atoms + atoms_n; it != ie; ++it) {</div><div class="line"> <span class="keywordtype">size_t</span> n;</div><div class="line"> <span class="keywordtype">char</span> *str_new;</div><div class="line"></div><div class="line"> <span class="comment">// determine size of the string representation of the next symbol in the model</span></div><div class="line"> <span class="keywordflow">if</span> (!<a name="a5"></a><a class="code" href="group__Symbols.html#gafba05ee305c698c687390ed2ee4f1562">clingo_symbol_to_string_size</a>(*it, &n)) { <span class="keywordflow">goto</span> error; }</div><div class="line"></div><div class="line"> <span class="keywordflow">if</span> (str_n < n) {</div><div class="line"> <span class="comment">// allocate required memory to hold the symbol's string</span></div><div class="line"> <span class="keywordflow">if</span> (!(str_new = (<span class="keywordtype">char</span>*)realloc(str, <span class="keyword">sizeof</span>(*str) * n))) {</div><div class="line"> <a class="code" href="group__BasicTypes.html#ga861bc86ccee813d744920ac0de35ee11">clingo_set_error</a>(<a class="code" href="group__BasicTypes.html#gga5adba1f5e366e7489ac4a3fb5266dba6aa14a0926eb3e653fcc13299b33d8d348">clingo_error_bad_alloc</a>, <span class="stringliteral">"could not allocate memory for symbol's string"</span>);</div><div class="line"> <span class="keywordflow">goto</span> error;</div><div class="line"> }</div><div class="line"></div><div class="line"> str = str_new;</div><div class="line"> str_n = n;</div><div class="line"> }</div><div class="line"></div><div class="line"> <span class="comment">// retrieve the symbol's string</span></div><div class="line"> <span class="keywordflow">if</span> (!<a name="a6"></a><a class="code" href="group__Symbols.html#ga7990b41b229320ac158fc428fa366000">clingo_symbol_to_string</a>(*it, str, n)) { <span class="keywordflow">goto</span> error; }</div><div class="line"></div><div class="line"> printf(<span class="stringliteral">" %s"</span>, str);</div><div class="line"> }</div><div class="line"></div><div class="line"> printf(<span class="stringliteral">"\n"</span>);</div><div class="line"> *goon = <span class="keyword">true</span>;</div><div class="line"> <span class="keywordflow">goto</span> out;</div><div class="line"></div><div class="line">error:</div><div class="line"> ret = <span class="keyword">false</span>;</div><div class="line"></div><div class="line">out:</div><div class="line"> <span class="keywordflow">if</span> (atoms) { free(atoms); }</div><div class="line"> <span class="keywordflow">if</span> (str) { free(str); }</div><div class="line"></div><div class="line"> <span class="keywordflow">return</span> ret;</div><div class="line">}</div><div class="line"></div><div class="line"><span class="keywordtype">int</span> main(<span class="keywordtype">int</span> argc, <span class="keywordtype">char</span> <span class="keyword">const</span> **argv) {</div><div class="line"> <span class="keywordtype">char</span> <span class="keyword">const</span> *error_message;</div><div class="line"> <span class="keywordtype">int</span> ret = 0;</div><div class="line"> <span class="keywordtype">size_t</span> offset;</div><div class="line"> <a class="code" href="group__Control.html#gae917a23b0591d181004ec88c4e3291c1">clingo_solve_result_bitset_t</a> solve_ret;</div><div class="line"> <a class="code" href="group__Control.html#gaf008e9db9dbb37b0b7ef039bb9d582f0">clingo_control_t</a> *ctl;</div><div class="line"> <a class="code" href="group__SymbolicAtoms.html#ga48f4026c13a49553efce213e76ab32aa">clingo_symbolic_atoms_t</a> *atoms;</div><div class="line"> <a class="code" href="group__ProgramBuilder.html#ga5e9458723e4631599ada46f8748ca577">clingo_backend_t</a> *backend;</div><div class="line"> <a class="code" href="group__BasicTypes.html#ga6480ac77dd156ea3c4bda71f161166c5">clingo_atom_t</a> atom_ids[4];</div><div class="line"> <span class="keywordtype">char</span> <span class="keyword">const</span> *atom_strings[] = {<span class="stringliteral">"a"</span>, <span class="stringliteral">"b"</span>, <span class="stringliteral">"c"</span>};</div><div class="line"> <a class="code" href="group__BasicTypes.html#gaa95dd19334e536397bbad174c8fa4ff8">clingo_literal_t</a> body[2];</div><div class="line"> <a name="_a7"></a><a class="code" href="structclingo__part.html">clingo_part_t</a> parts[] = {{ <span class="stringliteral">"base"</span>, NULL, 0 }};</div><div class="line"></div><div class="line"> <span class="comment">// create a control object and pass command line arguments</span></div><div class="line"> <span class="keywordflow">if</span> (!<a name="a8"></a><a class="code" href="group__Control.html#ga46254f92ddf864a5214cf1572402e2bc">clingo_control_new</a>(argv+1, argc-1, NULL, NULL, 20, &ctl) != 0) { <span class="keywordflow">goto</span> error; }</div><div class="line"></div><div class="line"> <span class="comment">// add a logic program to the base part</span></div><div class="line"> <span class="keywordflow">if</span> (!<a name="a9"></a><a class="code" href="group__Control.html#gab65cc073ff9dbf71e53b71fb14bc9ec7">clingo_control_add</a>(ctl, <span class="stringliteral">"base"</span>, NULL, 0, <span class="stringliteral">"{a; b; c}."</span>)) { <span class="keywordflow">goto</span> error; }</div><div class="line"></div><div class="line"> <span class="comment">// ground the base part</span></div><div class="line"> <span class="keywordflow">if</span> (!<a name="a10"></a><a class="code" href="group__Control.html#gad0d95c5766649ff2f00e4ee3507a679f">clingo_control_ground</a>(ctl, parts, 1, NULL, NULL)) { <span class="keywordflow">goto</span> error; }</div><div class="line"></div><div class="line"> <span class="comment">// get the container for symbolic atoms</span></div><div class="line"> <span class="keywordflow">if</span> (!<a name="a11"></a><a class="code" href="group__Control.html#gac1c7e54de01e03c4595446d6fe5632cc">clingo_control_symbolic_atoms</a>(ctl, &atoms)) { <span class="keywordflow">goto</span> error; }</div><div class="line"> <span class="comment">// get the ids of atoms a, b, and c</span></div><div class="line"> offset = 0;</div><div class="line"> <span class="keywordflow">for</span> (<span class="keywordtype">char</span> <span class="keyword">const</span> **it = atom_strings, **ie = it + <span class="keyword">sizeof</span>(atom_strings) / <span class="keyword">sizeof</span>(*atom_strings); it != ie; ++it) {</div><div class="line"> <a class="code" href="group__Symbols.html#ga6c75c60fa57c3b97505265ff08f6f951">clingo_symbol_t</a> sym;</div><div class="line"> <a class="code" href="group__SymbolicAtoms.html#ga0a773e6f798cb2395d0cdfab5b277ca2">clingo_symbolic_atom_iterator_t</a> atom_it, atom_ie;</div><div class="line"> <a class="code" href="group__BasicTypes.html#gaa95dd19334e536397bbad174c8fa4ff8">clingo_literal_t</a> lit;</div><div class="line"> <span class="keywordtype">bool</span> equal;</div><div class="line"></div><div class="line"> <span class="comment">// lookup the atom</span></div><div class="line"> <span class="keywordflow">if</span> (!<a name="a12"></a><a class="code" href="group__Symbols.html#ga17bb0c81e3d6e0bf1422dd969b0b9b9b">clingo_symbol_create_id</a>(*it, <span class="keyword">true</span>, &sym)) { <span class="keywordflow">goto</span> error; }</div><div class="line"> <span class="keywordflow">if</span> (!<a name="a13"></a><a class="code" href="group__SymbolicAtoms.html#ga2aaaf8cbd79db44ee7e165b0ef609b8d">clingo_symbolic_atoms_find</a>(atoms, sym, &atom_it)) { <span class="keywordflow">goto</span> error; }</div><div class="line"> <span class="keywordflow">if</span> (!<a name="a14"></a><a class="code" href="group__SymbolicAtoms.html#ga327968989c722c6df96b014d01f97edb">clingo_symbolic_atoms_end</a>(atoms, &atom_ie)) { <span class="keywordflow">goto</span> error; }</div><div class="line"> <span class="keywordflow">if</span> (!<a name="a15"></a><a class="code" href="group__SymbolicAtoms.html#ga1790be46a59ee51cea04055ebfcfc412">clingo_symbolic_atoms_iterator_is_equal_to</a>(atoms, atom_it, atom_ie, &equal)) { <span class="keywordflow">goto</span> error; }</div><div class="line"> assert(!equal); (void)equal;</div><div class="line"></div><div class="line"> <span class="comment">// get the atom's id</span></div><div class="line"> <span class="keywordflow">if</span> (!<a name="a16"></a><a class="code" href="group__SymbolicAtoms.html#gad848a8e3019450374adf396eb1298fec">clingo_symbolic_atoms_literal</a>(atoms, atom_it, &lit)) { <span class="keywordflow">goto</span> error; }</div><div class="line"> atom_ids[offset++] = lit;</div><div class="line"> }</div><div class="line"></div><div class="line"> <span class="comment">// get the backend</span></div><div class="line"> <span class="keywordflow">if</span> (!<a name="a17"></a><a class="code" href="group__Control.html#ga001eca3c6f6a0542cf04597d5e4ec58a">clingo_control_backend</a>(ctl, &backend)) { <span class="keywordflow">goto</span> error; }</div><div class="line"></div><div class="line"> <span class="comment">// add an additional atom (called d below)</span></div><div class="line"> <span class="keywordflow">if</span> (!<a name="a18"></a><a class="code" href="group__ProgramBuilder.html#gaa5ccab894645f7cf14a525d63efa6f86">clingo_backend_add_atom</a>(backend, &atom_ids[3])) { <span class="keywordflow">goto</span> error; }</div><div class="line"></div><div class="line"> <span class="comment">// add rule: d :- a, b.</span></div><div class="line"> body[0] = atom_ids[0];</div><div class="line"> body[1] = atom_ids[1];</div><div class="line"> <span class="keywordflow">if</span> (!<a name="a19"></a><a class="code" href="group__ProgramBuilder.html#ga4a6416ec7f95e40f441c1d110a96ea23">clingo_backend_rule</a>(backend, <span class="keyword">false</span>, &atom_ids[3], 1, body, <span class="keyword">sizeof</span>(body)/<span class="keyword">sizeof</span>(*body))) { <span class="keywordflow">goto</span> error; }</div><div class="line"></div><div class="line"> <span class="comment">// add rule: :- not d, c.</span></div><div class="line"> body[0] = -(<a name="a20"></a><a class="code" href="group__BasicTypes.html#gaa95dd19334e536397bbad174c8fa4ff8">clingo_literal_t</a>)atom_ids[3];</div><div class="line"> body[1] = atom_ids[2];</div><div class="line"> <span class="keywordflow">if</span> (!<a class="code" href="group__ProgramBuilder.html#ga4a6416ec7f95e40f441c1d110a96ea23">clingo_backend_rule</a>(backend, <span class="keyword">false</span>, NULL, 0, body, <span class="keyword">sizeof</span>(body)/<span class="keyword">sizeof</span>(*body))) { <span class="keywordflow">goto</span> error; }</div><div class="line"></div><div class="line"> <span class="comment">// solve using a model callback</span></div><div class="line"> <span class="keywordflow">if</span> (!<a name="a21"></a><a class="code" href="group__Control.html#ga32ecc4f42257562dde343621597a596d">clingo_control_solve</a>(ctl, on_model, NULL, NULL, 0, &solve_ret)) { <span class="keywordflow">goto</span> error; }</div><div class="line"></div><div class="line"> <span class="keywordflow">goto</span> out;</div><div class="line"></div><div class="line">error:</div><div class="line"> <span class="keywordflow">if</span> (!(error_message = <a name="a22"></a><a class="code" href="group__BasicTypes.html#ga9732caf39396e15e7a2eb0e01d1a71b0">clingo_error_message</a>())) { error_message = <span class="stringliteral">"error"</span>; }</div><div class="line"></div><div class="line"> printf(<span class="stringliteral">"%s\n"</span>, error_message);</div><div class="line"> ret = <a name="a23"></a><a class="code" href="group__BasicTypes.html#ga94ce73fd010c0dbad708901a472f2c7a">clingo_error_code</a>();</div><div class="line"></div><div class="line">out:</div><div class="line"> <span class="keywordflow">if</span> (ctl) { <a name="a24"></a><a class="code" href="group__Control.html#gad1c4e8f6be76e1d3d8e34d71363dd067">clingo_control_free</a>(ctl); }</div><div class="line"></div><div class="line"> <span class="keywordflow">return</span> ret;</div><div class="line">}</div><div class="line"></div></div><!-- fragment --> </div><!-- contents -->
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