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POc++: Getting a segfault when initialising a queue of pointers
 

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  1. POc++: Getting a segfault when initialising a queue of pointers
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    <p>I am trying to implement the BFS algorithm described in CLRS. And have the following:</p> <pre><code>#include &lt;iostream&gt; #include &lt;list&gt; #include &lt;queue&gt; #include &lt;string&gt; #include &lt;sstream&gt; using namespace std; struct Node{ char colour; int numNbr; Node* parent; int distance; int* neighbours; int* costs; int name; Node(int _numNbr,int _name){ name = _name; colour = 'w'; parent = 0; distance = -1; neighbours = new int[_numNbr]; costs = new int[_numNbr]; numNbr = _numNbr; } }; list&lt;Node*&gt; bfs(Node** &amp;nodes,int numNodes,int startNode) { cout &lt;&lt; "performing BFS\n"; for(int i = 0; i &lt; numNodes;i++) { nodes[i]-&gt;colour = 'w'; nodes[i]-&gt;parent = 0; } cout &lt;&lt; "All nodes painted white" &lt;&lt;endl; queue&lt;Node*&gt; q; // segfault occurs here cout &lt;&lt; "initialised a queue" &lt;&lt; endl; list&lt;Node*&gt; l; cout &lt;&lt; "initialised a list" &lt;&lt; endl; nodes[startNode]-&gt;colour = 'g'; nodes[startNode]-&gt;distance = 0; q.push(nodes[startNode]); Node* u; Node* v; while(!q.empty()){ u = q.front(); for(int i = 0;i &lt; u-&gt;numNbr; i++) { v = nodes[u-&gt;neighbours[i]]; if(v-&gt;colour == 'w'){ v-&gt;colour = 'g'; v-&gt;distance = (u-&gt;distance)+1; v-&gt;parent = u; q.push(v); } } l.push_front(u); u-&gt;colour = 'b'; q.pop(); } return l; } int main(){ int nodeCount; cin &gt;&gt; nodeCount; cin.ignore(); Node** nodes = new Node*[nodeCount+1]; for(int i = 0; i &lt; nodeCount; i++){ .... // build up the nodes in the adjacency list } list&lt;Node*&gt; l = bfs(nodes,nodeCount,1); cout &lt;&lt; "BFS of nodes\n"; for(list&lt;Node*&gt;::iterator it = l.begin();it != l.end();it++){ cout &lt;&lt; (*it)-&gt;name &lt;&lt; " "; } cout &lt;&lt; endl; return 0; } </code></pre> <p>When I run this however, I only get the following output followed by a <code>segfault</code> when the queue is initialised:</p> <pre><code>jonathan@debian:~/Code/cpp/dijkstra$ ./dijkstra 3 1 2 1 3 1 2 3 1 3 1 1 performing bfs All nodes painted white Segmentation fault </code></pre> <p>I am compiling with the following command:</p> <pre><code>g++ -Wall -o dijkstra dijkstra.cpp </code></pre>
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    1. USdsolimano
    1. USJonno_FTW
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    1. COUse your debugger and do a stack trace. As per: http://stackoverflow.com/questions/3911814/c-in-g-segmentation-fault-when-not-using-pointers/3911873#3911873
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      1. POc++: Getting a segfault when initialising a queue of pointers
      1. USnsanders
    2. COIf you have a segfault creating the queue container, then most likely the heap has been corrupted. You have a comment about creating the adjacency list, but that could be where the problem is. You are allocating an array of nodes, but are you also allocating the individual nodes?
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      1. POc++: Getting a segfault when initialising a queue of pointers
      1. USVaughn Cato
    3. COA comment regarding style: using the variable name "l" (lowercase L) will only lead to confusion (because everybody will assume it's a "1" (number one)). So please, do yourself a favour and use another name.
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      1. POc++: Getting a segfault when initialising a queue of pointers
      1. USFrank Schmitt
 

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