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POWeird bug in C++ involving new and delete[] functions
 

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    <p>I'm working on the first programming homework on Coursera's <em>Algorithms: Design and Analysis</em> course ( <a href="https://www.coursera.org/course/algo" rel="nofollow">https://www.coursera.org/course/algo</a> ). It involves using merge sort to count inversions ( <a href="http://en.wikipedia.org/wiki/Inversion_(discrete_mathematics" rel="nofollow">http://en.wikipedia.org/wiki/Inversion_(discrete_mathematics</a>) ). I thought this would be a relative no-brainer because I've encountered merge sort before (in school).</p> <pre><code>#include &lt;iostream&gt; #include &lt;fstream&gt; using namespace std; int *half(int *array, int n, int start, int end) { /* * Creates a new array which contains elements from ''array'' starting with ''start'' * and ending with ''end - 1''. */ int *new_array = new int[end-start]; for(int i = start; i &lt; end; i++) { new_array[i-start] = array[i]; } return new_array; } int merge(int *array1, int n1, int *array2, int n2, int *new_array) { /* * Merges arrays 1 and 2 (with lengths n1 and n2) into a new_array, counting * ''split inversions'' by the way. */ int i = 0, j = 0, count = 0; for(int k = 0; k &lt; n1 + n2; k++) { if(i &gt;= n1) { new_array[k] = array2[j]; j++; continue; } if(j &gt;= n2) { new_array[k] = array1[i]; i++; continue; } if( array1[i] &lt;= array2[j] ) { new_array[k] = array1[i]; i++; } else { new_array[k] = array2[j]; j++; count++; } } return count; } int mergesort(int *array, int n) { if(n == 1) return 0; //base case int x, y, z; int odd; if(n%2 == 0) odd = 0; else odd = 1; int *half1 = new int [n/2]; int *half2 = new int [n/2 + odd]; half1 = half(array, n, 0, n/2); half2 = half(array, n, n/2, n); x = mergesort(half1, n/2); y = mergesort(half2, n/2 + odd); //if n is odd, we add one z = merge(half1, n/2, half2, n/2 + odd, array); //we write a sorted array back in our starting array delete [] half1; delete [] half2; return x + y + z; } int main() { int n; int *array = new int[n]; cin &gt;&gt; n; for(int i = 0; i &lt; n; i++) { int x; cin &gt;&gt; x; array[i] = x; } for(int i = 0; i &lt; n; i++) cout &lt;&lt; array[i] &lt;&lt; " "; cout &lt;&lt; endl; cout &lt;&lt; "Number of inversions: " &lt;&lt; mergesort(array, n) &lt;&lt; endl; for(int i = 0; i &lt; n; i++) cout &lt;&lt; array[i] &lt;&lt; " "; cout &lt;&lt; endl; delete[] array; return 0; } </code></pre> <p>So, what is so weird here? First thing: For me, it works perfectly for some arrays, but crashes for other arrays (examples later). Second thing: I sent code to my friend who said that everything is working fine for him, even the examples that crash dramatically for me.</p> <p>So, examples:</p> <p>For array [1 2 3 4 5 6 7] g++ produces this:</p> <pre><code>malloc.c:2451: sYSMALLOc: Assertion `(old_top == (((mbinptr) (((char *) &amp;((av)-&gt;bins[((1) - 1) * 2])) - __builtin_offsetof (struct malloc_chunk, fd)))) &amp;&amp; old_size == 0) || ((unsigned long) (old_size) &gt;= (unsigned long)((((__builtin_offsetof (struct malloc_chunk, fd_nextsize))+((2 * (sizeof(size_t))) - 1)) &amp; ~((2 * (sizeof(size_t))) - 1))) &amp;&amp; ((old_top)-&gt;size &amp; 0x1) &amp;&amp; ((unsigned long)old_end &amp; pagemask) == 0)' failed. Aborted (core dumped) </code></pre> <p>When I ''backtrack'' it using gdb:</p> <pre><code>#0 0x00007ffff7753445 in __GI_raise (sig=&lt;optimized out&gt;) at ../nptl/sysdeps/unix/sysv/linux/raise.c:64 #1 0x00007ffff7756bab in __GI_abort () at abort.c:91 #2 0x00007ffff779abed in __malloc_assert (assertion=&lt;optimized out&gt;, file=&lt;optimized out&gt;, line=&lt;optimized out&gt;, function=&lt;optimized out&gt;) at malloc.c:300 #3 0x00007ffff779e0f4 in sYSMALLOc (av=0x7ffff7ad3720, nb=32) at malloc.c:2448 #4 _int_malloc (av=0x7ffff7ad3720, bytes=12) at malloc.c:3892 #5 0x00007ffff779fa45 in __GI___libc_malloc (bytes=12) at malloc.c:2924 #6 0x00007ffff7b8fded in operator new(unsigned long) () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6 #7 0x00007ffff7b8ff09 in operator new[](unsigned long) () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6 #8 0x0000000000400b12 in mergesort (array=0x603010, n=7) at jedan.cpp:81 #9 0x0000000000400cfe in main () at jedan.cpp:120 </code></pre> <p>It does something similar (but not <strong>the same</strong>!) for the array [1 2 3 4 5 6 7 8 9 10], again connected to the new and delete[] functions. I can post it here later if somebody thinks that would be helpful, but I don't want to bloat this post too much. And it works for most of the arrays I tried (I had no problems with arrays of size &lt;= 6, and for a pretty big number of bigger arrays).</p> <p>I'm using Ubuntu 12.04, installed yesterday... pretty clean and fresh. Help?</p> <p>P.S. If you find variable names a little weird, sorry... I translated them from my native language so the code could be more readable.</p>
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    1. COuse valgrind ...
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    2. CO`new[]` and `delete[]` are operators FYI.
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    3. COAgree with bmargulies; this sounds like you need a memory debugger like Valgrind.
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