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POPointers and Dynamic Memory
 

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  1. POPointers and Dynamic Memory
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    <p>I have a function that returns a pointer to an array. I'm running it in a loop and <code>free()</code> seems to be giving me problems. I'm not sure where, but it appears that somewhere in the main loop the memory that I'm trying to free is being used. I'm using Xcode 3.2.1 in 10.6 | Debug | x86_64 build.</p> <p>The program will run through the main loop one time; the second time it encounters the <code>free()</code> it gives me the following error:</p> <pre><code>malloc: *** error for object 0x100100180: incorrect checksum for freed object - object was probably modified after being freed. </code></pre> <p>Can someone point out (no pun intended) what I'm doing wrong with pointers here?</p> <p>Here is the program:</p> <pre><code>int main(int argc, char **argv) { int *partition; int lowerLimit; int upperLimit; // snip ... got lowerLimit and upperLimit from console arguments // this is the 'main loop': for (int i = lowerLimit; i &lt;= upperLimit; i += 2) { partition = goldbachPartition(i); printOutput(partition[0], partition[1], i); free(partition); // I get problems on the second iteration here } return 0; } int *goldbachPartition(int x) { int solved = 0; int y, z; int *primes; int *result; result = intAlloc(2); primes = atkinsPrimes(x); for (int i = intCount(primes)-1; i &gt;= 0; i--) { y = primes[i]; for (int j = 0; j &lt; y; j++) { z = primes[j]; if (z + y &gt;= x) { break; } } if (z + y == x) { solved = 1; result[0] = y; result[1] = z; break; } else if (y == z) { result[0] = 0; result[1] = 0; break; } } free(primes); return result; } int *atkinsPrimes(int limit) { int *primes; int *initialPrimes; int *filtered; int *results; int counter = 0; int sqrtLimit; int xLimit; int resultsSize; primes = intAlloc(limit+1); intFillArray(primes, limit+1, 0); sqrtLimit = floor(sqrt(limit)); xLimit = floor(sqrt((limit+1) / 2)); // these loops are part of the Atkins Sieve implementation for (int x = 1; x &lt; xLimit; x++) { int xx = x*x; for (int y = 1; y &lt; sqrtLimit; y++) { int yy = y*y; int n = 3*xx + yy; if (n &lt;= limit &amp;&amp; n % 12 == 7) { primes[n] = (primes[n] == 1) ? 0 : 1; } n += xx; if (n &lt;= limit &amp;&amp; (n % 12 == 1 || n % 12 == 5)) { primes[n] = (primes[n] == 1) ? 0 : 1; } if (x &gt; y) { n -= xx + 2*yy; if (n &lt;= limit &amp;&amp; n % 12 == 11) { primes[n] = (primes[n] == 1) ? 0 : 1; } } } } for (int n = 5; n &lt; limit; n++) { if (primes[n] == 1) { for (int k = n*n; k &lt; limit; k += n*n) { primes[k] = 0; } } } initialPrimes = intAlloc(2); if (limit &gt;= 2) { initialPrimes[counter++] = 2; } if (limit &gt;= 3) { initialPrimes[counter++] = 3; } filtered = intFilterArrayKeys(primes, limit+1); results = intMergeArrays(initialPrimes, filtered, counter, trueCount(primes, limit+1)); resultsSize = counter + trueCount(primes, limit+1); free(primes); free(initialPrimes); free(filtered); results[resultsSize] = 0; return results; } int trueCount(int *subject, int arraySize) { int count = 0; for (int i = 0; i &lt; arraySize; i++) { if (subject[i] == 1) { count++; } } return count; } int intCount(int *subject) { // warning: expects 0 terminated array. int count = 0; while (*subject++ != 0) { count++; } return count; } void intFillArray(int *subject, int arraySize, int value) { for (int i = 0; i &lt; arraySize; i++) { subject[i] = value; } } int *intFilterArrayKeys(int *subject, int arraySize) { int *filtered; int count = 0; filtered = intAlloc(trueCount(subject, arraySize)); for (int i = 0; i &lt; arraySize; i++) { if (subject[i] == 1) { filtered[count++] = i; } } return filtered; } int *intMergeArrays(int *subject1, int *subject2, int arraySize1, int arraySize2) { int *merge; int count = 0; merge = intAlloc(arraySize1 + arraySize2); for (int i = 0; i &lt; arraySize1; i++) { merge[count++] = subject1[i]; } for (int i = 0; i &lt; arraySize2; i++) { merge[count++] = subject2[i]; } return merge; } int *intAlloc(int amount) { int *ptr; ptr = (int *)malloc(amount * sizeof(int)); if (ptr == NULL) { printf("Error: NULL pointer\n"); } return ptr; } void printOutput(int num1, int num2, int rep) { if (num1 == 0) { printf("%d: No solution\n", rep); exit(0); } else { printf("%d = %d + %d\n", rep, num1, num2); } } </code></pre>
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    1. PTQuestion
    1. USRebecca Nelson
    1. USRebecca Nelson
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    1. This table or related slice is empty.
    1. This table or related slice is empty.
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      1. POPointers and Dynamic Memory
      1. USiankits
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    1. COWe would need to see atkinsPrimes() and printOutput() too, at a minimum. I removed the `xcode` tag because your IDE is not relevant to a programming logic question; that tag is (presumably) for discussing how to make the IDE do things for you a certain way.
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      1. POPointers and Dynamic Memory
      1. USKarl Knechtel
    2. COI'd like to see intAlloc() and printOutput() since those are the ones that play with the pointer you are free()'ing
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      1. POPointers and Dynamic Memory
      1. USSiegeX
    3. COOk, I've added those functions to the source code there.
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      1. POPointers and Dynamic Memory
      1. USRebecca Nelson
 

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