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POStrange (Undefined?) Behavior of Free in C
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copied!<p>This is really strange... and I can't debug it (tried for about two hours, debugger starts going haywire after a while...). Anyway, I'm trying to do something really simple:</p> <p><code>Free an array of strings</code>. The array is in the form:</p> <p><code>char **myStrings</code>. The array elements are initialized as:</p> <pre><code>myString[index] = malloc(strlen(word)); myString[index] = word; </code></pre> <p>and I'm calling a function like this:</p> <p><code>free_memory(myStrings, size);</code> where size is the length of the array (I know this is not the problem, I tested it extensively and everything except this function is working).</p> <p><code>free_memory</code> looks like this:</p> <pre><code>void free_memory(char **list, int size) { for (int i = 0; i < size; i ++) { free(list[i]); } free(list); } </code></pre> <p>Now here comes the weird part. <code>if (size> strlen(list[i]))</code> then the program crashes. For example, imagine that I have a list of strings that looks something like this:</p> <pre><code>myStrings[0] = "Some"; myStrings[1] = "random"; myStrings[2] = "strings"; </code></pre> <p>And thus the length of this array is <code>3</code>.</p> <p>If I pass this to my <code>free_memory</code> function, <code>strlen(myStrings[0]) > 3</code> (<strong>4 > 3</strong>), and the program crashes.</p> <p>However, if I change <code>myStrings[0]</code> to be <code>"So"</code> instead, then <code>strlen(myStrings[0]) < 3</code> (<strong>2 < 3</strong>) and the program does <em>not</em> crash.</p> <p>So it seems to me that <code>free(list[i])</code> is actually going through the <code>char[]</code> that is at that location and trying to free each character, which I <em>imagine</em> is <strong>undefined behavior</strong>.</p> <p>The only reason I say this is because I can play around with the size of the first element of <code>myStrings</code> and make the program crash whenever I feel like it, so I'm assuming that this is the problem area.</p> <p><strong>Note</strong>: I <em>did</em> try to debug this by stepping through the function that calls <code>free_memory</code>, noting any weird values and such, but the moment I step into the <code>free_memory</code> function, the debugger crashes, so I'm not really sure what is going on. Nothing is out of the ordinary until I enter the function, then the world explodes.</p> <p><strong>Another note</strong>: I also posted the shortened version of the source for this program (not too long; Pastebin) <a href="http://pastebin.com/vFr2tZDi" rel="nofollow">here</a>. I am compiling on MinGW with the c99 flag on.</p> <p><strong>PS</strong> - I just thought of this. I am indeed passing <code>numUniqueWords</code> to the free function, and I know that this does not actually free the entire piece of memory that I allocated. I've called it both ways, that's not the issue. And I left it how I did because that is the way that I will be calling it after I get it to work in the first place, I need to revise some of my logic in that function.</p> <p><strong>Source, as per request (on-site)</strong>:</p> <pre><code>#include <stdio.h> #include <string.h> #include <ctype.h> #include <stdlib.h> #include "words.h" int getNumUniqueWords(char text[], int size); int main(int argc, char* argv[]) { setvbuf(stdout, NULL, 4, _IONBF); // For Eclipse... stupid bug. --> does NOT affect the program, just the output to console! int nbr_words; char text[] = "Some - \"text, a stdin\". We'll have! also repeat? We'll also have a repeat!"; int length = sizeof(text); nbr_words = getNumUniqueWords(text, length); return 0; } void free_memory(char **list, int size) { for (int i = 0; i < size; i ++) { // You can see that printing the values is fine, as long as free is not called. // When free is called, the program will crash if (size > strlen(list[i])) //printf("Wanna free value %d w/len of %d: %s\n", i, strlen(list[i]), list[i]); free(list[i]); } free(list); } int getNumUniqueWords(char text[], int length) { int numTotalWords = 0; char *word; printf("Length: %d characters\n", length); char totalWords[length]; strcpy(totalWords, text); word = strtok(totalWords, " ,.-!?()\"0123456789"); while (word != NULL) { numTotalWords ++; printf("%s\n", word); word = strtok(NULL, " ,.-!?()\"0123456789"); } printf("Looks like we counted %d total words\n\n", numTotalWords); char *uniqueWords[numTotalWords]; char *tempWord; int wordAlreadyExists = 0; int numUniqueWords = 0; char totalWordsCopy[length]; strcpy(totalWordsCopy, text); for (int i = 0; i < numTotalWords; i++) { uniqueWords[i] = NULL; } // Tokenize until all the text is consumed. word = strtok(totalWordsCopy, " ,.-!?()\"0123456789"); while (word != NULL) { // Look through the word list for the current token. for (int j = 0; j < numTotalWords; j ++) { // Just for clarity, no real meaning. tempWord = uniqueWords[j]; // The word list is either empty or the current token is not in the list. if (tempWord == NULL) { break; } //printf("Comparing (%s) with (%s)\n", tempWord, word); // If the current token is the same as the current element in the word list, mark and break if (strcmp(tempWord, word) == 0) { printf("\nDuplicate: (%s)\n\n", word); wordAlreadyExists = 1; break; } } // Word does not exist, add it to the array. if (!wordAlreadyExists) { uniqueWords[numUniqueWords] = malloc(strlen(word)); uniqueWords[numUniqueWords] = word; numUniqueWords ++; printf("Unique: %s\n", word); } // Reset flags and continue. wordAlreadyExists = 0; word = strtok(NULL, " ,.-!?()\"0123456789"); } // Print out the array just for funsies - make sure it's working properly. for (int x = 0; x <numUniqueWords; x++) { printf("Unique list %d: %s\n", x, uniqueWords[x]); } printf("\nNumber of unique words: %d\n\n", numUniqueWords); // Right below is where things start to suck. free_memory(uniqueWords, numUniqueWords); return numUniqueWords; } </code></pre>

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