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    <p>The problem is not how you're freeing, but how you're creating the array. Consider this:</p> <pre><code>uniqueWords[numUniqueWords] = malloc(strlen(word)); uniqueWords[numUniqueWords] = word; ... word = strtok(NULL, " ,.-!?()\"0123456789"); </code></pre> <p>There are several issues here:</p> <p><code>word = strtok()</code>: what strtok returns is not something that you can free, because it has not been malloc'ed. ie it is not a copy, it just points to somewhere inside the underlying large string (the thing you called strtok with first). </p> <p><code>uniqueWords[numUniqueWords] = word</code>: this is not a copy; it just assigns the pointer. the pointer which is there before (which you malloc'ed) is overwritten.</p> <p><code>malloc(strlen(word))</code>: this allocates too little memory, should be strlen(word)+1</p> <p>How to fix:</p> <p>Option A: copy properly </p> <pre><code>// no malloc uniqueWords[numUniqueWords] = strdup(word); // what strdup returns can be free'd </code></pre> <p>Option B: copy properly, slightly more verbose</p> <pre><code>uniqueWords[numUniqueWords] = malloc(strlen(word)+1); strcpy(uniqueWords[numUniqueWords], word); // use the malloc'ed memory to copy to </code></pre> <p>Option C: don't copy, don't free</p> <pre><code>// no malloc uniqueWords[numUniqueWords] = word; // not a copy, this still points to the big string // don't free this, ie don't free(list[i]) in free_memory </code></pre> <p><em>EDIT</em> As other have pointed out, this is also problematic:</p> <pre><code> char *uniqueWords[numTotalWords]; </code></pre> <p>I believe this is a GNU99 extension (not even C99), and indeed you cannot (should not) free it. Try <code>char **uniqueWords = (char**)malloc(sizeof(char*) * numTotalWords)</code>. Again the problem is not the free() but the way you allocate. You are on the right track with the free, just need to match every free with a malloc, or with something that says it is equivalent to a malloc (like strdup).</p>
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    1. POStrange (Undefined?) Behavior of Free in C
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    1. POStrange (Undefined?) Behavior of Free in C
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    1. COI really did have a feeling that the assignment from `uniqueWords[numUniqueWords] = word` was just that, an assignment. And I also had a feeling that I should have just copied the string from the start. So it turns out that my initial concern with how to put those stupid things in the array in the first place was the problem. I can say this though, your explanation of the methods of manipulating those values was great. I just hope that someone else comes across this question in the future...
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      1. USChris Cirefice
    2. CO@ChrisCirefice: Thanks!
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    3. COThe `char *uniqueWords[numTotalWords];` declaration is a C99 standard VLA or variable-length array. They're also in C11, but are optional there (ISO/IEC 9899:2011 §6.10.8.3 Conditional feature macros). They are not just a GNU compiler extension.
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      1. USJonathan Leffler
 

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