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POWhy can I write and read memory when I haven't allocated space?
 

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  1. POWhy can I write and read memory when I haven't allocated space?
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    <p>I'm trying to build my own Hash Table in C from scratch as an exercise and I'm doing one little step at a time. But I'm having a little issue...</p> <p>I'm declaring the Hash Table structure as pointer so I can initialize it with the size I want and increase it's size whenever the load factor is high.</p> <p>The problem is that I'm creating a table with only 2 elements (it's just for testing purposes), I'm allocating memory for just those 2 elements but I'm still able to write to memory locations that I shouldn't. And I also can read memory locations that I haven't written to.</p> <p>Here's my current code:</p> <pre><code>#include &lt;stdio.h&gt; #include &lt;stdlib.h&gt; #define HASHSIZE 2 typedef char *HashKey; typedef int HashValue; typedef struct sHashTable { HashKey key; HashValue value; } HashEntry; typedef HashEntry *HashTable; void hashInsert(HashTable table, HashKey key, HashValue value) { } void hashInitialize(HashTable *table, int tabSize) { *table = malloc(sizeof(HashEntry) * tabSize); if(!*table) { perror("malloc"); exit(1); } (*table)[0].key = "ABC"; (*table)[0].value = 45; (*table)[1].key = "XYZ"; (*table)[1].value = 82; (*table)[2].key = "JKL"; (*table)[2].value = 13; } int main(void) { HashTable t1 = NULL; hashInitialize(&amp;t1, HASHSIZE); printf("PAIR(%d): %s, %d\n", 0, t1[0].key, t1[0].value); printf("PAIR(%d): %s, %d\n", 1, t1[1].key, t1[1].value); printf("PAIR(%d): %s, %d\n", 3, t1[2].key, t1[2].value); printf("PAIR(%d): %s, %d\n", 3, t1[3].key, t1[3].value); return 0; } </code></pre> <p>You can easily see that I haven't allocated space for <code>(*table)[2].key = "JKL";</code> nor <code>(*table)[2].value = 13;</code>. I also shouldn't be able read the memory locations in the last 2 <code>printfs</code> in <code>main()</code>.</p> <p>Can someone please explain this to me and if I can/should do anything about it?</p> <p><strong>EDIT:</strong><br> Ok, I've realized a few things about my code above, which is a mess... But I have a class right now and can't update my question. I'll update this when I have the time. Sorry about that.</p> <p><strong>EDIT 2:</strong><br> I'm sorry, but I shouldn't have posted this question because I don't want my code like I posted above. I want to do things slightly different which makes this question a bit irrelevant. So, I'm just going to assume this was question that I needed an answer for and accept one of the correct answers below. I'll then post my proper questions...</p>
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    1. USRicardo Amaral
    1. USRicardo Amaral
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      1. POWhy can I write and read memory when I haven't allocated space?
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    1. CORelated: http://stackoverflow.com/search?q=%5Bc%5D+allocation+errors. Possible Duplicate: http://stackoverflow.com/questions/828108/please-help-me-to-solve-this-memory-allocation-problem
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      1. POWhy can I write and read memory when I haven't allocated space?
      1. USS.Lott
    2. COhttp://stackoverflow.com/questions/1240970/c-why-do-you-specify-the-size-when-using-malloc
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      1. POWhy can I write and read memory when I haven't allocated space?
      1. USAndreas Grech
 

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