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PODynamically growing Stack in C
 

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  1. PODynamically growing Stack in C
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    <p>I'm experiencing some trouble with an assignment. I am supposed to implement a dynamically growing stack that doubles its size when it's full and halves it when it's 1/4 full. Since I am a total C beginner and are unfamiliar with pointers, I've looked through some examples and this is the Code I came up with.</p> <p>It actually compiles in gcc without warnings, but produces a "Segmentation fault" when I try to run it. I found out that this probably has to do with broken pointers, but I don't see any mistakes and would be glad if someone could point it out for me.</p> <p>Cheers</p> <pre><code># ifndef STACK_H # define STACK_H # include "stdlib.h" typedef struct stack { int *stack; int used; int size; } stack; stack* stck_construct() { stack *stck; stck-&gt;stack = (int *)malloc(10 * sizeof(int)); stck-&gt;used = 0; stck-&gt;size = 10; return stck; } void stck_destruct(stack *stck) { stck-&gt;stack = 0; stck-&gt;used = stck-&gt;size = 0; free(stck); } int stck_push(stack *stck, int val) { if (stck-&gt;used == stck-&gt;size) { stck-&gt;size *= 2; stck-&gt;stack = (int *)realloc(stck-&gt;stack, stck-&gt;size * sizeof(int)); } stck-&gt;stack[stck-&gt;used] = val; stck-&gt;used++; return 1; } int stck_pop(stack *stck, int *val) { *val = stck-&gt;stack[stck-&gt;used]; free(stck-&gt;stack); stck-&gt;used--; if (stck-&gt;used &lt;= (stck-&gt;size)/4) { if (stck-&gt;size &lt;=40) stck-&gt;size = 10; else stck-&gt;size /= 2; stck-&gt;stack = (int *)realloc(stck-&gt;stack, stck-&gt;size * sizeof(int)); } return 1; } int main(){ stack* test; test=stck_construct(); int i; int out; for (i =1; i&lt;=10; i++) stck_push(test, i); for (i =1; i&lt;=10; i++) { stck_pop(test,&amp;out); printf("%i\n", out); } stck_destruct(test); return 0; } # endif </code></pre>
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    1. USLion Härtner
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    1. COIn C, [don't cast the return value of `malloc()` and `realloc()`](http://stackoverflow.com/a/605858/28169).
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    2. COIn addition to what Alberto Miranda pointed out (which is likely the immediate cause of the segfault), you have a memory leak in `stck_destruct` (you only set `stck->stack` to null without freeing it). Also, you are freeing `stck->stack` in `stck_pop` before only conditionally reallocating it, which means you're destroying the previous contents and making it possible for `stck->stack` to remain unallocated.
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      1. USDavid Brown
    3. CO@unwind For all my years programming in C, I had never considered this. It makes so much sense. Thanks!
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      1. USAlberto Miranda
 

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