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POCan a variable be used to define the size of an array on the stack in c?
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  1. POCan a variable be used to define the size of an array on the stack in c?
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    copied!<p>I have a situation where I want my program to read in some numbers that will define the size of a two dimensional array (used as a matrix). I originally argued that the only way to do this would be to use a <code>malloc</code> call to put the array on the heap, something like this:</p> <pre><code>matrixElement* matrix = malloc(sizeof(matrixElement) * numRows * numCols); </code></pre> <p>where numCols and numRows are the integers which were read in earlier, and matrixElement is some arbitrary type. My reasoning was that simply writing:</p> <pre><code>matrixElement matrix[numRows][numCols]; </code></pre> <p>would not work since the compiler would have no way of knowing how much stack space to allocate to the function call. It turns out I was wrong, since the following code compiles and runs:</p> <pre><code>#include &lt;stdio.h&gt; #include &lt;stdlib.h&gt; int main(void) { int x, y; scanf("%d", &amp;x); scanf("%d", &amp;y); double arr[x][y]; printf("Made an array of size %d by %d, total memory %fKb\n", sizeof(arr) / sizeof(arr[0]), sizeof(arr[0]) / sizeof(arr[0][0]), (float) sizeof(arr) / 1024.0f); return 0; } </code></pre> <p>With big enough numbers input for x and y, this will eventually give a segfault, but I was very surprised too see that I could create a 1000x1000 array with this code. </p> <p>Can anyone explain what is going on here? </p> <p>Is the compiler just allocating a bunch of space for the array, even though it has no idea how much will be used? </p> <p>Is this behavior specified by ANSI C or is it just something that gcc is doing on its own?</p>
 

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