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    <p>Standard <code>malloc</code> is defined in the C standard to allocate a contiguous block of memory (at least it appears so to you) - it will return a null pointer if the allocation fails.</p> <p>At a lower level, the OS will be doing something like what <a href="https://stackoverflow.com/questions/3954188/how-malloc-works/3954215#3954215">kotlinski</a> or <a href="https://stackoverflow.com/questions/3954188/how-malloc-works/3954221#3954221">Blank Xavier</a> have described in their respective answers.</p> <p>From <strong>§7.20.3</strong> of the <strong>ISO/IEC 9899-1999 C Standard</strong>:</p> <blockquote> <p>The pointer returned if the allocation (by <code>calloc</code>, <code>realloc</code>, or <code>malloc</code>) succeeds is suitably aligned so that it may be assigned to a pointer to any type of object and then <strong>used to access such an object or an array of such objects in the space allocated</strong> (until the space is explicitly deallocated).</p> </blockquote> <p>It is not that explicit, but the paragraph mentions 'accessing an array of such objects', and in the C standard, arrays are:</p> <blockquote> <p>An array type describes a contiguously allocated nonempty set of objects with a particular member object type, called the element type. (from <strong>§6.2.5</strong>)</p> </blockquote> <p>Also note that subsequent calls to <code>calloc</code>, <code>realloc</code>, and <code>malloc</code> do not guarantee contiguity or ordering of memory (with other memory blocks already allocated).</p> <p>This point is also specified in <strong>§7.20.3</strong>.</p> <blockquote> <p>The order and contiguity of storage allocated by successive calls to the <code>calloc</code>, <code>malloc</code>, and <code>realloc</code> functions is unspecified.</p> </blockquote>
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    1. COSubsequent call to `realloc` WILL either return a pointer to contiguous memory of that size of fail. It will also protect your existing data in that area if correctly allocated before. The pointer can change, and the absolute address of your data can change, but if `realloc` is successful you have your original data and the resized block as one block. Your very last statement is false with respect to `realloc`...
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    2. COMy last statement may have been unclear - I meant that repeated calls to any of the allocation functions do not guarantee contiguity/ordering respective to other blocks of memory that have already been allocated, not that the memory allocated by them would not be contiguous.
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