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    <p>No, the code should not (necessarily) give a segfault. A segfault occurs when you attempt to access a virtual memory page that is not allocated to your process.</p> <p>The "heap" or "free store" is a region of virtual memory pages owned by your process. The <code>malloc()</code> API sub-divides this region into blocks and returns a pointer to the block.</p> <p>If you address beyond the end of the block to which you have a pointer, you will usually access memory that is part of the heap, but not part of your allocated block. In this way, you can corrupt other heap blocks or even the data structures which <code>malloc()</code> uses to define the heap.</p> <p>For more information on heap corruption, and methods to detect it in the debug version of your code, this is a great book:</p> <p><a href="https://rads.stackoverflow.com/amzn/click/1556155514" rel="nofollow noreferrer">Writing Solid Code: Microsoft's Techniques for Developing Bug-Free C Programs by Steve Maguire</a> <a href="https://rads.stackoverflow.com/amzn/click/1556155514" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/vyYUx.jpg" alt="alt text"></a></p> <p>An addendum for the pedantic: In rare cases, by accessing memory beyond the end of a heap block, you may access memory that is not part of the heap. In these cases, you may get the segmentation fault you expected. You might also corrupt some other data structure than the heap. It's really a matter of chance. However, the heap itself is very large compared to typical heap blocks, so 99% of the time code such as your example will corrupt the heap. The example you provide falls into that 99% case.</p>
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    1. COActually, since he asked to change 20 bytes past the end of the stack, depending on the architecture and surrounding code he could instead hose the stack, or even change the program itself. The behavior is truly undefined.
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    2. COT.E.D. That is only "sort of" true. He is no doubt using a machine with a virtual memory page size. Presumably executable code has different VM protection than heap. I would wager he is on such a system because the C standard library malloc() function exists. What you write is true, however, in particular on the Radio Shack Model III running TRS-DOS on a Z-80. ;)
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    3. COAnd yes, even on a VM page system, what you write could possible happen. It just isn't that likely and it isn't very pedagogical to go into that detail that rarely matters. Address the problem at the level at which it is stated. Avoid pedantry.
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