How do functions like alloca allocate memory from the stack?

This article explains how the _alloca function works in conjunction with the _chkstk function to allocate memory from the stack while ensuring that large allocations do not skip over the guard page. It includes a code example demonstrating the process.

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A little while ago, I talked about how compilers ensure that large stack allocations do not skip over the guard page . Shawn Van Ness was curious how this works with _alloca . “Does it do the necessary _chkstk() probing?”

Yes, the _alloca() function calls the same _chkstk() function to probe the stack before adjusting the stack pointer for the allocated memory.

#include <malloc.h> void consume(void*,void*); void f(int n) { char buffer[16384]; consume(alloca(n), buffer); } On x86-64, this results in

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