How do functions like alloca allocate memory from the stack?
This article explains the technical mechanism behind how the alloca function allocates memory on the stack in C. It details the use of the __chkstk function to ensure stack safety and prevent skipping over guard pages.
Why it matters
Understanding low-level memory management is critical for systems programming and preventing security vulnerabilities like stack overflows.
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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