How do functions like alloca allocate memory from the stack?
This article explains how the C function alloca allocates memory on the stack by utilizing the _chkstk function. It provides a technical breakdown of the assembly code generated on x86-64 architectures to ensure stack safety.
Why it matters
Understanding low-level memory management is critical for systems programming and preventing stack overflow vulnerabilities in high-performance software.
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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