How do functions like alloca allocate memory from the stack?

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. Here’s an artificial example: #include void consume(void*,void*); void f(int n) { char buffer[16384]; consume(alloca(n), buffer); } On x86-64, this results in push rbp mov eax, 16416 ; probe for local frame call __chkstk sub rsp, rax ; create local frame lea rbp, [rsp+32] movsxd rax, ecx ; n lea rcx, [rax+15] ; round up to multiple of 16 and rcx, -16 mov rax, rcx ; special __chkstk calling convention call __chkstk sub rsp, rcx ; allocate n bytes lea rdx, [rbp] ; rdx -> buffer lea rcx, [rsp+32] ; rcx -> alloca'd memory call consume lea rsp, [rbp+16384] ; clean up local frame pop rbp ret 0 Observe that the same __chkstk function is used both for performing the initial stack probe when creating the local frame as well as for the alloca() . 0 comments Be the first to start the discussion.