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Raymond Chen
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)<br>char buffer[16384];<br>consume(alloca(n), buffer);
On x86-64, this results in
push rbp<br>mov eax, 16416 ; probe for local frame<br>call __chkstk<br>sub rsp, rax ; create local frame
lea rbp, [rsp+32]
movsxd rax, ecx ; n<br>lea rcx, [rax+15] ; round up to multiple of 16<br>and rcx, -16
mov rax, rcx ; special __chkstk calling convention<br>call __chkstk<br>sub rsp, rcx ; allocate n bytes
lea rdx, [rbp] ; rdx -> buffer<br>lea rcx, [rsp+32] ; rcx -> alloca'd memory<br>call consume
lea rsp, [rbp+16384] ; clean up local frame<br>pop rbp<br>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().
Category<br>Old New Thing
Topics<br>Code
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Author
Raymond Chen
Raymond has been involved in the evolution of Windows for more than 30 years. In 2003, he began a Web site known as The Old New Thing which has grown in popularity far beyond his wildest imagination, a development which still gives him the heebie-jeebies. The Web site spawned a book, coincidentally also titled The Old New Thing (Addison Wesley 2007). He occasionally appears on the Windows Dev Docs Twitter account to tell stories which convey no useful information.
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