x86 64 Bit Stack Boundaries

While inspecting disassembled code on my Macbook Pro, I couldn’t figure out why the stack allocated so much space when I only requested a little. After some investigation into GCC I figured it out.

According to Apple’s developer documentation on GCC, “If -mpreferred-stack-boundary is not specified, the default is 4 (16 bytes or 128 bits).” That means that the stack will always allocate chunks in factors of 16 (2^4) unless you specify otherwise.

That means that the following code

int main(int argc, char **argv) {
        char buf[1];
        return 0;
}

Translates to allocating 16 bytes on the stack even though it’s only 1 byte.

...
0x0000000100000ef2 :    sub    rsp,0x10 # 16 bytes
...

Update
This is not specific to Mac, all 64 bit platforms use this stack alignment. Here is the Windows documentation and here is the Linux documentation. Thanks Pascal!