c - Why doesn't the stack/base pointer change here?

Question

I've written the following function to play around with inline assembly a bit and print out various registers:

void run(void)
{
    long rsp, rbp;
    asm("mov %%rsp, %0;" "mov %%rbp, %1;" : "=r" (rsp), "=r" (rbp));
    printf("Middle
%%rsp = %#lx
" "%%rbp = %#lx
", rsp, rbp);
}
int main(void)
{
    long rsp, rbp;
    asm("mov %%rsp, %0;" "mov %%rbp, %1;" : "=r" (rsp), "=r" (rbp));
    printf("Start
%%rsp = %#lx
" "%%rbp = %#lx
", rsp, rbp);

    run();
    long a,b,c;
    char* d = "Hello";
    char e[10];

    asm("mov %%rsp, %0;" "mov %%rbp, %1;" : "=r" (rsp), "=r" (rbp));
    printf("End
%%rsp = %#lx
" "%%rbp = %#lx
", rsp, rbp);
}

And it prints out the following:

Start
%rsp = 0x7ffeec93bf60
%rbp = 0x7ffeec93bf90
Middle
%rsp = 0x7ffeec93bf40
%rbp = 0x7ffeec93bf50
End
%rsp = 0x7ffeec93bf60 * same as start
%rbp = 0x7ffeec93bf90 * same as start

So in the above case the stack/base pointer changes when going into another function (Middle) but why doesn't it change when defining those 5 variables in the middle of the main function? Other than calling a function, what types of operations result in changing the stack? Or is that totally up to the compiler, and you really don't have any control of it when not writing directly in asm?

question from:https://stackoverflow.com/questions/65836151/why-doesnt-the-stack-base-pointer-change-here

Answer 1

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