C ++设置堆栈指针

Question

In my C++ / C project I want to set the stack pointer equal to the base pointer ... Intuitively I would use something like this:

asm volatile(
    "movl %%ebp %%esp"
);

However, when I execute this, I get this error message:

Error: bad register name `%%ebp %%esp'

I use gcc / g++ version 4.9.1 compiler.

I dont know whether I need to set specific g++ or gcc flag though... There should be a way to manipulate the esp and ebp registers but I just don't know the right way to do it.

Doe anybody know how to manipulate these two registers in c++? Maybe I should do it with hexed OP codes?

Answer 1

You're using GNU C Basic Asm syntax (no input/output/clobber constraints), so % is not special and therefore, it shouldn't be escaped.

It's only in Extended Asm (with constraints) that % needs to be escaped to end up with a single % in front of hard-coded register names in the compiler's asm output ( as required in AT&T syntax ).

You also have to separate the operands with a comma:

asm volatile(
    "movl %ebp, %esp"
);

asm statements with no output operands are implicitly volatile, but it doesn't hurt to write an explicit volatile .

Note, however, that putting this statement inside a function will likely interfere with the way the compiler handles the stack frame .