[Dwarf-Discuss] how to find local variable stored in register

[Jim Blandy]

On Feb 5, 2008 8:57 PM, Neeraj kushwaha <kushneeraj at gmail.com> wrote:
> How GDB finds the value of variable which are stored in register for
> each function stack frame?
> As GDB uses dwarf debug info internally and is capable of finding them.

It depends.  For native Linux debugging, GDB uses the 'ptrace' system
call to read and write a process's registers.  See
i386_linux_fetch_inferior_registers.  Other debuggers probably do
something similar.

DWARF only shows the relationship between a particular pile of machine
code and the source code from which it was generated.  So while DWARF
does tell GDB which register a variable lives in, or how to calculate
the variable's address, DWARF doesn't tell GDB how to actually read
that register or access that memory.

GDB could be debugging a program running on a simulator; it could be
using the remote protocol (see Appendix D of the GDB manual); it could
be doing something else.  In each of these cases, the way GDB actually
reads the register that DWARF has directed it to would be entirely
different.