What's the difference between GetLocation, AddressOf and GetLoadAddress

Subprojects LLDB
1

I want to know if two pointer point to the same variable, so I want to get the location of each variable and the value of the pointer. In the API doc, I found three functions that seem to do the work, they are:

  • SBValue::GetLocation → const char*
  • SBValue::AddressOf → SBValue
  • SBValue::GetLoadLocation → addr_t (uint64_t)

I did a simple experiment, and find out that they contain the same address (only in different format). So I wonder what’s the difference between them, are they just return address in different format? If not, which one should I use? thank in advance.

2

If your variables are both pointers, then you just get the value as unsigned:

SBValue value1 = ...;
SBValue value2 = ...;
if (value1.GetType().IsPointerType() && value2.GetType().IsPointerType())
{
    if (value1.GetValueAsUnsigned() == value2.GetValueAsUnsigned())
    {
        // Pointers are the same
    }
}

I want to know if two pointer point to the same variable, so I want to get the location of each variable and the value of the pointer.
In the API doc, I found three functions that seem to do the work, they are:

- SBValue::GetLocation -> const char*
- SBValue::AddressOf -> SBValue
- SBValue::GetLoadLocation -> addr_t (uint64_t)

Why do you want the location? A pointer will often be in a register like "rax". And if a pointer is on the stack, why do you want the address of the pointer (which is what SBValue::GetLocation() would tell you)?

So: SBValue::GetLocation() will tell you where a variable is stored. It might be something like "rax" when it is in a register. Or some address when it is on the stack, heap or .data section. But this is the location of the value itself (address of the pointer).

SBValue::AddressOf() will return a new SBValue that represents the address of the value itself. If your variable is in a register, this will be invalid. If you already have a pointer like a "Foo *", then calling AddressOf will return you a value that is a "Foo **".

If you want to know if something is loaded in memory and not in a register, you can call SBValue::GetLoadLocation() which will only return a valid address _if_ the value is actually in memory. If you have a pointer that is in "rax" you will get back LLDB_INVALID_ADDRESS.

So to sum up: if you have a SBValue that represents a pointer, you can ask the value for its value as unsigned using SBValue::GetValueAsUnsigned() since that will be the pointer value itself. You don't want the location (the address of) the value. The SBValue::AddressOf() can be used on a variable that is actually an instance of "Foo". So if you have a variable whose type is "Foo", and a pointer to a "Foo":

SBValue foo1 = get_instance_of_foo();
SBValue foo2 = get_pointer_to_foo();

You can still check if these point to the same instance if you wanted to:

lldb::addr_t foo1_load_location = LLDB_INVALID_ADDRESS;
lldb::addr_t foo2_load_location = LLDB_INVALID_ADDRESS;
if (foo1.GetType().IsPointerType())
    foo1_load_location = foo1.GetValueAsUnsigned();
else
    foo1_load_location = foo1.GetLoadLocation();

if (foo2.GetType().IsPointerType())
    foo2_load_location = foo2.GetValueAsUnsigned();
else
    foo2_load_location = foo2.GetLoadLocation();

if (foo1_load_location == foo2_load_location && foo2_load_location != LLDB_INVALID_ADDRESS)
{
    // Two SBValue represent the same value in memory
}

Again you really need to understand what you are asking of the value.

Greg Clayton

3

Every time the process stops, I use SBFrame::GetVariables to get to variable list and display them in the “variable” window. Since variable within different scope may have same name, I want to use the address of the variable to check if they are same. For example, when the process first stop, I got a variable “a” at address 0x123. While on the next stop, I got a variable “a” at address 0x456, so I know these two “a” are different variables.

If variable can also be in register, how can I know if two variables got from two stop are the same. Is there a way to get the identity of variables?

PS: I know there’s a function SBValue::GetID, but that seems to return different result on different stop.

4

Every time the process stops, I use SBFrame::GetVariables to get to variable list and display them in the "variable" window. Since variable within different scope may have same name, I want to use the address of the variable to check if they are same. For example, when the process first stop, I got a variable "a" at address 0x123. While on the next stop, I got a variable "a" at address 0x456, so I know these two "a" are different variables.

If variable can also be in register, how can I know if two variables got from two stop are the same. Is there a way to get the identity of variables?

My question to you: why are you trying to remove some variables? Are you trying to show all variables for all frames at the same time? If not, you shouldn't be removing any variables. So my question is why are you trying to remove anything?

PS: I know there's a function SBValue::GetID, but that seems to return different result on different stop.

This isn't going to help you unique anything I would avoid using it.