Stack traces

Limitations

Unlike debuggers for desktop operating system which can request backtraces using system calls such as backtrace(3), neither PROS or VexOS offer a way to obtain such information from the user program.

Our Solution

C++ offers a feature to classes called destructors which do the opposite of their constructor and usually ends up freeing resources.

v5dbg uses these to its advantage with the V5DbgFunction class. When this class is constructed its fed in location information obtained from the __PRETTY__FUNCTION__, __FILE__, and __LINE__ macros. In the constructor we access the current virtual callstack vector for our thread and push all this debug info into it at the last index.

The C++ compiler will automatically call the destructor once this class instance falls out of scope in the current function, inside our destructor we call pop_back on the current virtual callstack vector.

With this method, a function's debug information is inserted onto the virtual callstack at the proper location and is available for the entire duration of its execution. When we wish to inspect the callstack, like for DEBUGGER_MESSAGE_VSTACK_FOR messages, we simply loop through the proper callstack vector to remove the last inserted frame.

Since collecting debug info and constructing classes is annoying to do manually for every function its been compressed into a singular macro called $function, when this macro is used at the top of a function it generates the code required to generate stack traces and places the V5DbgFunction class instance in a local variable named _v5dbg_stack_func

Future Alternatives

In the future libunwind could be used to obtain a backtrace without the use of $function. Since libunwind only returns addresses it would be up to the debugger to convert them into human readable locations with tools such as addr2line.