lldb C local variable not printing

Question

Value eval(Value arg, Table env) {
if (arg.tag == ConsCell) {
    Value operator = car(arg);
    Value operands = cdr(arg); // <- debugger stopped here

If I print the argument arg with p arg, I get:

(lldb) p arg
(Value) $0 = {
  data = {
    number = 1068272
    list = 0x0000000100104cf0
    symbol = 0x0000000100104cf0 "?L\x10"
  }
  tag = ConsCell
}

But if p operator, I get:

(lldb) p operator
error: expected a type
error: 1 errors parsing expression

However, using frame variable operator works:

(lldb) frame variable operator
(Value) operator = {
  data = {
  number = 1068208
  list = 0x0000000100104cb0
  symbol = 0x0000000100104cb0 "\x10L\x10"
}
  tag = ConsCell
}

What's going wrong when I'm using p operator?

Answer 1

lldb evaluates expression in hybrid of C++ and Objective-C. operator, the name of your variable, is a reserved keyword in C++. When you use the p command (which is an alias to expression command), lldb passes your expression to clang to parse and evaluate in C++/Objective-C (or, if you're debugging a Swift method, parse and evaluate in Swift). Even though your program is written in straight C, your expressions are evaluated as C++ expressions and that's not a valid C++ expression.

frame variable (fr v for short) doesn't go through a compiler for evaluation, it tries to do simple parsing of the variable path provided. It can do simple dereferencing and following of pointers, but it can't cast values for instance.