Trouble calling a function from a debugger

[alexcrichton]

Output of a session of mine:

fn factorial(x: int) -> int {
    if x <= 0 { 1 } else { x * factorial(x - 1) }
}

fn main() {
    let x = 4;
    println!("{}", factorial(x));
}

$ rustc -g inspect.rs  
$ lldb ./inspect        
Current executable set to './inspect' (x86_64).
(lldb) b main
Breakpoint 1: 2 locations.
(lldb) r
Process 45203 launched: './inspect' (x86_64)
Process 45203 stopped
* thread #1: tid = 0x370ce1, 0x0000000100001780 inspect`main, queue = 'com.apple.main-thread', stop reason = breakpoint 1.2
    frame #0: 0x0000000100001780 inspect`main
inspect`main:
-> 0x100001780:  cmpq   %gs:0x330, %rsp
   0x100001789:  ja     0x1000017a5               ; main + 37
   0x10000178b:  movabsq $0x18, %r10
   0x100001795:  movabsq $0x0, %r11
(lldb) c
Process 45203 resuming
Process 45203 stopped
* thread #2: tid = 0x370cf7, 0x00000001000016b4 inspect`inspect::main + 52 at inspect.rs:6, stop reason = breakpoint 1.1
    frame #0: 0x00000001000016b4 inspect`inspect::main + 52 at inspect.rs:6
   3    }
   4   
   5    fn main() {
-> 6        let x = 4;
   7        println!("{}", factorial(x));
   8    }
(lldb) print factorial(5)
Error [IRForTarget]: Size of result type 'long (long)' couldn't be determined
error: warning: function 'factorial' has internal linkage but is not defined
note: used here
error: The expression could not be prepared to run in the target
(lldb) 

cc @michaelwoerister

[alexcrichton]

I'm not sure if this is the fault of some debuginfo, or possibly that the default OSX lldb is a little out of date. Some other things that I did:

// lldb thinks it's returning a function, so call it again!
(lldb) print factorial(4)(3)
zsh: segmentation fault  lldb ./inspect

// cast the return value to an integer.
(lldb) print (long) factorial(4)
Instruction returns a non-scalar type!
  %9 = call i64 (i64) inttoptr (i64 4294972928 to i64 (i64) (i64)*)(i64 4), !lldb.call.realName !9
PtrToInt source must be pointer
  %10 = ptrtoint i64 (i64) %9 to i64
Broken module found, compilation aborted!
zsh: abort      lldb ./inspect

[alexcrichton]

This may also be an lldb bug, I'm unsure

[jdm]

If Rust is still dealing with return values through an outparam, DWARF does not appear to have a way to represent that. Function return values in gdb are always meaningless.

[thestinger]

Rust only uses an out parameter for what it calls non-immediate types. An int is an immediate type.

[michaelwoerister]

Mind that calling functions in the debugger is explicitly not supported at the moment. This will definitely be interesting in the future but it isn't a high priority right now.

[steveklabnik]

Triage: today, a different error:

(lldb) print factorial(5)
error: warning: duplicate 'signed' declaration specifier
error: cannot combine with previous 'unsigned' declaration specifier
error: use of undeclared identifier 'factorial'
error: 2 errors parsing expression

[brson]

Updated test

fn factorial(x: isize) -> isize {
    if x <= 0 { 1 } else { x * factorial(x - 1) }
}

fn main() {
    let x = 4;
    println!("{}", factorial(x));
}

It looks to me like the paths in the debugger want to be crate-prefixed today.

Results in gdb on Ubuntu 16.04:

(gdb) print test::factorial(10)
$1 = 3628800
(gdb) print factorial(10)
No symbol "factorial" in current context.
(gdb)

Results in lldb on Ubuntu 16.04:

(lldb) print test::factorial(10)
error: use of undeclared identifier 'test'
error: 1 errors parsing expression
(lldb) print factorial(10)
error: use of undeclared identifier 'factorial'
error: 1 errors parsing expression