Crystal: LLDB don't show frame variables while debugging

[faustinoaq]

Compile the following code with crystal build -d a.cr:

a = 0
b = 1
b = a

Using GDB:

$ gdb -q a
Reading symbols from a...done.
(gdb) b a.cr:1
Breakpoint 1 at 0x432ec9: file /home/main/Projects/Experiments/a.cr, line 1.
(gdb) r
Starting program: /home/main/Projects/Experiments/a
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/usr/lib/libthread_db.so.1".
[New Thread 0x7ffff68ca700 (LWP 17885)]

Thread 1 "a" hit Breakpoint 1, __crystal_main () at /home/main/Projects/Experiments/a.cr:1
1       a = 0
(gdb) n
2       b = 1
(gdb) n
3       b = a
(gdb) info locals
a = 0
b = 1
(gdb) c
Continuing.
(gdb) [Thread 0x7ffff68ca700 (LWP 17885) exited]
[Inferior 1 (process 17881) exited normally]

Using LLDB:

$ lldb a
(lldb) target create "a"
Current executable set to 'a' (x86_64).
(lldb) b /home/main/Projects/Experiments/a.cr:1
Breakpoint 1: where = a`__crystal_main + 1529, address = 0x0000000000432ec9
(lldb) r
Process 17664 launched: '/home/main/Projects/Experiments/a' (x86_64)
Process 17664 stopped
* thread #1, name = 'a', stop reason = breakpoint 1.1
    frame #0: a`__crystal_main at a.cr:1
-> 1    a = 0
   2    b = 1
   3    b = a
(lldb) s
Process 17664 stopped
* thread #1, name = 'a', stop reason = step in
    frame #0: a`__crystal_main at a.cr:2
   1    a = 0
-> 2    b = 1
   3    b = a
(lldb) fr v
(lldb) frame variable
(lldb) fr v
(lldb) c
Process 17664 resuming
Process 17664 exited with status = 0 (0x00000000)

Testing a C file gcc -ggdb a.c:

int main() {
  int a = 0;
  int b = 1;
  b = a;
  return 0;
}

$ lldb a.out
(lldb) target create "a.out"
Current executable set to 'a.out' (x86_64).
(lldb) b /home/main/Projects/Experiments/a.c:2
Breakpoint 1: where = a.out`main + 4 at a.c:2, address = 0x00000000004004aa
(lldb) r
Process 18050 launched: '/home/main/Projects/Experiments/a.out' (x86_64)
Process 18050 stopped
* thread #1, name = 'a.out', stop reason = breakpoint 1.1
    frame #0: a.out`main at a.c:2
   1    int main() {
-> 2      int a = 0;
   3      int b = 1;
   4      b = a;
   5      return 0;
   6    }
(lldb) n
Process 18050 stopped
* thread #1, name = 'a.out', stop reason = step over
    frame #0: a.out`main at a.c:3
   1    int main() {
   2      int a = 0;
-> 3      int b = 1;
   4      b = a;
   5      return 0;
   6    }
(lldb) n
Process 18050 stopped
* thread #1, name = 'a.out', stop reason = step over
    frame #0: a.out`main at a.c:4
   1    int main() {
   2      int a = 0;
   3      int b = 1;
-> 4      b = a;
   5      return 0;
   6    }
(lldb) fr v
(int) a = 0
(int) b = 1
(lldb) c
Process 18050 resuming
Process 18050 exited with status = 0 (0x00000000)

Can you reproduce this issue?

[faustinoaq]

Versions used:

$ crystal -v
Crystal 0.22.0 (2017-04-22) LLVM 4.0.0
$ lldb --version
lldb version 4.0.0

Maybe related: https://github.com/rust-lang/rust/issues/33062

[at-grandpa]

I think this issue is important, because debugging like "pry" is desiring among people interested in Crystal. (me, too.)

see: https://stackoverflow.com/questions/35491883/is-it-any-crystal-debuggers-like-pry

And, I think this issue is also related to IDE support.

[ysbaddaden]

This sounds like an lldb issue as per https://github.com/rust-lang/rust/issues/33062#issuecomment-211601447

If LLDB doesn't understand the DW_AT_language, it won't default to a C-like language, and thus won't work. A solution would be to fix lldb so it supports Crystal (e.g. as an alias for C or C++), or just use gdb instead.

[at-grandpa]

Oh ... Thanks, @ysbaddaden.

However, is gdb also insufficiently? For example, I got the following result. I can't print Union Types variable.

$ crystal -v
Crystal 0.22.0 [3c71228] (2017-04-20) LLVM 3.5.0
$ cat sample.cr
debugger
x = 1
y = 2
z = (y < 1) ? 1 : "a"

puts typeof(z)
puts x
$ crystal build -d sample.cr
$ gdb ./sample
GNU gdb (Ubuntu 7.7.1-0ubuntu5~14.04.2) 7.7.1
Copyright (C) 2014 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
<http://www.gnu.org/software/gdb/documentation/>.
For help, type "help".
Type "apropos word" to search for commands related to "word"...
Reading symbols from ./sample...done.
(gdb) run
Starting program: /root/how_to_debug_crystal/sample
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
[New Thread 0x7ffff71d1700 (LWP 417)]

Program received signal SIGTRAP, Trace/breakpoint trap.
__crystal_main () at /root/how_to_debug_crystal/sample.cr:2
2   x = 1
(gdb) n
3   y = 2
(gdb) n
4   z = (y < 1) ? 1 : "a"
(gdb) n
6   puts typeof(z)
(gdb) n
(Int32 | String)
7   puts x
(gdb) print z
No symbol "z" in current context.
(gdb) info locals
x = 1
y = 2
(gdb)

Can not debug any more with gdb?

[ysbaddaden]

Debug support is minimal: file line and basic types are defined. If I recall correctly we don't detail the layout of more complex types, so gdb can't print them, because it doesn't know how to, and since we don't detail the type it's likely it doesn't know about the variable at all.

[at-grandpa]

I see.

I'm interested in debugging, so I'll think about it for a while.
Thanks!

[faustinoaq]

BTW, I'm using This extension to debug binaries in VSCode.

Maybe using gdb prettyprinters, it can improve something.

[faustinoaq]

I discovered some tricks to debug crystal using GDB, see here 👉 https://stackoverflow.com/questions/46369703/crystal-debugging-with-gdb

[baelter]

@ysbaddaden Does not seem like a LLDB issue as I tried compiling crystal with DW_AT_language set to C(0x02) and C(0x04)++ and neither gave any output for frame variable.

[gazsp]

Is there any update on this issue?

[wontruefree]

I am having this problem and it is really frustrating to not have a debugger.

[chances]

I'm having success seeing local variables when debugging with https://github.com/vadimcn/vscode-lldb.

Breakpoints set in the GUI seem to work, too. 🎉️

[bcardiff]

@chances and you can use the custom formatters too https://dev.to/bcardiff/debug-crystal-in-vscode-via-codelldb-3lf