Skip to content

Pony LLDB Cheat Sheet


macOS Setup

% brew install llvm
% sudo /usr/sbin/DevToolsSecurity --enable


Official LLDB tutorial

GDB ↔ LLDB Rosetta Stone

Building an Executable You Can Debug

% ponyc --debug

Running an Executable

% lldb EXECUTABLE -- arg1 arg2 arg3 …
% lldb -- EXECUTABLE arg1 arg2 arg3 ...
(lldb) run

Setting Breakpoints

(lldb) breakpoint set --file foo.c --line 12
(lldb) breakpoint set --name foo
(lldb) b foo.c:12
(lldb) b Main_create
(lldb) b map.pony:73

When using a file name/line combination to set the breakpoint, you can specify any file in the program without having to specify a path to that file, as long as the file name is unambiguous.

List Breakpoints

(lldb) breakpoint list
(lldb) b


(lldb) thread backtrace
(lldb) bt

If you would like to limit the size of the backtrace (for example, when you are debugging a case of runaway recursion that blows out the stack and generates a very long backtrace), you can use the --count or -c option to specify the number of frames to print, starting with the most recent one.

(lldb) thread backtrace --count 5
(lldb) bt 5


(lldb) thread step-in
(lldb) s
(lldb) sif funToStepInto
(lldb) thread step-over
(lldb) n
(lldb) thread step-out
(lldb) finish
(lldb) thread until LINE


Select a frame:

(lldb) frame select NUMBER
(lldb) f NUMBER

Show the variables in the current frame:

(lldb) frame variable

Show a specific variable from the current frame:

(lldb) frame variable VARIABLE
(lldb) p VARIABLE

Pointer, array, structure, and cast operation from C can be applied to frame variables.

&variable : address of a variable

*variable : dereference a pointer

variable.field : access a field within a structure

pointer_variable->field : access a field within a structure pointed to by a pointer

array_pointer[index] : access an index within an array

(type_t *)variable : cast variable to a pointer to a type_t


Example: Examining an object

(lldb) frame variable
(OscMessage *const) this = <variable not available>
(Array_U8_val *) parts = 0x0000000108fcf4c0
(OscString *) oscAddress = <variable not available>
(OscString *) types = 0x00007fff5fbff878
(lldb) frame variable *types
(OscString) *types = {
  _data = 0x0000000108fcf560
(lldb) frame variable *types->_data
(String) *types->_data = (_size = 2, _alloc = 3, _ptr = ",f")

(pony_test_TestHelper *) this = 0x0000000110ff5020
(lldb) p this
(pony_test_TestHelper *) $2 = 0x0000000110ff5020
(lldb) p *$2
(pony_test_TestHelper) $3 = {
  _runner = 0x0000000110ff7200
  env = 0x0000000108ffec00
(lldb) p $3.env
(Env *) $4 = 0x0000000108ffec00
(lldb) p *$4
(Env) $5 = {
  root = 0x000000010004f3e0
  input = 0x0000000108fd6800
  out = 0x0000000108fff500
  err = 0x0000000108fff600
  args = 0x0000000108fd6400
  _envp = 0x00007fff5fbffa98
  _vars = 0x000000010004f3b0

Using p lets you work from the resulting lldb temporary variables, which can be very convenient when you want to examine an intermediate result.

Example: Printing A String

Assume you have a Pony String called x. You can print the string like this:

(lldb) p (char *)(x->_ptr)
(char *) $3 = 0x00000001000106f8 "abc"

Example: Printing The Values In An Array Of Numbers

Assume you have an Array[U64] called x with 6 elements. You can print the values of the values of the elements like this:

(lldb) p *(long(*)[6])(x->_ptr)
(long [6]) $3 = ([0] = 1, [1] = 2, [2] = 3, [3] = 4, [4] = 5, [5] = 6)

You can adjust the type (long) to appropriately print the types that correspond to the array values, and adjust the size (6) to print more or fewer of the elements.

Pony Stuff

Method Name Mangling

{{< warning title=”Future breakage possible!” >}} While currently necessary for some debugging, you should know that name mangling might change in the future. Do not depend on this remaining static. {{< /warning >}}

Method names get mangled by the compiler. The general format for the mangling is:


In the above:

package : the name of the package (for methods brought in through use expressions)

type : the name of the type to which the method belongs

typearg1, typearg2 … : the type parameters of the method (for methods that use type parameters)

rcap : the reference capability of the method

methodname : the name of the method

mangling : a mangling string where each character indicates the type of each method parameter and the method’s return type according to the following conversion:

Type Mangling
object o
Bool b
I8 c
I16 s
I32 i
I64 w
I128 q
ILong l
ISize z
U8 C
U16 S
U32 I
U64 W
U128 Q
ULong L
USize Z
F32 f
F64 d

These rules can be used to determine the name of function in order to specify that you would like to place a breakpoint on it. You can also type b <characters><tab> to see a tab-complete list of all the available functions that start with <characters>. To set a breakpoint on a function:

(lldb) breakpoint set --name Bar_foo
(lldb) b Bar_foo

Calling Methods

As of April 5, 2017, issue #1813 is preventing this from working.

Behaviors and Messages

Each behavior has an associated send method (__send) that is used to send a message to the receiver to trigger that behavior. Each actor has a dispatch method (_Dispatch) that translates messages into calls to the appropriate functions in the receiver.