[lldb-dev] Question regarding argument types of "BreakpointHitCallback"

[Jim Ingham via lldb-dev]

> On Apr 30, 2020, at 12:43 PM, Vangelis Tsiatsianas via lldb-dev <lldb-dev at lists.llvm.org> wrote:
> 
> Thank you for the answer, Greg.
> 
> I personally managed to work around the problem, although it confused me a bit at first and took a while to figure out the cause. May I suggest the addition of a note in the documentation of "{Breakpoint, Watchpoint}::{Invoke, Set}Callback()" and possibly other relevant functions as a warning to future developers that may stumble upon the same issue?
> 
> Regarding the public C++ API, would defining "break_id_t" as "int64_t" be a viable solution or that change would also break the API? It seems that making both types 64-bit alleviates the issue, despite the sign difference.

Mangled names don’t encode typedef names, but the bare type.  For instance:

 > cat signatures.cpp 
#include <stdint.h>
#include <stdio.h>

typedef int64_t break_id_t;

struct Foo {
  void GiveMeABreak(break_id_t id) { printf("Got: %d\n", id); }
};

int
main()
{
  Foo myFoo;
  myFoo.GiveMeABreak(100);
  return 0;
}
 > clang++ -g -O0 signatures.cpp
 > nm a.out | grep GiveMeABreak
0000000100000f50 T __ZN3Foo12GiveMeABreakEx
 > c++filt __ZN3Foo12GiveMeABreakEx
Foo::GiveMeABreak(long long)

So this change would change the mangled names of any methods taking a break_id_t, and mean old clients would get missing symbol errors when running with the new API’s.  So this isn’t something we can do.

Jim


> 
> 
> ― Vangelis
> 
> 
>> On 30 Apr 2020, at 22:22, Greg Clayton <clayborg at gmail.com <mailto:clayborg at gmail.com>> wrote:
>> 
>> 
>> 
>>> On Apr 30, 2020, at 8:50 AM, Vangelis Tsiatsianas via lldb-dev <lldb-dev at lists.llvm.org <mailto:lldb-dev at lists.llvm.org>> wrote:
>>> 
>>> Hello,
>>> 
>>> I would like to ask a question regarding "BreakpointHitCallback", which is declared as such:
>>> 
>>> bool (*BreakpointHitCallback)(void *baton,
>>>                               StoppointCallbackContext *context,
>>>                               lldb::user_id_t break_id,
>>>                               lldb::user_id_t break_loc_id);
>>> 
>>> Is there any particular reason that "break_id" and "break_loc_id" are of type "user_id_t" (64-bit unsigned) instead of "break_id_t" (32-bit signed), which is used both for "Stoppoint::m_bid" and "StoppointLocation::m_loc_id"?
>> 
>> I believe this callback predated the time when we added break_id and break_loc_id, and since arguments are part of the signature of C++ functions, we didn't change it in order to keep the public API from changing. Or this could have just been a mistake. Either way, we have a stable API and can't really change it.
>>> 
>>> This causes an issue mainly with internal breakpoints, since the callback of an internal breakpoint with (ID == 0xfffffffe) is called with (break_id == 0xfffffffffffffffe), forcing the callback to cast the argument back to a 32-bit signed in order to use it correctly, e.g. when the IDs are stored and need to be looked up.
>>> 
>>> A small example attempting to illustrate the problem: https://godbolt.org/z/y8LbK2 <https://godbolt.org/z/y8LbK2>
>> Sorry for the issue, but I think we are stuck with it now.
>> 
>> 
> 
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