How to interpret the template function signature of backtrace in GDB? - c++

Question

The output of backtrace of GDB is pretty messy, especially for template.
For instance:
Thread 2 (LWP 100146 of process 1245):
#0 thr_new () at thr_new.S:3
#1 0x000000080025c3da in _pthread_create (thread=0x7fffdfffd880, attr=<optimized out>, start_routine=0x205500 <void* std::__1::__thread_proxy<std::__1::tuple<std::__1::unique_ptr<std::__1::__thread_struct, std::__1::default_delete<std::__1::__thread_struct> >, void (std::__1::__async_assoc_state<void, std::__1::__async_func<func2(std::__1::atomic<long>&)::$_0> >::*)(), std::__1::__async_assoc_state<void, std::__1::__async_func<func2(std::__1::atomic<long>&)::$_0> >*> >(void*)>, arg=0x8007fa8e0) at /usr/src/lib/libthr/thread/thr_create.c:188
#2 0x0000000000204e40 in std::__1::thread::thread<void (std::__1::__async_assoc_state<void, std::__1::__async_func<func2(std::__1::atomic<long>&)::$_0> >::*)(), std::__1::__async_assoc_state<void, std::__1::__async_func<func2(std::__1::atomic<long>&)::$_0> >*, void>(void (std::__1::__async_assoc_state<void, std::__1::__async_func<func2(std::__1::atomic<long>&)::$_0> >::*&&)(), std::__1::__async_assoc_state<void, std::__1::__async_func<func2(std::__1::atomic<long>&)::$_0> >*&&) ()
#3 0x0000000000204309 in std::__1::future<void> std::__1::__make_async_assoc_state<void, std::__1::__async_func<func2(std::__1::atomic<long>&)::$_0> >(std::__1::__async_func<func2(std::__1::atomic<long>&)::$_0>&&) ()
#4 0x00000000002035ea in std::__1::future<std::__1::__invoke_of<std::__1::decay<func2(std::__1::atomic<long>&)::$_0>::type>::type> std::__1::async<func2(std::__1::atomic<long>&)::$_0>(std::__1::launch, func2(std::__1::atomic<long>&)::$_0&&) ()
#5 0x0000000000203462 in func2(std::__1::atomic<long>&) ()
#6 0x0000000000206f18 in main::$_1::operator()() const ()
#7 0x0000000000206eed in void std::__1::__async_func<main::$_1>::__execute<>(std::__1::__tuple_indices<>) ()
#8 0x0000000000206ea5 in std::__1::__async_func<main::$_1>::operator()() ()
#9 0x0000000000206df3 in std::__1::__async_assoc_state<void, std::__1::__async_func<main::$_1> >::__execute() ()
#10 0x0000000000207183 in void* std::__1::__thread_proxy<std::__1::tuple<std::__1::unique_ptr<std::__1::__thread_struct, std::__1::default_delete<std::__1::__thread_struct> >, void (std::__1::__async_assoc_state<void, std::__1::__async_func<main::$_1> >::*)(), std::__1::__async_assoc_state<void, std::__1::__async_func<main::$_1> >*> >(void*) ()
#11 0x000000080025c776 in thread_start (curthread=0x8007de500) at /usr/src/lib/libthr/thread/thr_create.c:292
#12 0x0000000000000000 in ?? ()
Backtrace stopped: Cannot access memory at address 0x7fffdfffe000
In frame #8, there are three pairs of parentheses, std::__1::__async_func<main::$_1>::operator()() () what do they mean exactly?

Answer 1

Frame #8 doesn't have debug info, so GDB can't accurately describe it.
Consider this test case:
struct Foo {
int operator()(void) {
return 1; // line 3
}
};
int main()
{
return Foo()();
}
When compiled with g++ -g t.cc and at breakpoint on line 3, this is what GDB displays:
Breakpoint 1, Foo::operator() (this=0x7fffffffdcff) at t.cc:3
3 return 1;
(gdb) bt
#0 Foo::operator() (this=0x7fffffffdcff) at t.cc:3
#1 0x0000555555555139 in main () at t.cc:10
But compile the same source without -g, set a breakpoint on _ZN3FooclEv, and this is what you will see:
Breakpoint 1, 0x0000555555555140 in Foo::operator()() ()
(gdb) bt
#0 0x0000555555555140 in Foo::operator()() ()
#1 0x0000555555555139 in main ()
The first two sets of parenthesis come from demangling the symbol:
c++filt _ZN3FooclEv
Foo::operator()()
The third set is added by GDB because the symbol being displayed is in the .text section and is assumed to be a function.