Unexpected breakpoint in ntdll when debuggin VERY SIMPLE C++ programs

[rovinidest]

Recently I've occasionally encountered strange breakpoints in Qt Creator when debugging some simple plain C++ programs.

According Qt Creator, The breakpoint occurs in Disassembler (ntdll!DbgBreakPoint). More precisely, in debugger log:

Program received signal
SIGTRAP, Trace/breakpoint trap.
[Switching to Thread 4632.0x13e4]
0x7718000d in ntdll!DbgBreakPoint () from C:\Windows\SysWOW64\ntdll.dll
*stopped,reason="signal-received",signal-name="SIGTRAP",signal-meaning="Trace/breakpoint trap",frame={addr="0x7718000d",func="ntdll!DbgBreakPoint",args=[],from="C:\\Windows\\SysWOW64\\ntdll.dll"},thread-id="2",stopped-threads="all"
NOTE: INFERIOR SPONTANEOUS STOP
Stopped.
State changed from InferiorRunOk(11) to InferiorStopOk(14) [master]
105importPlainDumpers off
SIGTRAP CONSIDERED HARMLESS. CONTINUING.
Stopped: "signal-received".
=thread-selected,id="2"
Thread 2 selected
......
Dump of assembler code for function ntdll!DbgBreakPoint:
   0x7718000c <+0>:    cc             int3
=> 0x7718000d <+1>:    c3             ret     **where the code breaks**
   0x7718000e <+2>:    90             nop
   0x7718000f <+3>:    90             nop
   0x77180010 <+4>:    90             nop
   0x77180011 <+5>:    90             nop
   0x77180012 <+6>:    90             nop
   0x77180013 <+7>:    90             nop
   0x77180014 <+8>:    90             nop
   0x77180015 <+9>:    90             nop
   0x77180016 <+10>:   90             nop
   0x77180017 <+11>:   90             nop
   0x77180018 <+12>:   90             nop
   0x77180019 <+13>:   90             nop
   0x7718001a <+14>:   90             nop
   0x7718001b <+15>:   90             nop
   0x7718001c <+16>:   90             nop
   0x7718001d <+17>:   90             nop
   0x7718001e <+18>:   90             nop
   0x7718001f <+19>:   90             nop
   0x77180020 <+20>:   8b 4c 24 04    mov    0x4(%esp),%ecx
   0x77180024 <+24>:   f6 41 04 06    testb  $0x6,0x4(%ecx)
   0x77180028 <+28>:   74 05          je     0x7718002f <ntdll!DbgBreakPoint+35>
   0x7718002a <+30>:   e8 a1 1d 01 00 call   0x77191dd0 <ntdll!ZwTestAlert>
   0x7718002f <+35>:   b8 01 00 00 00 mov    $0x1,%eax
   0x77180034 <+40>:   c2 10 00       ret    $0x10
   0x77180037 <+43>:   90             nop
End of assembler dump.

Program received signal
SIGTRAP, Trace/breakpoint trap.
[Switching to Thread 4632.0x13e4]
0x7718000d in ntdll!DbgBreakPoint () from C:\Windows\SysWOW64\ntdll.dll
*stopped,reason="signal-received",signal-name="SIGTRAP",signal-meaning="Trace/breakpoint trap",frame={addr="0x7718000d",func="ntdll!DbgBreakPoint",args=[],from="C:\\Windows\\SysWOW64\\ntdll.dll"},thread-id="2",stopped-threads="all"
NOTE: INFERIOR SPONTANEOUS STOP
Stopped.
State changed from InferiorRunOk(11) to InferiorStopOk(14) [master]
105importPlainDumpers off
SIGTRAP CONSIDERED HARMLESS. CONTINUING.
Stopped: "signal-received".
=thread-selected,id="2"
Thread 2 selected
......
Dump of assembler code for function ntdll!DbgBreakPoint:
   0x7718000c <+0>:    cc             int3
=> 0x7718000d <+1>:    c3             ret     **where the code breaks**
   0x7718000e <+2>:    90             nop
   0x7718000f <+3>:    90             nop
   0x77180010 <+4>:    90             nop
   0x77180011 <+5>:    90             nop
   0x77180012 <+6>:    90             nop
   0x77180013 <+7>:    90             nop
   0x77180014 <+8>:    90             nop
   0x77180015 <+9>:    90             nop
   0x77180016 <+10>:   90             nop
   0x77180017 <+11>:   90             nop
   0x77180018 <+12>:   90             nop
   0x77180019 <+13>:   90             nop
   0x7718001a <+14>:   90             nop
   0x7718001b <+15>:   90             nop
   0x7718001c <+16>:   90             nop
   0x7718001d <+17>:   90             nop
   0x7718001e <+18>:   90             nop
   0x7718001f <+19>:   90             nop
   0x77180020 <+20>:   8b 4c 24 04    mov    0x4(%esp),%ecx
   0x77180024 <+24>:   f6 41 04 06    testb  $0x6,0x4(%ecx)
   0x77180028 <+28>:   74 05          je     0x7718002f <ntdll!DbgBreakPoint+35>
   0x7718002a <+30>:   e8 a1 1d 01 00 call   0x77191dd0 <ntdll!ZwTestAlert>
   0x7718002f <+35>:   b8 01 00 00 00 mov    $0x1,%eax
   0x77180034 <+40>:   c2 10 00       ret    $0x10
   0x77180037 <+43>:   90             nop
End of assembler dump.

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1. This behavior only occurs occasionally. I've not found a way to reproduce it consistently.
2. The C++ code is VERY simple. Two entire examples shown below.
3. This behavior is only observed when debugging the code in Qt Creator (using MinGW 5.3.0 32-bit).
4. The breakpoint occurs in a 2nd thread, not the one where the main function is. This 2nd thread seems to only exist when debugging the code.
5. I've tried to reproduce this behavior outside of Qt Creator, such as using gdb directly from cmd, but with no success.
6. I've read posts/articles on similar problems such as here and here. Both points to heap corruption. However, my example code is SO SIMPLE that I don't see how heap corruption can occur in my code. Or am I missing something extremely basic?
7. Qt version 5.7.0 (probably irrelevant)
   Qt Creator version 4.1.0
   OS: Windows 7 Pro SP1 64-bit

Any comments / suggestions / solutions welcomed.
Thanks in advance!

Example code 1:

#include <vector>
#include <iostream>
using namespace std;
 
int main()
{
    vector<int> v;
    for(long long i=0; i<1000000; i++)
        v.push_back(i);
    cout << v.size() << endl;
}

#include <vector>
#include <iostream>
using namespace std;

int main()
{
    vector<int> v;
    for(long long i=0; i<1000000; i++)
        v.push_back(i);
    cout << v.size() << endl;
}

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Example code 2:

#include <iostream>
#include <fstream>
using namespace std;
 
int main()
{
    ofstream f("..\\fio\\a.txt");
    if( !f.is_open() ) {
        cerr << "failed to open file" << endl;
        exit(EXIT_FAILURE);
    }
    for(int i=0; i<10000; i++)
        f << i << endl;
    f.close();
}

#include <iostream>
#include <fstream>
using namespace std;

int main()
{
    ofstream f("..\\fio\\a.txt");
    if( !f.is_open() ) {
        cerr << "failed to open file" << endl;
        exit(EXIT_FAILURE);
    }
    for(int i=0; i<10000; i++)
        f << i << endl;
    f.close();
}

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[ferdna]

I have this same issue. Only happens when running it from Qt Creator but other than that I can compile and run it with out problems.

- ntdll!DbgBreakpoint
- ntdll!DbgUiRemoteBreakin

Qt 5.10.0
Qt Creator 4.5.0
win7 ultimate sp1 x64 (inside virtualbox)