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Last updated：2026-10-12

C++ STL Debug Mode

The Problem

During the Project stage, the fields in the schema changed. fields is a vector node whose members are std::shared_ptr<arrow::Field>; comparing the vector pointers before and after, I found that the _M_start position of _M_impl had changed, so I needed to set a gdb watch on it to see where it was modified.

The Vector Structure

The vector code:

https://github.com/gcc-mirror/gcc/blob/master/libstdc%2B%2B-v3/include/bits/stl_vector.h

typedef typename __gnu_cxx::__alloc_traits<_Tp_alloc_type>::pointer
        pointer;
struct _Vector_impl_data
{
    pointer _M_start;
    pointer _M_finish;
    pointer _M_end_of_storage;
}

A few important pieces of information:

  • _M_start, _M_finish, and so on are all of pointer type
  • The pointer type is a raw pointer type

With this information, we can use watch to conveniently observe what changed about _M_start. Note that it is the pointer _M_start itself that changed, not its value.

Watch

First, print its address:

(gdb) p &((arrow::Schema::Impl *)this->output_schema_->impl_)->fields_._M_impl._M_start
$10 = (std::_Vector_base<std::shared_ptr<arrow::Field>, std::allocator<std::shared_ptr<arrow::Field> > >::pointer *) 0xb11dd0

We obtained the address of the pointer*; now watch it.

(gdb) watch *((std::_Vector_base<std::shared_ptr<arrow::Field>, std::allocator<std::shared_ptr<arrow::Field> > >::pointer *) 0xb11dd0)
Hardware watchpoint 2: *((std::_Vector_base<std::shared_ptr<arrow::Field>, std::allocator<std::shared_ptr<arrow::Field> > >::pointer *) 0xb11dd0)

Then we continue, and we land at the point of the change:

(gdb) c
Continuing.


Thread 1 "postgres" hit Hardware watchpoint 2: *((std::_Vector_base<std::shared_ptr<arrow::Field>, std::allocator<std::shared_ptr<arrow::Field> > >::pointer *) 0xb11dd0)

Old value = (std::_Vector_base<std::shared_ptr<arrow::Field>, std::allocator<std::shared_ptr<arrow::Field> > >::pointer) 0x979470
New value = (std::_Vector_base<std::shared_ptr<arrow::Field>, std::allocator<std::shared_ptr<arrow::Field> > >::pointer) 0x9fe1e0
VecSeqNext (node=0x9fdc40) at src/backend/vecexecutor/nodeSeqscan.c:210
210     if (table_scan_getnextslot(scandesc, direction, slot))
(gdb) bt
#0  VecSeqNext (node=0x9fdc40) at src/backend/vecexecutor/nodeSeqscan.c:210
#1  0x00007f45994e5c70 in ExecVecScanFetchInternal
    (node=0x9fdc40, accessMtd=0x7f45994e6335 <VecSeqNext>, recheckMtd=0x7f45994e6430 <VecSeqRecheck>) at src/backend/vecexecutor/execScan.c:146
#2  0x00007f45994e58da in ExecVecScanFetch (node=0x9fdc40, accessMtd=0x7f45994e6335 <VecSeqNext>, recheckMtd=0x7f45994e6430 <VecSeqRecheck>)
    at src/backend/vecexecutor/execScan.c:40
#3  0x00007f45994e5d0b in ExecVecScan
    (node=0x9fdc40, vestate=0x9fdd48, accessMtd=0x7f45994e6335 <VecSeqNext>, recheckMtd=0x7f45994e6430 <VecSeqRecheck>)
    at src/backend/vecexecutor/execScan.c:200
#4  0x00007f45994e6487 in ExecVecSeqScan (pstate=0x9fdc40) at src/backend/vecexecutor/nodeSeqscan.c:246
#5  0x00007f45994c40ce in VecExecProcNodeGPDB (node=0x9fdc40) at src/backend/hook/executor.c:102
#6  0x00007f45994c41c2 in VecExecProcNodeFirst (node=0x9fdc40) at src/backend/hook/executor.c:153
#7  0x00007f45994dc610 in ExecProcNode (node=0x9fdc40) at /home/gpadmin/install/cbdb/include/postgresql/server/executor/executor.h:281
#8  0x00007f45994e2065 in get_current_next_batch (node=0x9fdc40) at src/backend/vecexecutor/execMain.c:1851
#9  0x00007f4599375c76 in get_arrow_record_batch(GetNextCallback, void*) (fn=0x7f45994e204d <get_current_next_batch>, arg=0x9fdc40)
    at c_glib/arrow-glib/reader.cpp:216

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