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- const README: rewrite the space-saving claim per issue #5 follow-ups (constant-expression const vs #define immediates; which const qualify as constant expressions) - const README: fix the class-section diagnosis per issue #338 (overload resolution picks the const overload; the restriction only surfaces once that overload is removed) - sizeof README: virtual-inheritance result is compiler-dependent (GCC merges vbptr+vfptr -> 16, MSVC keeps them separate -> 32) #246 - static README: new section on internal linkage / access restriction with anonymous-namespace comparison #142 - 10-day practice: full problem list table + fixed day links #136 - new design_pattern/singleton/singleton_default.hpp: reusable thread-safe CRTP singleton template, compile+concurrency tested #303
80 lines
3.6 KiB
Markdown
80 lines
3.6 KiB
Markdown
> 来自 [C++那些事网站](https://light-city.github.io/stories_things/cpp-tools/stl-debug-mode/) 的文章，由站点同步脚本导出。
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# C++ STL Debug 模式
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## 问题
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Project阶段的schema里面的fields发生了变化，fields是一个vector节点，每个成员是`std::shared_ptr<arrow::Field>`，对比前后的vector指针发现`_M_impl`的`_M_start`位置发生了变化，因此需要gdb watch其变化，查看何处改了它。
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## Vector结构
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vecotr代码：
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> https://github.com/gcc-mirror/gcc/blob/master/libstdc%2B%2B-v3/include/bits/stl_vector.h
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```cpp
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typedef typename __gnu_cxx::__alloc_traits<_Tp_alloc_type>::pointer
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pointer;
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struct _Vector_impl_data
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{
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pointer _M_start;
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pointer _M_finish;
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pointer _M_end_of_storage;
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}
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```
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需要知道几个重要信息：
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- `_M_start`与`_M_finish`等都是pointer类型
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- pointer类型是原始指针类型
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有了这个信息，我们watch便可以非常方便的观察这个`_M_start`发生了什么变化，注意：是这个指针`_M_start`发生了变化，而不是其值发生了变化。
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## Watch
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首先打印其地址:
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```cpp
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(gdb) p &((arrow::Schema::Impl *)this->output_schema_->impl_)->fields_._M_impl._M_start
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$10 = (std::_Vector_base<std::shared_ptr<arrow::Field>, std::allocator<std::shared_ptr<arrow::Field> > >::pointer *) 0xb11dd0
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```
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我们得到了`pointer* `的地址，然后watch它。
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```cpp
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(gdb) watch *((std::_Vector_base<std::shared_ptr<arrow::Field>, std::allocator<std::shared_ptr<arrow::Field> > >::pointer *) 0xb11dd0)
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Hardware watchpoint 2: *((std::_Vector_base<std::shared_ptr<arrow::Field>, std::allocator<std::shared_ptr<arrow::Field> > >::pointer *) 0xb11dd0)
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```
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随后我们continue，变得了变更点：
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```cpp
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(gdb) c
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Continuing.
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Thread 1 "postgres" hit Hardware watchpoint 2: *((std::_Vector_base<std::shared_ptr<arrow::Field>, std::allocator<std::shared_ptr<arrow::Field> > >::pointer *) 0xb11dd0)
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Old value = (std::_Vector_base<std::shared_ptr<arrow::Field>, std::allocator<std::shared_ptr<arrow::Field> > >::pointer) 0x979470
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New value = (std::_Vector_base<std::shared_ptr<arrow::Field>, std::allocator<std::shared_ptr<arrow::Field> > >::pointer) 0x9fe1e0
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VecSeqNext (node=0x9fdc40) at src/backend/vecexecutor/nodeSeqscan.c:210
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210 if (table_scan_getnextslot(scandesc, direction, slot))
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(gdb) bt
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#0 VecSeqNext (node=0x9fdc40) at src/backend/vecexecutor/nodeSeqscan.c:210
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#1 0x00007f45994e5c70 in ExecVecScanFetchInternal
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(node=0x9fdc40, accessMtd=0x7f45994e6335 <VecSeqNext>, recheckMtd=0x7f45994e6430 <VecSeqRecheck>) at src/backend/vecexecutor/execScan.c:146
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#2 0x00007f45994e58da in ExecVecScanFetch (node=0x9fdc40, accessMtd=0x7f45994e6335 <VecSeqNext>, recheckMtd=0x7f45994e6430 <VecSeqRecheck>)
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at src/backend/vecexecutor/execScan.c:40
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#3 0x00007f45994e5d0b in ExecVecScan
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(node=0x9fdc40, vestate=0x9fdd48, accessMtd=0x7f45994e6335 <VecSeqNext>, recheckMtd=0x7f45994e6430 <VecSeqRecheck>)
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at src/backend/vecexecutor/execScan.c:200
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#4 0x00007f45994e6487 in ExecVecSeqScan (pstate=0x9fdc40) at src/backend/vecexecutor/nodeSeqscan.c:246
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#5 0x00007f45994c40ce in VecExecProcNodeGPDB (node=0x9fdc40) at src/backend/hook/executor.c:102
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#6 0x00007f45994c41c2 in VecExecProcNodeFirst (node=0x9fdc40) at src/backend/hook/executor.c:153
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#7 0x00007f45994dc610 in ExecProcNode (node=0x9fdc40) at /home/gpadmin/install/cbdb/include/postgresql/server/executor/executor.h:281
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#8 0x00007f45994e2065 in get_current_next_batch (node=0x9fdc40) at src/backend/vecexecutor/execMain.c:1851
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#9 0x00007f4599375c76 in get_arrow_record_batch(GetNextCallback, void*) (fn=0x7f45994e204d <get_current_next_batch>, arg=0x9fdc40)
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at c_glib/arrow-glib/reader.cpp:216
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``` |