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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
3.6 KiB
3.6 KiB
来自 C++那些事网站 的文章，由站点同步脚本导出。
C++ STL Debug 模式
问题
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。
Vector结构
vecotr代码：
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;
}
需要知道几个重要信息：
_M_start与_M_finish等都是pointer类型- pointer类型是原始指针类型
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。
Watch
首先打印其地址:
(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
我们得到了pointer* 的地址，然后watch它。
(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)
随后我们continue，变得了变更点：
(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