c – 最小程序在g上产生Valgrind警告,但不产生clang或VS.

我一直在疯狂追踪中型项目中的内存错误.我将一个最小化到下面的代码,并验证这导致Valgrind警告在注释行号处无效读取.我正在使用g 4.8.2(Debian 4.8.2-16)进行项目,使用-std = c 11 -O0进行编译. g 4.7.0也产生错误,4.4.6略微修改版本(删除C 11). clang不会导致错误,VS2013也不会.

我的问题是：

>我是否在不知不觉中从事未定义的行为？
>这是Valgrind的假阳性,如果是这样的话,我怎么能在将来保证自己这样/这样的情况呢？

这段代码相当脆弱 – 特别是,让foo通过引用获取coord消除了这个问题,就像为Coord选择更大的类型一样,它的内存大小超过8个字节.

// Types are a little strange because I wanted the minimal failing
// case -- eg,row/col used to be ints.
#include <cassert>
#include <iostream>
#include <vector>

class Coord {
  public:
    bool row,col,layer;
};

void foo(Coord wtf) { }

std::vector<Coord> baz() {
  std::vector<Coord> bees;
  for (int i = 0; i < 4; ++i) {
    bees.push_back({1,1,1});
  }
  return bees;
}


int main(int argc,char** argv) {
  auto vec = baz();
  assert(vec.size() > 3);
  Coord bbb = vec[3];
  foo(bbb);
  static int i = 0;
  foo(vec.at(3)); // Causes Valgrind warning.

  Coord& ccc = vec[3];
  foo(ccc);      // causes Valgrind warning
}

Valgrind输出：

==17700== Memcheck,a memory error detector
==17700== Copyright (C) 2002-2013,and GNU GPL'd,by Julian Seward et al.
==17700== Using Valgrind-3.9.0 and LibVEX; rerun with -h for copyright info
==17700== Command: ./a.out
==17700== 
==17700== Invalid read of size 8
==17700==    at 0x400BC9: main (in /home/alexr/projects/suncatcher/a.out)
==17700==  Address 0x59f80e9 is 9 bytes inside a block of size 12 alloc'd
==17700==    at 0x4C27A00: operator new(unsigned long) (vg_replace_malloc.c:319)
==17700==    by 0x40150A: __gnu_cxx::new_allocator<Coord>::allocate(unsigned long,void const*) (in /home/alexr/projects/suncatcher/a.out)
==17700==    by 0x4013A2: std::_Vector_base<Coord,std::allocator<Coord> >::_M_allocate(unsigned long) (in /home/alexr/projects/suncatcher/a.out)
==17700==    by 0x401061: void std::vector<Coord,std::allocator<Coord> >::_M_emplace_back_aux<Coord>(Coord&&) (in /home/alexr/projects/suncatcher/a.out)
==17700==    by 0x400F02: void std::vector<Coord,std::allocator<Coord> >::emplace_back<Coord>(Coord&&) (in /home/alexr/projects/suncatcher/a.out)
==17700==    by 0x400D25: std::vector<Coord,std::allocator<Coord> >::push_back(Coord&&) (in /home/alexr/projects/suncatcher/a.out)
==17700==    by 0x400B17: baz() (in /home/alexr/projects/suncatcher/a.out)
==17700==    by 0x400B64: main (in /home/alexr/projects/suncatcher/a.out)
==17700== 
==17700== Invalid read of size 8
==17700==    at 0x400BEA: main (in /home/alexr/projects/suncatcher/a.out)
==17700==  Address 0x59f80e9 is 9 bytes inside a block of size 12 alloc'd
==17700==    at 0x4C27A00: operator new(unsigned long) (vg_replace_malloc.c:319)
==17700==    by 0x40150A: __gnu_cxx::new_allocator<Coord>::allocate(unsigned long,std::allocator<Coord> >::push_back(Coord&&) (in /home/alexr/projects/suncatcher/a.out)
==17700==    by 0x400B17: baz() (in /home/alexr/projects/suncatcher/a.out)
==17700==    by 0x400B64: main (in /home/alexr/projects/suncatcher/a.out)
==17700== 
==17700== 
==17700== HEAP SUMMARY:
==17700==     in use at exit: 0 bytes in 0 blocks
==17700==   total heap usage: 3 allocs,3 frees,21 bytes allocated
==17700== 
==17700== All heap blocks were freed -- no leaks are possible
==17700== 
==17700== For counts of detected and suppressed errors,rerun with: -v
==17700== ERROR SUMMARY: 2 errors from 2 contexts (suppressed: 2 from 2)

#ifdef SOMETHING_SOMETHING
#define SUPRESS_VALGRIND_FALSE_POSITIVES __attribute__((__aligned__(8)))
#else
#define SUPRESS_VALGRIND_FALSE_POSITIVES
#endif

...
class Coord {
  public:
    bool row,layer;
} SUPRESS_VALGRIND_FALSE_POSITIVES ;