valgrind：多次 std::vector::resize 调用

Question

我一直在追踪大型 C++ 代码库中的一个差一问题。出于某种原因，我无法理解以下 Valgrind 行为。有人可以在这里阐明一下吗？

代码是：

% cat foo.cxx
#include <cstring>
#include <string>
#include <vector>

int main() {
  std::vector<char> v;
#ifdef RESIZE9
  v.resize(9);
#endif
  v.resize(10);

  std::string s(10, 'x');
  std::strcpy(&v[0], s.c_str());

  return 0;
}

这是预期的 Valgrind 输出：

% g++ foo.cxx && valgrind ./a.out
==21886== Memcheck, a memory error detector
==21886== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==21886== Using Valgrind-3.14.0 and LibVEX; rerun with -h for copyright info
==21886== Command: ./a.out
==21886==
==21886== Invalid write of size 1
==21886==    at 0x4838DD7: strcpy (in /usr/lib/x86_64-linux-gnu/valgrind/vgpreload_memcheck-amd64-linux.so)
==21886==    by 0x1092CA: main (in /tmp/a.out)
==21886==  Address 0x4d84c8a is 0 bytes after a block of size 10 alloc'd
==21886==    at 0x4835DEF: operator new(unsigned long) (in /usr/lib/x86_64-linux-gnu/valgrind/vgpreload_memcheck-amd64-linux.so)
==21886==    by 0x109BCD: __gnu_cxx::new_allocator<char>::allocate(unsigned long, void const*) (in /tmp/a.out)
==21886==    by 0x109AD5: std::allocator_traits<std::allocator<char> >::allocate(std::allocator<char>&, unsigned long) (in /tmp/a.out)
==21886==    by 0x109997: std::_Vector_base<char, std::allocator<char> >::_M_allocate(unsigned long) (in /tmp/a.out)
==21886==    by 0x1095E8: std::vector<char, std::allocator<char> >::_M_default_append(unsigned long) (in /tmp/a.out)
==21886==    by 0x1093CC: std::vector<char, std::allocator<char> >::resize(unsigned long) (in /tmp/a.out)
==21886==    by 0x10926A: main (in /tmp/a.out)
==21886==
==21886==
==21886== HEAP SUMMARY:
==21886==     in use at exit: 0 bytes in 0 blocks
==21886==   total heap usage: 2 allocs, 2 frees, 72,714 bytes allocated
==21886==
==21886== All heap blocks were freed -- no leaks are possible
==21886==
==21886== For counts of detected and suppressed errors, rerun with: -v
==21886== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0)

但现在考虑以下问题：

% g++ -DRESIZE9 foo.cxx && valgrind ./a.out
==21904== Memcheck, a memory error detector
==21904== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==21904== Using Valgrind-3.14.0 and LibVEX; rerun with -h for copyright info
==21904== Command: ./a.out
==21904==
==21904==
==21904== HEAP SUMMARY:
==21904==     in use at exit: 0 bytes in 0 blocks
==21904==   total heap usage: 3 allocs, 3 frees, 72,731 bytes allocated
==21904==
==21904== All heap blocks were freed -- no leaks are possible
==21904==
==21904== For counts of detected and suppressed errors, rerun with: -v
==21904== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

系统是 Debian/10.9，其中：

% g++ --version
g++ (Debian 8.3.0-6) 8.3.0
Copyright (C) 2018 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

和

% valgrind --version
valgrind-3.14.0

Answer 1

虽然我无法确认这是一个完整（跨平台）'solution'，但添加一行以显示实际的容量调整大小操作后的矢量可能会有所启发：

#include <cstring>
#include <string>
#include <vector>
#include <iostream>

//#define RESIZE9 1

int main()
{
    std::vector<char> v;
    #ifdef RESIZE9
    v.resize(9);
    #endif
    v.resize(10);

    std::cout << v.capacity() << std::endl; // Show the actual allocated size

    std::string s(10, 'x');
    std::strcpy(&v[0], s.c_str());

    return 0;
}

运行此代码原样 (Visual Studio, MSVC, Windows 10, 64-bit) 显示容量为 10（意料之中）。但是，当取消注释 #define RESIZE9 1 行时，显示的容量（在两次调整大小调用之后）是 13.

添加额外的容量，我相信，在标准的要求之内：只要新分配的 vector 有 足够的容量 来满足新的大小，没有什么是坏的。分配 4 个额外字节（而不是一个字节）最有可能优化内存管理。

valgrind：多次 std::vector::resize 调用

valgrind: Multiple std::vector::resize calls

c++

valgrind