c - Valgrind 报告无效的 Realloc

Question

我正在努力补充我的 C 内存管理知识。我的背景主要是脚本编写和管理，我想了解更多有关 C 和 C++ 的知识。为此，我读了几本书，其中包括使用 realloc 修剪一串空格的示例:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

char* trim(char* phrase)
{
  char* old = phrase;
  char* new = phrase;

  while(*old == ' ') {
    old++;
  }

  while(*old) {
    *(new++) = *(old++);
  }

  *new = 0;

  return (char*)realloc(phrase, strlen(phrase)+1);
}

int main ()
{
  char* buffer = (char*)malloc(strlen("  cat")+1);
  strcpy(buffer, "  cat");

  printf("%s\n", trim(buffer));

  free(buffer);
  buffer=NULL;

  return 0;
}

我忠实地复制了示例，并使用 c99 -Wall -Wpointer-arith -O3 -pedantic -march=native 编译。我没有遇到任何编译错误，并且该应用程序运行并按照书中所 promise 的进行，但是当我针对 valgrind 运行它时，我收到有关无效 realloc 的错误。

==21601== Memcheck, a memory error detector
==21601== Copyright (C) 2002-2013, and GNU GPL'd, by Julian Seward et al.
==21601== Using Valgrind-3.10.0.SVN and LibVEX; rerun with -h for copyright info
==21601== Command: ./trim
==21601== 
==21601== Invalid free() / delete / delete[] / realloc()
==21601==    at 0x402B3D8: free (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
==21601==    by 0x804844E: main (in /home/mo/programming/learning_pointers/trim)
==21601==  Address 0x4202028 is 0 bytes inside a block of size 6 free'd
==21601==    at 0x402C324: realloc (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
==21601==    by 0x80485A9: trim (in /home/mo/programming/learning_pointers/trim)
==21601==    by 0x804842E: main (in /home/mo/programming/learning_pointers/trim)
==21601== 
==21601== 
==21601== HEAP SUMMARY:
==21601==     in use at exit: 4 bytes in 1 blocks
==21601==   total heap usage: 2 allocs, 2 frees, 10 bytes allocated
==21601== 
==21601== 4 bytes in 1 blocks are definitely lost in loss record 1 of 1
==21601==    at 0x402C324: realloc (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
==21601==    by 0x80485A9: trim (in /home/mo/programming/learning_pointers/trim)
==21601==    by 0x804842E: main (in /home/mo/programming/learning_pointers/trim)
==21601== 
==21601== LEAK SUMMARY:
==21601==    definitely lost: 4 bytes in 1 blocks
==21601==    indirectly lost: 0 bytes in 0 blocks
==21601==      possibly lost: 0 bytes in 0 blocks
==21601==    still reachable: 0 bytes in 0 blocks
==21601==         suppressed: 0 bytes in 0 blocks
==21601== 
==21601== For counts of detected and suppressed errors, rerun with: -v
==21601== ERROR SUMMARY: 2 errors from 2 contexts (suppressed: 0 from 0)

所以请帮助我理解为什么它被认为是无效的重新分配。这个例子是废话吗？有什么我想念的吗？我知道根据规范，realloc 期望指针先前已由 malloc 创建，这是因为 realloc 在另一个函数中吗？还是因为 valgrind 在不同的函数中而感到困惑？我不是一个彻头彻尾的白痴(大多数时候)，但现在我有点像一个没有看到问题的人。

提前致谢!

Answer 1

您正在尝试释放 原始指针，而不是reallocd 指针。您可以通过以下方式修复它:

buffer = trim(buffer)