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这段代码中的内存泄漏在哪里?如何修复它?

valgrind memory-leaks c

citronas · 技术社区 · 14 年前

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main(int argc, char* argv[] )
{
 char* output = "\0";
 int counter = 5;
  while(counter > 0)
  {
      char buffer[20];
      sprintf(buffer, "%u", counter);
      char* temp;
      temp = malloc((strlen(output) + strlen(buffer) + 1));
      strcpy(temp, buffer);
      strcat(temp, output);
      char* oldmemory = output;
      output = temp;
      free(oldmemory);
      counter--;
  }
printf("output: %s\n", output);
free(output);
return 0;
}

退货:

==7125== Memcheck, a memory error detector
==7125== Copyright (C) 2002-2009, and GNU GPL'd, by Julian Seward et al.
==7125== Using Valgrind-3.6.0.SVN-Debian and LibVEX; rerun with -h for copyright info
==7125== Command: ./foo
==7125== Parent PID: 4455
==7125== 
==7125== Invalid free() / delete / delete[]
==7125==    at 0x4024B3A: free (vg_replace_malloc.c:366)
==7125==    by 0x8048662: main (foo.c:20)
==7125==  Address 0x8048780 is not stack'd, malloc'd or (recently) free'd
==7125== 
==7125== 
==7125== HEAP SUMMARY:
==7125==     in use at exit: 0 bytes in 0 blocks
==7125==   total heap usage: 5 allocs, 6 frees, 20 bytes allocated
==7125== 
==7125== All heap blocks were freed -- no leaks are possible
==7125== 
==7125== For counts of detected and suppressed errors, rerun with: -v
==7125== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 15 from 8)

内存泄漏在哪里?如何修复?

4 回复 | 直到 14 年前

Philip Potter 14 年前

#include <stdio.h>
#include <stdlib.h>
int main(int argc, char* argv[] )
{
 char* output = "\0";

 int counter = 5;
  while(counter > 0)
  {
      char buffer[20];
      sprintf(buffer, "%u", counter);
      char* temp;
      temp = malloc((strlen(output) + strlen(buffer) + 1));
      strcpy(temp, buffer);
      strcat(temp, output);
      char* oldmemory = output;
      output = temp;
      free(oldmemory);

这是我第一次 free() 调用时,释放输出的初始值,即指向字符串文字的指针 "\0" . 打电话在从返回的有效指针以外的任何对象上 *alloc() 或 NULL 是未定义的行为。

      counter--;
  }
printf("output: %s\n", output);
free(output);
return 0;
}

valgrind报告:

==7125== Invalid free() / delete / delete[]
==7125==    at 0x4024B3A: free (vg_replace_malloc.c:366)
==7125==    by 0x8048662: main (foo.c:20)
==7125==  Address 0x8048780 is not stack'd, malloc'd or (recently) free'd

免费() .

sizzzzlerz 14 年前

你的密码坏了。第一步,将oldmemory设置为output,其中output指向堆上未分配的内存。稍后,您将尝试释放此内存。这将生成关于释放未通过malloc分配的内存的valgrind错误。因此,您分配的原始内存永远不会被释放。

bramp Madhur Ahuja 14 年前

不要使用malloc和strcpy,请看realloc,它可以做您想要做的一切,但更好:)但是您很可能希望向前构建字符串(counter=0;counter<5;count++),而不是向后

Logan Capaldo 14 年前

如果要使用此算法,则应使用malloc分配输出点的初始空间,因此:

 char *output = malloc(1);
 if(!output) { /* handle error */ }
 output[0] = '\0';
 ... rest of code as is ...

字符串文字未分配给 malloc ,因此不能 free 埃德,这是你问题的根源。