7

这是我的 C 程序：

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <time.h>
#include <ctype.h>
#define FALSE 0
#define TRUE 1

typedef struct _Frag
{
  struct _Frag *next;
  char *seq;
  int x1;
  int length;
} Frag;

typedef struct _Fragment
{
  int type;
  Frag *frag_list;
} Fragment;

static void
free_frags (Fragment * frags, int len)
{
  int i;
  for (i = 0; i < len; i++)
    {
      Fragment *fragment = &frags[i];
      Frag *current = fragment->frag_list;

      while (current != NULL)
    {
      free (current->seq);
      fragment->frag_list = current->next;
      free (current);
      current = fragment->frag_list;
    }

      /* to do : free fragment */
      free (fragment);
      fragment = NULL;
    }
  free (frags);
}

int
main ()
{
  Fragment *frags = (Fragment *) malloc (10 * sizeof (Fragment));
  int i, j;
  for (i = 0; i < 10; i++)
    {
      Fragment *fragment = &frags[i];
      fragment->frag_list = (Frag *) malloc (1 * sizeof (Frag));
      Frag *frag = fragment->frag_list;
      frag->seq = malloc (6 * sizeof (char));
      strcpy (frag->seq, "55555");
      frag->next = (Frag *) malloc (1 * sizeof (Frag));
      frag = frag->next;
      frag->seq = malloc (6 * sizeof (char));
      strcpy (frag->seq, "55555");
      frag->next=NULL;
    }
  free_frags (frags, 10);
  return 0;
}

当我用 gdb 调试它时，错误消息是：

(gdb) run a.out 
..........................
..........................
09574000-09595000 rwxp 00000000 00:00 0          [heap]
b7e00000-b7e21000 rwxp 00000000 00:00 0 
b7e21000-b7f00000 ---p 00000000 00:00 0 
b7f2e000-b7f4b000 r-xp 00000000 08:08 298454     /usr/lib/libgcc_s.so.1
b7f4b000-b7f4c000 rwxp 0001c000 08:08 298454     /usr/lib/libgcc_s.so.1
b7f4c000-b7f4d000 rwxp 00000000 00:00 0 
b7f4d000-b808d000 r-xp 00000000 08:08 67152259   /lib/libc-2.10.1.so
b808d000-b808f000 r-xp 0013f000 08:08 67152259   /lib/libc-2.10.1.so
b808f000-b8090000 rwxp 00141000 08:08 67152259   /lib/libc-2.10.1.so
b8090000-b8094000 rwxp 00000000 00:00 0 
b80ae000-b80af000 r-xp 00000000 00:00 0          [vdso]
b80af000-b80cb000 r-xp 00000000 08:08 67152744   /lib/ld-2.10.1.so
b80cb000-b80cc000 r-xp 0001b000 08:08 67152744   /lib/ld-2.10.1.so
b80cc000-b80cd000 rwxp 0001c000 08:08 67152744   /lib/ld-2.10.1.so
bfc0f000-bfc24000 rw-p 00000000 00:00 0          [stack]

Program received signal SIGABRT, Aborted.
0xb80ae424 in __kernel_vsyscall ()
(gdb) where
#0  0xb80ae424 in __kernel_vsyscall ()
#1  0xb7f77411 in raise () from /lib/libc.so.6
#2  0xb7f78c12 in abort () from /lib/libc.so.6
#3  0xb7fb271d in __libc_message () from /lib/libc.so.6
#4  0xb7fb8581 in malloc_printerr () from /lib/libc.so.6
#5  0xb7fb9c82 in _int_free () from /lib/libc.so.6
#6  0xb7fbcd4d in free () from /lib/libc.so.6
#7  0x08048488 in free_frags (frags=0x9574008, len=10) at main.c:41
#8  0x080485b3 in main () at main.c:65
(gdb)

valgrind 消息如下：

==2832== Memcheck, a memory error detector.
==2832== Copyright (C) 2002-2008, and GNU GPL'd, by Julian Seward et al.
==2832== Using LibVEX rev 1884, a library for dynamic binary translation.
==2832== Copyright (C) 2004-2008, and GNU GPL'd, by OpenWorks LLP.
==2832== Using valgrind-3.4.1, a dynamic binary instrumentation framework.
==2832== Copyright (C) 2000-2008, and GNU GPL'd, by Julian Seward et al.
==2832== For more details, rerun with: -v
==2832== 
==2832== Invalid read of size 4
==2832==    at 0x8048442: free_frags (main.c:31)
==2832==    by 0x80485B2: main (main.c:66)
==2832==  Address 0x418b034 is 12 bytes inside a block of size 80 free'd
==2832==    at 0x4023EBA: free (in /usr/lib/valgrind/x86-linux/vgpreload_memcheck.so)
==2832==    by 0x8048487: free_frags (main.c:42)
==2832==    by 0x80485B2: main (main.c:66)
==2832== 
==2832== Invalid write of size 4
==2832==    at 0x8048460: free_frags (main.c:36)
==2832==    by 0x80485B2: main (main.c:66)
==2832==  Address 0x418b034 is 12 bytes inside a block of size 80 free'd
==2832==    at 0x4023EBA: free (in /usr/lib/valgrind/x86-linux/vgpreload_memcheck.so)
==2832==    by 0x8048487: free_frags (main.c:42)
==2832==    by 0x80485B2: main (main.c:66)
==2832== 
==2832== Invalid read of size 4
==2832==    at 0x8048471: free_frags (main.c:38)
==2832==    by 0x80485B2: main (main.c:66)
==2832==  Address 0x418b034 is 12 bytes inside a block of size 80 free'd
==2832==    at 0x4023EBA: free (in /usr/lib/valgrind/x86-linux/vgpreload_memcheck.so)
==2832==    by 0x8048487: free_frags (main.c:42)
==2832==    by 0x80485B2: main (main.c:66)
==2832== 
==2832== Invalid free() / delete / delete[]
==2832==    at 0x4023EBA: free (in /usr/lib/valgrind/x86-linux/vgpreload_memcheck.so)
==2832==    by 0x8048487: free_frags (main.c:42)
==2832==    by 0x80485B2: main (main.c:66)
==2832==  Address 0x418b030 is 8 bytes inside a block of size 80 free'd
==2832==    at 0x4023EBA: free (in /usr/lib/valgrind/x86-linux/vgpreload_memcheck.so)
==2832==    by 0x8048487: free_frags (main.c:42)
==2832==    by 0x80485B2: main (main.c:66)
==2832== 
==2832== Invalid free() / delete / delete[]
==2832==    at 0x4023EBA: free (in /usr/lib/valgrind/x86-linux/vgpreload_memcheck.so)
==2832==    by 0x80484A5: free_frags (main.c:45)
==2832==    by 0x80485B2: main (main.c:66)
==2832==  Address 0x418b028 is 0 bytes inside a block of size 80 free'd
==2832==    at 0x4023EBA: free (in /usr/lib/valgrind/x86-linux/vgpreload_memcheck.so)
==2832==    by 0x8048487: free_frags (main.c:42)
==2832==    by 0x80485B2: main (main.c:66)
==2832== 
==2832== ERROR SUMMARY: 55 errors from 5 contexts (suppressed: 13 from 1)
==2832== malloc/free: in use at exit: 0 bytes in 0 blocks.
==2832== malloc/free: 41 allocs, 51 frees, 520 bytes allocated.
==2832== For counts of detected errors, rerun with: -v
==2832== All heap blocks were freed -- no leaks are possible.

请帮我修复它们，谢谢。

4

6 回答 6

10

你有多个问题。

在这里，您为 5 个字符分配空间，但复制 6 个字符（字符串末尾的 nul 终止符也需要一个空格）：

  frag->seq = malloc (5 * sizeof (char));
  strcpy (frag->seq, "55555");

同时，您永远不会在分配frag->next的那一秒内设置。frag您需要将其设置为NULL，以便例程中的while循环free_frag不会跑到杂草中。

第三个问题在这里：

  /* to do : free fragment */
  free (fragment);

你释放fragment了，但它不是一个完整的块malloc——它只是你一次性分配的 10 个片段的单个块之一。后者free(frags)会正确释放该块，因此您只需要删除该错误行。

于 2009-09-03T06:52:41.667 回答
7

您正在尝试释放阵列的中间部分。

Fragment *fragment = &frags[i];
...
...
/* to do : free fragment */
free (fragment);
fragment = NULL;
于 2009-09-03T06:44:52.607 回答
7

这些行似乎有Bufferoverflow

  frag->seq = malloc (5 * sizeof (char));
  strcpy (frag->seq, "55555");

因为该字符串55555还将包含一个终止零字符，该字符也将写入分配的 5 个字节之外的内存中。

相反，您可以使用strdup()which 分配和复制字符串

  frag->seq = strdup("55555");
于 2009-09-03T06:49:23.063 回答
3

您将frag_list其视为Frag指针的链接列表，但在创建列表时并没有放入终止符。

尝试这个：

int
main ()
{
  Fragment *frags = (Fragment *) malloc (10 * sizeof (Fragment));
  int i, j;
  for (i = 0; i < 10; i++)
    {
      Fragment *fragment = &frags[i];
      fragment->frag_list = (Frag *) malloc (1 * sizeof (Frag));
      Frag *frag = fragment->frag_list;
      frag->seq = malloc (5 * sizeof (char));
      strcpy (frag->seq, "55555");
      frag->next = (Frag *) malloc (1 * sizeof (Frag));
      frag = frag->next;
      frag->seq = malloc (5 * sizeof (char));
      strcpy (frag->seq, "55555");
      frag->next = NULL; // <--------------------- This is what you need to do
    }
  free_frags (frags, 10);
  return 0;
}

问题是，当你malloc()有一个新的内存块时，编译器和/或操作系统可能会为你清空它，但更有可能它只会给你垃圾。当你尝试去free()那个垃圾时，你会崩溃。

于 2009-09-03T06:45:01.017 回答
2

如果您打算使用Frag.next指针作为哨兵（在 free_frags() 中），那么您需要在代码中的某处将其设置为 NULL。

另外，请注意 - 您正在使用 malloc() 为Frag.seq分配 5 个字符，并且您正在该空间中复制一个非 NULL 终止的字符串。

于 2009-09-03T06:43:48.040 回答
0

您正在引用一个不属于您的内存地址。这发生在 free_frags() 函数中，特别是在第 41 行，free (fragment); .

于 2009-09-03T06:43:49.257 回答