C内存管理错误？

Question

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,错误信息是:

(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.

请帮我修理一下,谢谢.

Answer 1

你有很多问题.

在这里,你为5个字符分配空间,但是复制6个(字符串末尾的nul-terminator也需要一个空格):

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

同时,你永远不会frag->next在frag你分配的第二个中设置.您需要将其设置为NULL,以便例程中的while循环free_frag不会流入杂草.

第三个问题在这里:

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

你是自由的fragment,但它并不是一个完整的块malloc- 它只是你一次分配的10个片段中的一个.后者free(frags)释放了正确阻止,所以你只需要删除那个错误的线.

Answer 2

您正试图释放阵列的中间位置.

Fragment *fragment = &frags[i];
...
...
/* to do : free fragment */
free (fragment);
fragment = NULL;

Answer 3

这些行似乎有一个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,该字符也被写入分配的5个字节之外的内存中.

相反,您可以使用strdup()分配和复制字符串

  frag->seq = strdup("55555");

Answer 4

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个位置将其设置为 NULL。

另外，要小心 - 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。