c - 通用链表

Question

我正在尝试用 ansic (c89) 编写最简单的通用链表。

代码如下：

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

struct node {
  struct node *previous;
  struct node *next;
  void *value;
};

struct node*
allocate_node(void);

void
free_node(struct node *freeable_node);

void
init_node(struct node *initializable_node);

void
print_node(struct node *printable_node);

struct node*
allocate_node(void)
{
  struct node *allocatable_node =
    (struct node*) calloc(1, sizeof(struct node));

  allocatable_node -> previous = NULL;
  allocatable_node -> next = NULL;
  return allocatable_node;
}

void
free_node(struct node *freeable_node)
{
  free(freeable_node -> previous);
  free(freeable_node -> next);
  free(freeable_node -> value);
  free(freeable_node);
}

void
init_node(struct node *initializable_node)
{
  int *initializable_value = (int*) (initializable_node -> value);
  initializable_value = calloc(1, sizeof(int*));
  *initializable_value = rand() % 100;
}

void
print_node(struct node *printable_node)
{
  printf("%d\n", *((int*) (printable_node -> value)));
}

int
main(void)
{
  struct node *demo_list = NULL;

  srand((unsigned int) time(NULL));
  demo_list = allocate_node();
  init_node(demo_list);
  print_node(demo_list);
  free(demo_list);

  return 0;
}

通过clang编译成功，但运行后打印segfault。

和valgrind 输出：

$ valgrind ./build/app                                                                  
==23061== Memcheck, a memory error detector
==23061== Copyright (C) 2002-2015, and GNU GPL'd, by Julian Seward et al.
==23061== Using Valgrind-3.12.0.SVN and LibVEX; rerun with -h for copyright info
==23061== Command: ./build/app
==23061==
==23061== Invalid read of size 4
==23061==    at 0x4007AE: print_node (app.c:56)
==23061==    by 0x400813: main (app.c:67)
==23061==  Address 0x0 is not stack'd, malloc'd or (recently) free'd
==23061==
==23061==
==23061== Process terminating with default action of signal 11 (SIGSEGV)
==23061==  Access not within mapped region at address 0x0
==23061==    at 0x4007AE: print_node (app.c:56)
==23061==    by 0x400813: main (app.c:67)
==23061==  If you believe this happened as a result of a stack
==23061==  overflow in your program's main thread (unlikely but
==23061==  possible), you can try to increase the size of the
==23061==  main thread stack using the --main-stacksize= flag.
==23061==  The main thread stack size used in this run was 8388608.
==23061==
==23061== HEAP SUMMARY:
==23061==     in use at exit: 32 bytes in 2 blocks
==23061==   total heap usage: 2 allocs, 0 frees, 32 bytes allocated
==23061==
==23061== LEAK SUMMARY:
==23061==    definitely lost: 8 bytes in 1 blocks
==23061==    indirectly lost: 0 bytes in 0 blocks
==23061==      possibly lost: 0 bytes in 0 blocks
==23061==    still reachable: 24 bytes in 1 blocks
==23061==         suppressed: 0 bytes in 0 blocks
==23061== Rerun with --leak-check=full to see details of leaked memory
==23061==
==23061== For counts of detected and suppressed errors, rerun with: -v
==23061== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0)
Segmentation fault

有什么解决办法吗？

Answer 1

问题很简单，init_node 函数中少了一行。

void
init_node(struct node *initializable_node)
{
  int *initializable_value = (int*) (initializable_node -> value);
  initializable_value = calloc(1, sizeof(int*));
  *initializable_value = rand() % 100;
  //This line puts the variable you just created in the structure
  initializable_node->value=initializable_value;
}

顺便说一句，在main 函数中，我认为您的意思是使用free_node(demo_list); 而不是free(demo_list);

如果您需要更多帮助，请随时提出！

编辑：意识到不需要init_node 函数的第一行，因为您正在为int 指针分配一个NULL 值。

因此，请这样做：

void
init_node(struct node *initializable_node)
{
  int *initializable_value = calloc(1, sizeof(int*));
  *initializable_value = rand() % 100;
  //This line puts the variable you just created in the structure
  initializable_node->value=initializable_value;
}

Answer 2

free(freeable_node -> value);
这个void * value 永远不会被初始化。所以value 默认情况下会有垃圾值，你正在尝试释放该内存。