为什么在尝试编辑此链接列表时遇到段错误

Question

I have a C data structure that consists of a nested series of linked lists. Imagine it like this.

[]->[]->[]->[]->[]
|
V
[]->[]->[]->[]->[]
|
V
[]->[]->[]->[]->[]
|
V
[]->[]->[]->[]->[]

Top Level Nodes have a structure like this

typedef struct _StackTop
{
    struct _StackTop *next;
    StackNode *head;
} StackTop;

Where StackNode is a pointer to the head of the Lower Level List which has a structure like this

typedef struct _StackNode
{
  int number;
  struct _StackNode *next;
} StackNode;

For part of the business logic, I need to change which of the nodes the Top Level points to, ie change StackNode *head.

This seems reasonable.

However, when I try and execute the code as follows:

*note that stackKing is the first Top Level Node

StackTop *currStackNode = &stackKing;

currStackNode->head = currStackNode->head->next;

I am getting a seg fault.

At first, I figured the issue was with editing the Top Level Node. However, if I set it to something else

currStackNode->head = NULL;

all is well. I can even change what it points to.

currStackNode->head = currStackNode->next->head;

without issue.

I'm fairly inexperience with dynamically allocated data systems, so I'm really hoping there's something very obvious I'm doing wrong, but I haven't been able to narrow it down on google or SO.

I'm looking at the information produced by valgrind, and I'm still not 100% sure what it means

==5038== Invalid read of size 4
==5038==    at 0x8048955: cachesim_access (in /home/rrollins/3056/Assignment-  5/assignment5/cachesim)
==5038==    by 0x40604D2: (below main) (libc-start.c:226)
==5038==  Address 0x4 is not stack'd, malloc'd or (recently) free'd
==5038== 
==5038== 
==5038== Process terminating with default action of signal 11 (SIGSEGV)
==5038==  Access not within mapped region at address 0x4

It sounds like maybe this means there is an error with my initialization, since I'm trying to access some memory that hasn't been correctly malloc'd?

Since it several comments and my own analysis indicate the initialization is the problem, here's that code as well

  i = 0;
  j = 0;
  StackTop *currStackNode = &stackKing;
  currStackNode->next = NULL;
  currStackNode->head = NULL;
  StackNode *innerCurrStackNode;
  //for each set
  while (i<numSets) {
    //create a linked list of StackNodes, one for each "way"
    j = 0;
    while (j<assoc) {
      if (j == 0) {
        currStackNode->head = malloc(sizeof(StackNode));
        innerCurrStackNode = currStackNode->head;
      } else {
        innerCurrStackNode->next = malloc(sizeof(StackNode));
        innerCurrStackNode = innerCurrStackNode->next;
      }
      //set this to negative 1, to show that no number is least recently used
      innerCurrStackNode->number = -1;
      innerCurrStackNode->next = NULL;
      j++;
    }
    currStackNode->next = malloc(sizeof(StackTop));
    currStackNode = currStackNode->next;
    currStackNode->next = NULL;
    currStackNode->head = NULL;
    i++;
  }
}

However, I have no problem "iterating" through each linked list. It's only trying to set this value that creates a serious issue

Answer 1

currStackNode->head->next失败，而currStackNode->next->head却没有，向我表明currStackNode的head指针可能未正确初始化，或者指向了奇怪的地方，因此当上面的第一行尝试引用时head-> ，它会导致段错误。