带链接列表的段错误

Question

I am having trouble with linked lists in C, I have only done data structures such as this in c++.

Gdb is giving me a

Program received signal SIGSEGV, Segmentation fault. 0x0804a23c in addArg (base=0x1, argument=0x804e410 "is") at myshell.c:42 42 while ( (curr != NULL) && (curr->n != NULL) )

I am familiar with segmentation faults having to do with memory, however I thought I have allocated memory correctly. What am I doing wrong?

addArg is being called as addArg(currentCmd->args, lexeme); and currentCmd is a pointer to a node struct

struct lnode {
 char *x;
 struct lnode *n;
};


struct node
  {
    char *command;
    struct lnode *args;
    int input;
    int output;
    int error;
    char *in;
    char *out;
    char *err;
    struct node *next;
    struct node *prev;
  };



void addArg(struct lnode *base, char *argument)
 {
 struct lnode *curr = base;

//this is line 42
  while ( (curr != NULL) && (curr->n != NULL) )
    curr = curr->n;

   curr -> n = malloc(sizeof(struct lnode));
   curr = curr->n;
   curr->x = strdup(argument);
   curr->n = NULL;
 }




struct node* createNode(char *command_, int input_, int output_, int error_, char *in_, char *out_, char *err_, struct node *prev_)
  {
  struct node *n;
  n = malloc(sizeof (struct node));
  n->command = strdup(command_);
  n->prev = prev_;
  n->next = NULL;
  n->input = input_;
  n->output = output_;
  n->error = error_;
  n->in = in_;
  n->out = out_;
  n->err = err_;
  n->args=malloc(sizeof(struct lnode));

 return n;
  }

Answer 1

It looks like currentCmd->args is an invalid pointer. Perhaps a pointer to free() d memory. Or an uninitialized pointer, or a pointer to a local variable that's gone out of scope (though these latter two don't appear to be the case here).

Or perhaps you've accidentally overwritten out-of-bounds memory somewhere else in your program. Pointer issues aren't always at the point of failure; sometimes they're in earlier code, unrelated code even.

Answer 2

我通过将lnode *args为lnode args并对内存管理进行了必要的更改来解决了这个问题。

Answer 3

我从您的gdb输出中看到的是while ( (curr != NULL) && (curr->n != NULL) )是，如果curr == NULL您仍在尝试访问curr-> n进行比较，因此您应该更改该条件以仅比较curr，并且仅在curr不为null时处理curr-> n，如果curr-> n == NULL，则可能会立即中断子句。