Segmentation Fault: 11 - C Linked-List

Question

I am trying to implement linked-list in C. I used a structure called NODE. For some reason, I get a Segmentation Error 11 when I call the function createList() . I debugged my code and I am pretty confident that the error occurs here:

/* Write into node */
strcpy(head->username, cur_username);
strcpy(head->password, cur_pw);
strcpy(head->type, cur_type);
printf("%s %s %s\n", head->username, head->password, head->type);

I think the error is because memory was not assigned correctly. But I initialized an array of 50 chars for each node value, so shouldn't the program manage the memory by itself?

    char *filename = "password.csv";
int n_users = 0;

struct NODE {
  char username[50];
  char password[50];
  char type[50];
  struct NODE *next;
} *head=NULL, *curr=NULL;

/* Global placeholders so they can be used by other both add() and creatList() */
char line[50], *cur_username, *cur_pw, *cur_type;
char linecopy[50];

void createList(void) {
  printf("Creating list\n");

  bool success;

  /* Open the file in read-only and copy content into array */

  /* File pointer */
  FILE *f;

  /* Node pointer */
  struct NODE * ptr = (struct NODE*) malloc(sizeof(struct NODE));
  if (NULL == ptr) {
    printf("\nError: was unable to initialize password validation information!! \n");
    return;
  }

  head = curr = ptr;

  /* Open the file in read */
  f = fopen("password.csv", "r");

  /* if f == NULL, there was an error - the file probably does not exist. We don't care, we will create the file later */
  if (f != NULL) {
    /* Read the file line by line */
    while(fgets(line, 49, f) != NULL) {

      struct NODE * p = (struct NODE*) malloc(sizeof(struct NODE));

      /* Copy line into linecopy because strtok is destructive */
      strcpy(linecopy, line);

      /* Extract the ids from linecopy */
      cur_username = strtok(linecopy, ",");;
      cur_pw = strtok(NULL, ",");
      cur_type = strtok(NULL, ",");

      if(head==NULL)
         add(head);
      else
         add(p);

      if (!success) {
        printf("\nError: was unable to initialize password validation information!! \n");
        return;
      }
    }
  }  

  /* Close file */
  fclose(f);
}

** HEAD is NULL **

While I don't have any segmentation fault anymore, my head does not get initialized... I am reading inputs from a file and create a node for each line of the file. Therefore, I am testing for head==NULL and sending head to add(struct *NODE). Shouldn't that initialize head?

struct NODE {
  char username[50];
  char password[50];
  char type[50];
  struct NODE *next;
} *head=NULL, *curr=NULL;

/* Global placeholders so they can be used by other both add() and creatList() */
char line[50], *cur_username, *cur_pw, *cur_type;
char linecopy[50];

void createList(void) {
  printf("Creating list\n");

  bool success;

  /* Open the file in read-only and copy content into array */

  /* File pointer */
  FILE *f;

  /* Node pointer */
  struct NODE * ptr = (struct NODE*) malloc(sizeof(struct NODE));
  if (NULL == ptr) {
    printf("\nError: was unable to initialize password validation information!! \n");
    return;
  }

  head = curr = ptr;

  /* Open the file in read */
  f = fopen("password.csv", "r");

  /* if f == NULL, there was an error - the file probably does not exist. We don't care, we will create the file later */
  if (f != NULL) {
    /* Read the file line by line */
    while(fgets(line, 49, f) != NULL) {

      struct NODE * p = (struct NODE*) malloc(sizeof(struct NODE));

      /* Copy line into linecopy because strtok is destructive */
      strcpy(linecopy, line);

      /* Extract the ids from linecopy */
      cur_username = strtok(linecopy, ",");;
      cur_pw = strtok(NULL, ",");
      cur_type = strtok(NULL, ",");

      if (head == NULL) 
        success = add(head);
      else 
        success = add(p);

      if (!success) {
        printf("\nError: was unable to initialize password validation information!! \n");
        return;
      }
    }
  }  

  /* Close file */
  fclose(f);
}

Answer 1

Yes, this would be a problem.

  /* Is list empty? */
  if (head == NULL) {

    /* Write into node */
    strcpy(head->username, cur_username);
    strcpy(head->password, cur_pw);
    strcpy(head->type, cur_type);
    printf("%s %s %s\n", head->username, head->password, head->type);

  } 

Dereferencing NULL will generally cause a segmentation fault.

In this case, you always want to allocate a new node. Try this:

struct NODE *node = malloc(sizeof(struct NODE));

if (node != NULL)
{
    strcpy(node->username, cur_username);
    strcpy(node->password, cur_pw);
    strcpy(node->type, cur_type);

    /* push node onto list head */
    node->next = head;
    head = node;
}

Answer 2

在我看来，人头没有分配