I'm trying to point a pointer in one structure to a node of another structure. I've been stuck on this 10 hours now. Can someone help me fix my code? I'm getting Segmentation fault at curr_users -> playlist = p_playlists;. Am I pointing it wrong?

struct playlist_ {
  int album;
  int track_num;
  struct playlist_ *next;
};
typedef struct playlist_  playlists;

struct users_ {
  int user_ID;
  struct playlist_ *playlist;
  struct users_ *next;
};
typedef struct users_ users;

int transaction(FILE *transaction_file,album *all_album){
  int transaction_id,i;
  int album_ID,
      account_number,
      add_playlist_user,
      add_playlist_album,
      add_playlist_track;

  users *head_users,*curr_users,*p_users,*users_pointer;
  playlists *head_playlists,*curr_playlists,*p_playlists,*playlist_pointer;

  head_users = NULL;

  fscanf(transaction_file,"%d\n",&account_number);

  /*Checks for empty list, if true creates the first user*/
  if( !(head_users)){
    p_users = malloc(sizeof(users ));
    p_users -> user_ID = account_number;
    head_users = p_users;
    head_users -> next = NULL;
    users_pointer = head_users;

  /*If list is not empty create new user and puts it in front of list*/
  }else{
    p_users = malloc(sizeof(users));
    p_users -> user_ID = account_number;
    curr_users = p_users;
    curr_users -> next = head_users;
    head_users = curr_users;
    users_pointer = head_users;
    }
  /*Create an empty playlist for user and set everything to null*/

  p_playlists = malloc(sizeof(playlists *));
  curr_playlists = p_playlists;
  curr_playlists -> album = 5;
  curr_playlists -> track_num = 5;
  curr_playlists -> next = NULL;
  curr_users -> playlist = p_playlists; 

The error message received when I run this code:

Program received signal SIGSEGV, Segmentation fault.
0x00011050 in transaction (transaction_file=0xff3675cc, all_album=0x226b0)
    at functions.c:94
94            curr_users -> playlist = p_playlists;

• c
• pointers
• linked-list
• structure

The error seems to be in this line:

p_playlists = malloc(sizeof(playlists *));

You are allocating enough memory for a pointer to a playlist_ structure, not enough memory for an entire playlist_ structure. Change the line to:

p_playlists = malloc(sizeof(playlists));

to allocate enough memory for the playlist_ struct.

EDIT
As indicated in the comments below, you also need to assign something to curr_users in the else block. Then, barring any other errors in your program, it should work :)

People already gave the answer, but I thought I would make it more complete with a suggestion:

To minimize the confusion, to make sure you get it right, and to minimize maintenance effort in case of certain changes, always use malloc like this:

type *pointer = malloc(count * sizeof(*pointer));

Note that in this case, the type of pointer is only mentioned once. If it changes, you don't need to touch the rest of the code. Also, sizeof(*pointer) always correctly shows the size of elements that can exist in pointer.

Now to get back to your code, have you noticed that you have the following local variables:

users *head_users, *curr_users, *p_users, *users_pointer;

that are not initialized, and you are checking

if( !(head_users))

? Since your comment says if list is empty, create the first user, I am guessing what you need is to make head_users global, or pass it to transaction and initialize it to NULL at program start.