Thread: Doubly Linked List Segmentation Fault

So i've never really worked with doubly linked lists in terms of coding, and I didn't do great with singly linked lists either. I have it all coded out but it's giving me a segmentation fault when I run it. I have looked over all my code and have no idea whats wrong, in add event i malloc one node of space then find where to add it and add it. i only add 1 per call to add event so that shouldnt be an issue. is there an initial part im missing when you first start the list?

[note all of the variables and definitions you see that aren't in the code are in one of the 2 external files if u need more info from them to help let me know.


osdefs.h

Code:

struct event_list
{
	/** Type of event. */
	event_type          event;

	/** User or device causing the event. */
	int                 agent;

	/** Time event occurs. */
	struct time_type    time;

	/** Next event node in list. */
	struct event_list*  prev;

	/** Previous event node in list. */
	struct event_list*  next;
};
typedef struct event_list event_list;

externs.h

Code:

extern event_list*    Event_List;  ///< Pointer to head of event list

obj1.c

Code:

void Add_Event( int event, int agent, struct time_type* time )
{
               //Header points to the head of the list.
              
              event_list* header = Event_List;
              
              
              
              event_list* temp;
              temp = malloc(sizeof(event_list));
              temp->event = event;
              temp->agent = agent;
              temp->time.nanosec = time->nanosec;
              temp->time.seconds = time->seconds;
              temp->next = NULL;
              temp->prev = NULL;
              
              if(Event_List==NULL)
              {
                    // header = temp;
                     Event_List = temp;
                     return;
                         
              }
              //While header is not the last node in the list.
              while(header!=NULL){
                  //First compare the temporary nodes seconds to the header's seconds. If they are the same:
                  //compare nanoseconds of each node, if we enter the loop then temp precedes header.
                  if(temp->time.seconds < header->time.seconds || (temp->time.seconds == header->time.seconds && temp->time.nanosec <= header->time.nanosec))                        
                   {       
                           //Make temp the new head of the list.
                           //temp->next now pointts to header.          
                           if(header->prev = NULL)
                           {
                              header->prev = temp;
                              temp->next = header;
                              temp->prev = NULL;  
                              Event_List = temp; 
                              header = NULL;         
                           
                           }
                           
                           else
                           {                                                                
                              //Insert into the middle of the list.
                              
                                header->prev->next = temp;
                                temp->prev = header->prev;
                                header->prev = temp;
                                temp->next = header;
                                header = NULL;         
                               
                           }             
                                                          
                  
                  }//end outer if
                  
            //Inserting at the end of the list temp->previous is set as header.      
           else if(header->next == NULL)
           {
             header->next = temp;
             temp->prev = header;
             temp->next = NULL;
             header = NULL;
                
           }  
           //Advance the header to the next node in the list.
           else
               header=header->next;
           }//end while
         
}

One little tip that may help you sort this out... you have two very similar names going on here... Event_List and event_list ... the first refers to your entire list the second refers to one event in the list... Rename the single event Event_Item throughout your code... then look at your code again.

Hint: One of the very best debugging tools you can have are the names of your variables.

I can't, that stuff in the headers was given to me and im not allowed to alter it.

through commenting out sections ive narrowed the segmentation fault down to this part:

Code:

  if(header->prev == NULL)
                           {
                              header->prev = temp;
                              temp->next = header;
                              temp->prev = NULL;  
                              Event_List = temp; 
                              header = NULL;         
                           
                           }
                         else
                           {                                                                
                              //Insert into the middle of the list.
                              
                                header->prev->next = temp;
                                temp->prev = header->prev;
                                header->prev = temp;
                                temp->next = header;
                                header = NULL;         
                               
                           }

Code:

if(header->prev = NULL)

= vs ==

yup. almost got the output right just a few more tweaks thanks for the help