CODEWITHSUNDEEP
cmemory
Galaxy
Galaxy

Reputation: 863

unable to access memory 0X.... C program (Ubuntu 12.10)

Purpose of code: TO maintain a unique element link list...UFID is the keyword for unique

Structure declaration:

    struct sharedFiles
    {
      char UFID[50];
      int valid;                    //valid 1 if someone have this file in write mode
      int shared;                   //no of user's reading this file
      struct sharedFiles *next;     //pointer to next node
    }*sfstart,*sfend;               //sfstart points to the first node of linked list and efend to the last node of linked list so that it will be easy to just insert at the end without traversing the linked list

Error Description: The below code gives segmentation fault when I invoking it 2nd time. I tried to debug with GDB and it says unable to access the location at line

if(strcmp(sftemp->UFID,ufid)==0)

In the above line it is unable to access sftemp->UFID

/*Function code*/


  int addShareList(char *ufid,int mode)      //mode=0 (read)  and mode=1 (Write request)
{
  struct sharedFiles *sftemp,*newnode;

  sftemp=sfstart;
   if(sfstart==NULL)   //if list is empty add first node
   {
       sfstart=(struct sharedFiles *) malloc(sizeof(struct sharedFiles));
       strcpy(sfstart->UFID,ufid);
       sfstart->valid=mode;
       sfstart->shared=1;
       sfstart->next=NULL;
       sfend=sfstart;             //this node will also be last node of Linked list
       return 0;
   }
   else                 //if list is not empty
   {

        while(sftemp->next != NULL)    //traverse till last node
        {
          if(strcmp(sftemp->UFID,ufid)==0)
          {
             //here if same node found some manupulation to the struct variables

          }
         sftemp=sftemp->next;
        } //while


  if(sftemp->next==NULL)  //procvess last node
       {

         if(strcmp(sftemp->UFID,ufid)!=0) //if last node not same add node at the end of   Linked list
         {
           newnode=(struct sharedFiles *) malloc(sizeof(struct sharedFiles));
           strcpy(newnode->UFID,ufid);
           newnode->valid=mode;
           newnode->shared=1;
            newnode->next=NULL;
           sftemp->next=newnode;
           sfend=newnode;
           return 0;
          }
          else //if last node is same
          {

                //some manipulations to struct variables 
          }
       } //if


   }

return -1;
}//addShareList

The above code works fine for inserting first element.When I invoke the same function for inserting second node in the linked list it unable to access the first node while comarision in the line if(strcmp(sftemp->UFID,ufid)==0). Hope now the purpose of code is clear.

Thanks in advance..

Upvotes: 0

Views: 289

Answers (2)

Guern
Guern

Reputation: 1257

In the while you check if sftemp!=NULL so we can be sure that in the second iteration, after line sftemp=sftemp->next; the pointer contain allocated memory.

But, since I don't know how the list it structured, I can't be sure that the content contains another node of sharedFiles type, it could contain an end-list node which not contains UFID attribute.

So, check in your list how to control if the list is finished.

Another solution can be to change your check this way:

while(sftemp->next!=NULL)

...

if(sftemp->next==NULL) {
    //add the node in the right way, consider the end-list node
}

EDIT:

Furthermore, change in the first if the line sftemp->next = NULL; to sfnext->next = NULL;.

And be sure to initialize stnext = NULL.

EDIT 2:

Now that you post struct declaration I still cannot see when you initialize sfstart. Try to do this:

Structure declaration:

struct sharedFiles
{
  char UFID[50];
  int valid;                    //valid 1 if someone have this file in write mode
  int shared;                   //no of user's reading this file
  struct sharedFiles *next;     //pointer to next node
}*sfstart = NULL,*sfend;

Upvotes: 1

Anonymouse
Anonymouse

Reputation: 945

Normally the answer for this is that the pointer is NULL, which in your case means malloc has failed because you're out of memory - you really should be checking the return value.

The most likely cause for your problem is that you have corrupted the heap, so that sftemp no longer points to a valid memory address. Check what it's value is that might give you some more clues.

Upvotes: 0

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