Bdtjtk

I'm trying to do a linked list database and for some reason the program won't let me access the memory, or at least that's what the debugger says. I'm using the gdb debugger, codeblocks.

I've already compiled the program on another machine, with the same operating system (windows). It worked flawlessly, BUT the debugger showed the same error.

void print_ListEl(ListEl* head)

{

 ListEl* current = malloc(sizeof(ListEl));

 current = head;

 if (head==NULL) exit(EXIT_FAILURE);

 if (current->next==NULL)

 {

     puts("No elements");

     return;

 }

 else

 {

     int i=1;

     while(current->next!=NULL)

     {

         printf("%d.%sn", i, current->name);

         current=current->next;

         ++i;

     }

 }

 free(current);

}

It's used like that:

ListEl* element = malloc(sizeof(ListEl));

print_listEl(element);

This is the function that seems to cause the problem according to the debugger. When I watch the "current->next" variable, the debugger says "Cannot access memory at address". If I change current->next to current, the debugger still shows the same function as causing the problem. The ListEl structure is just a regular single linked list with a char type data.

 struct ListEl

 {

     char name[MAX_CHAR];

     struct ListEl* next;

 };

I also use

typedef struct ListEl ListEl

Headers are safe with ifndef and endif and are all included, I've checked.

This function, instead of putting "No elements" when there's no elements in the list, spews out some random character and crashes the program.

1. You are not supposed to malloc for current. You are replacing the pointer obtained from it immediately with head and loose the allocation. The later free will free the last element of the list, but will not set the pointer of the previous element to NULL, causing undefined behavior if the list is used later.



2. 
   

   You are allocating memory for a node here:

   
   
   

   ListEl* element = malloc(sizeof(ListEl));
   
   

   
   
   

   but you never set any of its values before calling print_listEl(element);. Therefore the pointer element->next will be indeterminate and comparison against NULL will not be useful. You dereference this pointer later in print_listEl causing undefined behavior. Always initialize after malloc:

   
   
   

   ListEl* element = malloc(sizeof(ListEl));
   
   element->next = NULL;
   
   memset(element->name, '', sizeof(element->name));
   
   

   
   

may be

ListEl* current = malloc(sizeof(ListEl));

current = head;

must be

ListEl* current = malloc(sizeof(ListEl));

head = current;

else why to do the malloc and lost it immediately ?

I hope you have additional code before

if (current->next==NULL)

because current is only allocated, not initialized

When I watch the "current->next" variable, the debugger says "Cannot access memory at address".

That means current was broken because for instance it was deleted, or you just never initialized the field next. the problem is you do not give Minimal, Complete, and Verifiable example

If you code is really that :

ListEl* element = malloc(sizeof(ListEl));

print_listEl(element);

element->name nor element->next are not initialized, so in print_listEl it is the same