C Copying a linked list with two pointers - c

Question

This is for a school project and I need to finish a function to copy a linked list of employees that looks like this
typedef struct TEmployee
{
struct TEmployee * m_Next;
struct TEmployee * m_Bak;
char * m_Name;
} TEMPLOYEE;
this is my function for copying
TEMPLOYEE * cloneList(TEMPLOYEE * src)
{
TEMPLOYEE* current = src;
TEMPLOYEE* newList = NULL;
TEMPLOYEE* tail = NULL;
while(current != NULL)
{
if(newList==NULL)
{
newList = (TEMPLOYEE*)malloc(sizeof(TEMPLOYEE));
newList -> m_Name = current -> m_Name;
newList -> m_Next = NULL;
newList -> m_Bak = NULL;
tail = newList;
}
else
{
tail -> m_Next = (TEMPLOYEE*)malloc(sizeof(TEMPLOYEE));
tail = tail -> m_Next;
tail -> m_Name = current -> m_Name;
tail -> m_Next = NULL;
tail -> m_Bak = current -> m_Bak;
}
current = current -> m_Next;
}
return newList;
}
this works fine but it doesn't copy the m_Bak properly so when I try this assert the m_Next and m_Bak should be the same but aren't
assert ( b && ! strcmp ( b -> m_Name, "Maria" ) && b -> m_Bak == b -> m_Next );
Any help is appreciated.

Answer 1

This line
tail -> m_Bak = current -> m_Bak;
is wrong as it makes the new list point back to the current list.
You rather need:
else
{
tail -> m_Next = (TEMPLOYEE*)malloc(sizeof(TEMPLOYEE));
tail -> m_Next -> m_Bak = tail; // New line
tail = tail -> m_Next;
tail -> m_Name = current -> m_Name;
tail -> m_Next = NULL;
// tail -> m_Bak = current -> m_Bak; Delete this line
}
Notice that your code does not copy the names so both lists will point to the same name object. If you want to do a real copy of the names take a look at strdup
Also notice:
You don't need to cast the value returned by malloc. However, you should check if malloc returned NULL

Answer 2

this is my solution:
typedef struct TEmployee
{
struct TEmployee * m_Next;
struct TEmployee * m_Bak;
char * m_Name;
} TEMPLOYEE;
void *Malloc(size_t size)
{
void *ptr = NULL;
if ((ptr = malloc(size)) == NULL)
{
perror("alloc");
exit(1);
}
return ptr;
}
TEMPLOYEE *cloneList(TEMPLOYEE *src)
{
TEMPLOYEE *newList = (TEMPLOYEE *)Malloc(sizeof(TEMPLOYEE));
newList->m_Bak = NULL;
TEMPLOYEE *copy = newList;
TEMPLOYEE *current = src;
while (current != NULL)
{
newList->m_Name = strdup(current->m_Name); // need free
newList->m_Next = NULL;
if (current = current->m_Next)
{
newList->m_Next = (TEMPLOYEE *)Malloc(sizeof(TEMPLOYEE));
newList->m_Next->m_Bak = newList;
newList = newList->m_Next;
}
}
return copy;
}
Hope I understand your issue correctly...