C链表-指针值更改

Question

So I'm fairly new to C and pointers and I have a project for school to finish functions for adding to a linked list

this is how it looks:

TEMPLOYEE * newEmployee(const char * name, TEMPLOYEE * next)
{
    TEMPLOYEE* head = NULL;
    head = malloc(sizeof(TEMPLOYEE));
    if(head==NULL)
    {
        return 1;
    }

    head -> m_Name = name;
    head -> m_Next = next;
    head -> m_Bak = NULL;
}

this works when entering name as a string for example a = newEmployee ( "Peter", a ); but when I try to add using a temporary value the m_Name in like this

strncpy ( tmp, "Victoria", sizeof ( tmp ) );
a = newEmployee ( tmp, a );
strncpy ( tmp, "Peter", sizeof ( tmp ) );
a = newEmployee ( tmp, a );

the list will change with the value so it will have two employees with the name Peter instead of Peter and Victoria and I can't find how to do this. Any help is welcome.

Answer 1

In the function newEmployee() , you are doing:

head -> m_Name = name;

In both newEmployee() call, you are passing tmp :

a = newEmployee ( tmp, a );
a = newEmployee ( tmp, a );

So, the name pointer of both the nodes are pointing to same location tmp which contains the name. If you make any change in the value of tmp , it will be reflected in both.

The in memory view would be something like this:

---------------------
|m_Name|m_Next|m_Bak|
---------------------
    | 
    \/    tmp
   -----------
   | Peter   |
   -----------
    /\
    |
---------------------
|m_Name|m_Next|m_Bak|
---------------------

// Both the node m_Name is pointing to same location.
// Make any change in tmp and the changes will reflect in both the pointers
// as they are pointing to same location.

To solve this problem, you should allocate memory to m_Name and copy the tmp to it. You can use strdup() to resolve this issue. In the newEmployee() , replace this:

head -> m_Name = name;

with this:

head -> m_Name = strdup(name);

The strdup() function returns a pointer to a new string which is a duplicate of the string passed to it. strdup() allocates a memory for the new string and returns its pointer. Make sure to free it using free() once you are done with it. Note that strdup() is not part of the C Standard.

The newEmployee() function return type is TEMPLOYEE * but you are not returning anything from this function if the malloc is successful. Also, if malloc fails for some reason you are returning integer constant 1 from it which is incorrect. Instead, you should return NULL in case of failure.