Creating a linked list in C

Question

I am currently trying to make a linked list and initialize it within a function. It looks like this:

void add_forest(node_t *head, unsigned char value)
{
    int key;
    node_t *current = head;
    while (current->next != NULL)
    {
        current = current->next;
    }

}

int main()
{
    node_t *head;
    *head = init_forest(); //error here
}

I am currently getting a segfault at the following area in my code and can not figure out why. I am creating the head in init_forest() and then passing back in the main. When I go through the init_forest() the tree does become built. Any suggestions?

Please add the definition of the structure type, and indent the code! — Jonathan Leffler
– Jonathan Leffler, Commented Feb 16, 2015 at 20:50
I'm suspicious that the line current->next = head = (node_t *)malloc(sizeof(node_t)); assigning to head in add_forest() is to blame. — Jonathan Leffler
– Jonathan Leffler, Commented Feb 16, 2015 at 20:52
I am using gdb but can not figure out why it is giving me the seg fault there. I also updated my code above. — user081608
– user081608, Commented Feb 16, 2015 at 20:54
return *head; seems a little sketchy, since you change head in current->next = head = (node_t *)malloc(sizeof(node_t));. I take that back, the second time you enter add_forest, in while(current->next != NULL) you are dereferencing a likely null object (since you changed the contents of head in the previous call to add_forest). Maybe? — ttacon
– ttacon, Commented Feb 16, 2015 at 20:55

Jonathan Leffler · Accepted Answer · 2015-02-16 20:57:24Z

3

The immediate problem is in main():

node_t *head;
*head = init_forest();

head is an uninitialized pointer; you can't dereference it, and you don't actually want to. You should be using:

node_t *head = init_forest();

But you then need to ensure that init_forest() returns a node_t *, rather than a node_t value. This is the more orthodox style for such linked list management functions. You allocated the structure in the function; if you return a copy of the structure rather than a pointer to the structure, you're leaking memory (and you end up with other problems later).

answered Feb 16, 2015 at 20:57

Jonathan Leffler

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2 Comments

user081608 Over a year ago

I tried to do that and I get the following compile error: error: incompatible types when initializing type ‘struct node_t *’ using type ‘node_t’

Jonathan Leffler Over a year ago

See the update...you need to fix init_forest() to return a node_t *, with at least the consequential change that return *head; becomes return head;.

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