Pointer in insert function giving error in C

Question

I manipulated my code to be able to use pred_p but have run into problems since. My code stops at line "pred_p->next_p = temp_p;" and gives me the message "Thread 1: EXC_BAD_ACCESS (code=1, address=0x8).Not sure where to go from here.

struct list_node_s {
    int    data;
    struct list_node_s* next_p;
};

struct list_node_s* Insert(struct list_node_s* head_p, int val);
void Print(struct list_node_s* head_p);
char Get_command(void);
int  Get_value(void);

/*-----------------------------------------------------------------*/
int main(void) {
    char command;
    int  value;
    struct list_node_s* head_p = NULL;
    /* start with empty list */

    command = Get_command();
    while (command != 'q' && command != 'Q') {
        switch (command) {
            case 'i':
            case 'I':
                value = Get_value();
                head_p = Insert(head_p, value);
                break;
            case 'p':
            case 'P':
                Print(head_p);
                break;
            default:
                printf("There is no %c command\n", command);
                printf("Please try again\n");
        }
        command = Get_command();
    }

    return 0;
}  /* main */


/*-----------------------------------------------------------------*/
struct list_node_s* Insert(struct list_node_s* head_p, int val) {
    struct list_node_s* curr_p = head_p;
    struct list_node_s* pred_p = NULL;
    struct list_node_s* temp_p;

    while (curr_p != NULL) {
        if (curr_p->data >= val)
            break;
        pred_p = curr_p;
        curr_p = curr_p->next_p;
    }

    // Create new node
    temp_p = malloc(sizeof(struct list_node_s));
    temp_p->data = val;
    temp_p->next_p = curr_p;
    if (pred_p = NULL) {
        head_p = temp_p;
    }
    else {
        pred_p->next_p = temp_p;
    }

    return head_p;
}  /* Insert */

Is your code multi-threaded? It might be due to a concurrent access — Joel
– Joel, Commented Sep 19, 2014 at 21:35
You may find this alternative an interesting piece of code to walk through. — WhozCraig
– WhozCraig, Commented Sep 19, 2014 at 21:42
There are lots of problems with your code, here is one: the line if (pred_p = NULL) { is assigning NULL to pred_p rather than comparing pred_p to NULL. (a common mistake and the reason the literal value should be written first, so the compiler would catch the problem. — user3629249
– user3629249, Commented Sep 22, 2014 at 7:16

83457 · Accepted Answer · 2014-09-19 21:49:08Z

2

 if (pred_p = NULL)

This should be

if (pred_p == NULL)

You technically repeated yourself, as one =, simply assigned NULL to pred_p again

Also

You need to allocate memory to pred_p using pred_p=malloc(sizeof struct list_node_s).

The above would only have worked as it is if head_p wasn't NULL, meaning curr_p wouldn't be NULL and in turn pred_p but then you would never have noticed the pitfall.

edited Sep 19, 2014 at 21:49

answered Sep 19, 2014 at 21:34

83457

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