C, Segmentation Fault in linked list

Question

I have a problem with the function below, when (*ptr) is NULL instead of ending the while loop the function crashes. I've tried to use if((*ptr)->next==NULL) break; but it fails anyway.

boolean search_times(struct list ** ptr, float *V, int n, struct list_times ** new ){

    struct list * tmp_ptr=NULL;

    if( V!=NULL && n>=0 ) {
        while((*ptr)!=NULL) {

            int times=0;

            for (int i = 0; i < n; i++) {
                if (isequal((*ptr)->value, V[i]))
                    times++;
            }

            if((suf_insert_times(new, (*ptr)->value, times))==FALSE)
                return FALSE;

            tmp_ptr=*ptr;
            ptr=&(*ptr)->next;
            free(tmp_ptr);
        }
        return TRUE;
    }
    else
        return FALSE;
}

Time to learn how to debug your program https://stackoverflow.com/questions/25385173/what-is-a-debugger-and-how-can-it-help-me-diagnose-problems — rbf, Jul 03 '18 at 15:02
I've already debugged the program, otherwise I couldn't have known that it crashes at the while. Anyway thanks for the solution. — Emanuele, Jul 03 '18 at 15:15

score 1 · Accepted Answer · answered Jul 03 '18 at 15:12

ptr=&(*ptr)->next; saves a pointer to the member next of an object, which is freed in the next line. Trying to dereference ptr in the next iteration will lead to undefined behaviour.

*ptr = (*ptr)->next; should be correct. This will advance the list to the next item, while the previous one will be freed in the next line.

C, Segmentation Fault in linked list

1 Answers1