Runtime error sorting a linked list of strings

Question

void sort(struct node **s2) {
    struct node *x, *y;
    x = *s2;
    char *str;
    while (x != NULL) {
        y = x->n;
        while (y != NULL) {
            if (strcmp(x->name, y->name) > 0) {
                strcpy(str, x->name);
                strcpy(x->name, y->name);
                strcpy(y->name, str);
            }
            y = y->n;
        }
        x = x->n;
    }
}

This is showing a run time error. I don't know whats wrong I believe the sorting is correct It's selection sort The structure of my node is:

struct node {
    char *name;
    struct node *n;
};

It's showing runtime error.

oh i m sorry i missed out a } here...i have all the } in my original program — SynAck
– SynAck, Commented Aug 17, 2016 at 15:20
Your algorithm is an unstable variant of selection sort. Its time-complexity is O(N^2). Try and implement merge sort, a much more efficient algorithm with worst-case time-complexity of O(N.log(N)). — chqrlie
– chqrlie, Commented Aug 17, 2016 at 15:33

chux · Accepted Answer · 2016-08-17 15:25:14Z

5

char *str; ... strcpy(str,x->name); copies data pointed to by name to an someplace (it is undefined behavior) as str is an uninitialized pointer.

Just swap pointers.

       if (strcmp(x->name,y->name) > 0) {
         char *temp = x->name;
         x->name = y->name;
         y->name = temp;
       }

answered Aug 17, 2016 at 15:25

chux

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3 Comments

SynAck Over a year ago

thanks ,swapping the pointers works but I'm confused-what is wrong with swapping the data?.

Weather Vane Over a year ago

Three reasons: any string might not have the space to store a longer string; and you have not allocated any memory to your scratch space *str; and execution time.

SynAck Over a year ago

got it.Thank's a lot

trahane · Accepted Answer · 2016-08-20 10:29:41Z

1

char *str; This statement only create a pointer. There is no memory is associated with that;

strcpy(char* dest, char* src) simply copies the the content in the memory pointed by src to memory point by dest

In your case str not pointing to any memory location;

strcpy(str, x->name)

since str pointing to nothing, this statement endup with an error because strcpy fails to copy to str

instead of char* str; use char str[SIZE_MAX];

edited Aug 20, 2016 at 10:29

trahane

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answered Aug 17, 2016 at 17:16

Shailesh

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1 Comment

chqrlie Over a year ago

I'm afraid char str[SIZE_MAX]; is not an option because SIZE_MAX, the maximum value of type size_t, is much too large, likely larger than available memory and definitely too much for an automatic variable.

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