指向结构的指针中的结构的指针数组

Question

The code below attempts to initialise a node and in doing so, dynamically initialise an array of pointers to child nodes. However, I get a Segmentation fault: 11 when I try to access the children. I realise that I should not be getting any meaningful values (ie it will just be junk in memory) but I don't know why I'm getting the segmentation fault.

#include <stdlib.h>
#include <assert.h>
#include <stdio.h>
#include <string.h>

#define INIT_SIZE 10

typedef struct node_t node_t;

struct node_t {
  char *word;
  node_t **children;
  int arr_len;
};

void set_array_vals(node_t *root);

int main(int argc, char const *argv[]) {
    char *word = "hello";

    node_t *root = malloc(sizeof(node_t));
    assert(root);

    root->children = malloc(sizeof(node_t *) * INIT_SIZE);
    assert(root->children);
    root->arr_len = INIT_SIZE;

    root->word = malloc((sizeof(char)) * (strlen(word) + 1));
    assert(root->word);
    strcpy(root->word, word);

    set_array_vals(root);
    printf("Arr len: %d\n", root->arr_len);

    return 0;
}

void set_array_vals(node_t *root) {
    int i;
    for (i=1; i<root->arr_len; i++) {
        node_t *this_node = root->children[i];
        printf("%d: %d\n", i, this_node->arr_len);
    }
}

Answer 1

In set_array_vals you get the pointers from the "array" root->children , but that array is not initialized, and the pointers will be indeterminate and seemingly random. Dereferencing those pointers leads to undefined behavior .

Also, you seem to forget that array indexes start at zero . And once you make all pointers in the root->children array valid, you have to remember to initialize the structures they point to as well, otherwise the value of this_node->arr_len will be indeterminate.

Answer 2

As pointed by others, firstly children is of node_t ** type & you have allocated memory only for root->children not for root->children[row] . Allocate memory dynamically for root->children[row] & assign some values. It may looks like

 root->arr_len = INIT_SIZE;
 for(int row = 0; row < root->arr_len ;row++) {
         root->children[row] = malloc(sizeof(node_t));/* allocate memory for each children */
         root->children[row]->arr_len = row + 99;/* ?? assign some values into member of struct so that you can print in
                                                                            set_array_vals & verify  */
            }

And in set_array_vals() start printing from i=0 as above you allocated memory from root->children[0] to root->children[9] & accessing beyond size may results in undefined behavior.

void set_array_vals(node_t *root) {
        for (int i = 0; i < root->arr_len; i++) { /* start printing from i=0 , not i=1 */
                #if 0
                node_t *this_node = root->children[i]; /* no need of any temporary pointer */
                printf("%d: %d\n", i, this_node->arr_len);
                #endif
                printf("%d: %d\n", i, root->children[i]->arr_len);
        }
}