I'm having difficulty tracking down a problem with pointers returning null in my node structure. I simplified the program to the most basic functions so you could see exactly where the problem is.
This is a basic node tree set up where nodes can be added to each other to create a tree like structure. When creating and adding nodes everything works fine.
I have a print function which prints the node name and the names of it's children as well and here is where the problem is. Whenever I pass a node to the function the pointers to its children always return null. However, I can access the children pointer don't return null when accessed directly from the parent they only return null when the parent is copied into the function.
I'm compiling in C on Linux Mint 19. I included the header, code, and a basic example.
----header----
#ifndef node_h
#define node_h
typedef struct Node node;
struct Node {
node* parent;
node** children;
char* name;
int count;
};
node* create_node(char* name);
void add_child_node(node* parent, node* child);
void print_node(node* n);
#endif
----c file----
#include <stdio.h>
#include <stdlib.h>
#include "node.h"
node* create_node(char* name) {
node* n = malloc(sizeof(node));
if (n == 0) {
printf("Unable to create node %s.\n", name);
exit(1);
}
n->name = name;
n->parent = 0;
n->children = 0;
//n->map = create_stringmap();
n->count = 0;
return n;
}
void add_child_node(node* parent, node* child) {
child->parent = parent;
int count = parent->count + 1;
parent->children = realloc(parent->children, sizeof(node) * count);
if (parent->children == 0) {
printf("Unable to add child node %s to parent %s.\n", child->name, parent->name);
exit(1);
}
if (child == 0) {
printf("The child node you are attempting to add to %s is null.\n", parent->name);
exit(1);
}
// Add child to end of array.
parent->children[parent->count] = child;
parent->count = count;
}
void print_node(node* n) {
//print_node_path(n);
printf("%s\n", n->name);
int count = n->count;
for (int i = 0; i < count; i++) {
printf("%p\n", n->children[count]);
printf("%s\n", n->children[count]->name); // Crashes because children are null.
//print_node(n->children[count]);
}
}
----test file----
#include <stdio.h>
//#include <unistd.h>
#include <stdlib.h>
#include "node.h"
// gcc -Wall node.c stringstest.c -o stringstest && ./stringstest
int main() {
//while (1) {
// Create tree
node* root = create_node("root");
node* branch1 = create_node("branch1");
node* branch2 = create_node("branch2");
// Add children
add_child_node(root, branch1);
add_child_node(root, branch2);
printf("%s\n" , root->children[0]->name);
printf("%s\n" , root->children[1]->name);
printf("%p\n", root->children[0]);
printf("%p\n", root->children[1]);
print_node(root); // Crashes because children are return null.
// This works since no children are accessed in the loop.
//print_node(branch2);
//usleep(100);
//}
return 0;
}
In the example the nodes children work fine when accessed directly the problem is in the print_node function. When the node is passed to it the children now become null aka 0.
This is the exact method I use to store and access arrays of pointers in other libraries. Perhaps I'm missing something simple. Either way it always helps to get an outside fresh perspective.
