无法释放分配的内存

Question

Im having some issues with my code: I cannot recreate my graph using the function I wrote.

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

#define maxNodes 4

typedef struct edge {
    int target;
    struct edge* next;
} edge;

typedef struct node {
    edge* start;
} node;

int isInvalidInput(int input) {
    if(input > 3 && input < 0) {
      puts("Invalid input");
      return 1;
    }
    return 0;
}

int recreateGraph(node* graph) {
  edge* roam;
  for(int index = 0; index < maxNodes; index++) {
    roam = graph[index].start;
    while(roam) {
      edge* aux = roam;
      roam = roam->next;
      free(aux);
      aux = NULL;
    }

  }
  return 0;
}

int deleteEdge(node* graph, int node, int target) {

  if(!graph[node].start) {
    return 1;
  }

  edge* roam = graph[node].start;
  if(roam->target == target) {
    edge* aux = roam;
    graph[node].start = roam->next;
    free(aux);
    return 0;
  }

  while(roam->next) {
    if(roam->target == target) {
      edge* aux = roam;
      roam = roam->next;
      free(aux);
      return 0;
    }
    roam = roam->next;
  }
  return 1;
}

int insertEdge(node* graph, int from, int to) {
    if(isInvalidInput(from) == 1 && isInvalidInput(to) == 1) return 1;
    if(!graph[from].start) {
        graph[from].start = (edge*) malloc(sizeof(edge));
        graph[from].start->target = to;
        return 1;
    }
    edge* roam = graph[from].start;
    while(roam->next) {
        roam = roam->next;
    }
    roam->next = (edge*) malloc(sizeof(edge));
    roam->next->target = to;
    return 0;
}

node* createGraph() {
    node* graph = (node*) malloc(sizeof(graph) * maxNodes);
    for(int index = 0; index < maxNodes; index++) {
        graph[index].start = (edge*) malloc(sizeof(edge));
        graph[index].start = NULL;
    }
    return graph;
}

int main()
{
    node* graph = createGraph();
    insertEdge(graph, 0, 2);
    insertEdge(graph, 0, 1);
    insertEdge(graph, 2, 1);

    //deleteEdge(graph, 0, 1);
    recreateGraph(graph);

    printf("%i ", graph[0].start->next);
    return 0;
}

This function was supposed to free all the memory allocated to my structure. However It doesn't work because I can print any addresses of my structure after running this function. I can't diagnose the source of this issue as Im still new to C even after googling It for hours. I appreciate any help given. Thank you!

Answer 1

Once you free memory, you are not allowed to dereference the pointers that once pointed to it. Doing so invokes undefined behavior , which includes the data appearing to still be there.

If you really want to check that memory was handled properly, run your code through a memory checker like valgrind.