Pointer to Struct Value Memory Leak

Question

I am designing an ECS and I have a struct that holds two values: an array of integers and a double pointer. I also have another structure that holds a pointer to the previous struct.

I'm able to access to the pointer to the first struct. However, whenever I try to access any of the pointers in the struct, I end up having a memory leak.

Simple Implementation:

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

#define MAX_ENTITIES 4096
#define MAX_COMPONENTS 16

typedef unsigned int Entity;
typedef struct ENTITYMANAGER {
  Queue* available;
  unsigned int count;
} ENTITYMANAGER;

typedef void* Component;
typedef struct COMPONENTMANAGER {
  unsigned int* count;
  Component** available;
} COMPONENTMANAGER;

typedef struct COORDINATOR {
  ENTITYMANAGER* entities;
  COMPONENTMANAGER* components;
} COORDINATOR;

typedef struct Test {
  int num;
  char* string;
} Test;

Test* createTestComponent(char* string, int number) {
  Test* test = (Test*) malloc(sizeof(Test));
  test -> num = number;
  test -> string = (string) ? string : "Test Component";
  return test;
}

static ENTITYMANAGER* createEntityManager(void) {
  ENTITYMANAGER* entities = (ENTITYMANAGER*) malloc(sizeof(ENTITYMANAGER));
  entities -> available = queue(MAX_ENTITIES);
  entities -> count = 0;
  return entities;
}

static COMPONENTMANAGER* createComponentManager(void) {
  COMPONENTMANAGER* components = (COMPONENTMANAGER*) malloc(sizeof(COMPONENTMANAGER));
  components -> available = (Component**) malloc(sizeof(Component*) * MAX_ENTITIES);
  for (int element = 0; element < MAX_ENTITIES; element++)
    (components -> available)[element] = (Component*) calloc(MAX_COMPONENTS, sizeof(Component));

  components -> count = (int*) calloc(MAX_ENTITIES, sizeof(int));
  return components;
}

COORDINATOR* init(void) {
  COORDINATOR* coordinator = (COORDINATOR*) malloc(sizeof(COORDINATOR));
  coordinator -> entities = createEntityManager();
  coordinator -> components = createComponentManager();
  return coordinator;
}

Entity createEntity(COORDINATOR* coordinator) {
  if (!(coordinator && (coordinator -> entities -> count < MAX_ENTITIES)))
    return NULL_ITEM;

  Entity entity = front(coordinator -> entities -> available);
    dequeue(coordinator -> entities -> available);
    (coordinator -> entities -> count)++;

  return entity;
}

void createComponent(COORDINATOR* coordinator, Entity entity, int cnum, void* data) {
  if (!coordinator)
    return;

  (coordinator -> components -> available)[entity][cnum] = createTestComponent(data, 5);
  (coordinator -> components -> count)[entity]++;
}

int main(int argc, char const *argv[]) {
  COORDINATOR* coordinator = init();

  Entity test = createEntity(coordinator);
  createComponent(coordinator, test, 0, NULL);

  Test* component = (Test*) (coordinator -> components -> available)[test][0];
  printf("Test Component: (\"%s\", %d)", component -> string, component -> num);

  return 0;
}

Queue Implementation: Queue.h

Solution: I had to update my ENTITYMANAGER creation method, which was causing my Entity creation returning an out of bound value causing the memory leak. Specifically, I filled the queue which stores the available entity values with valid values.

static ENTITYMANAGER* createEntityManager(void) {
  ENTITYMANAGER* entities = (ENTITYMANAGER*) malloc(sizeof(ENTITYMANAGER));
  entities -> available = queue(MAX_ENTITIES); entities -> count = 0;
  for (Entity entity = 0; entity < MAX_ENTITIES; entity++)
    enqueue(entities -> available, entity);

  return entities;
}

"I'm assuming". Assuming is not a winning strategy in programming. Problem could be (and is likely to be) in code you have not shown. Please provide complete code as a minimal reproducible example. That is, a small complete example that anyone can take exactly as shown to reproduce the problem - includes code that calls the functions and uses the results in a way the results in the error condition. — kaylum
– kaylum, Commented Jul 29, 2022 at 23:03

yf_lw · Accepted Answer · 2022-07-30 01:04:13Z

0

I suppose there's no memory issue for above code.

I'm just confused you even are not able to access coordinator -> components -> count)[0]. It looks like type of Component is a pointer from (components -> available)[itr][stp] = NULL;. If it's not initialized definitely there will be an error to access it. But there should be no issue to access (coordinator -> components -> count)[0].

edited Jul 30, 2022 at 1:04

answered Jul 30, 2022 at 0:55

yf_lw

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1 Comment

PB020 Over a year ago

Thank you, you were exactly right. After a bit of testing, I found out that apparently, the issue wasn't my initialization of the pointers but how I was accessing the pointers. The queue I was using to create entities returned an invalid value outside the bounds of all the pointers which caused the leak on runtime.

user17592432 · Accepted Answer · 2022-07-29 23:40:19Z

0

That's quite a mouthful, and difficult to understand.

Since it appears that you want an unchanging 'hierarchy' of nested structs, simply declare them that way...

typedef struct {
    int countused;
    int elements[ 128 ];
} Bits_t;

typedef struct {
    int countused;
    Bits_t bits[ 32 ];
} DiffBits_t;

typedef struct {
    int countused;
    DiffBits_t random[ 16 ];
} Outter_t;

void main( void ) {
    Outter_t *topLevel = malloc( sizeof(*topLevel) );

    printf( "%d", sizeof(Outter_t) );

    topLevel->countused = 1;
    topLevel->random[4].countused = 1;
    /* or */
    DiffBits_t *tmp = &topLevel->random[4];
    tmp->countused = 1;
}

Further, use calloc() instead of malloc() and you won't have to zero-out the nested arrays...

answered Jul 29, 2022 at 23:40

user17592432

2 Comments

PB020 Over a year ago

I figured out what was wrong, but thanks for the tip on using calloc() instead of malloc(). I've reimplemented my initialization as seen in the edit.

user17592432 Over a year ago

Glad to help... Caution about using casting (twice) in that printf... The compiler knows the type of all of the nodes in the chain of ->. Imposing 'casting' on the ultimate struct type may lead to heartbreak (hard to find bugs)... Onward and upwanrd

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