Question

I’m pretty rust at my C still and I am just not figuring this out. What I am trying to do is to implement my own malloc so I can keep track of allocations and debug missing calls to free(). I have a header like this:

typedef struct MemoryInfo {
  mem_Kind kind;
  unsigned int id;
  struct MemoryInfo* prev;
  struct MemoryInfo* next;
} MemoryInfo;

And my custom malloc looks something like this:

void* my_malloc(mem_Kind kind, unsigned int size) {
  MemoryInfo* mem;

  allocCount++;
  mem = (MemoryInfo*)malloc(sizeof(MemoryInfo) + size);
  mem->id = id;
  mem->kind = kind;
  // set prev/next...

  return mem + sizeof(MemoryInfo); // return pointer to memory after header
}

But I’m clearly getting my pointer arithmetic wrong because it blows up pretty horribly very quickly. However if I add a void* memory to the end of my struct and do another malloc then it seems to do fine, the problem with that is that I can’t really find the header in my_free if I do that. I’m trying to basically prepend the header so I can do some reverse pointer arithmetic to get the header in free.

void my_free(void* memory) {
  MemoryInfo* mem = memory - sizeof(MemoryInfo); // not correct either
  allocCount--;
  free(mem);
}

What am I doing wrong here?

Answer 1

I think you have a problem with adding to pointer. It has to be like this:

return (char*)mem + sizeof(MemoryInfo); // return pointer to memory after header

and

void my_free(void* memory) {
  MemoryInfo* mem = (MemoryInfo*)((char*)memory - sizeof(MemoryInfo)); // not correct either
  allocCount--;
  free(mem);
}

By the way. Look at this program.

#include <stdio.h>

typedef struct MemoryInfo {
  int kind;
  unsigned int id;
  struct MemoryInfo* prev;
  struct MemoryInfo* next;
} MemoryInfo;



int main()
{
  MemoryInfo* ptr = 0;

  printf("sizeof: %d\n",sizeof(MemoryInfo));
  printf("%d\n",ptr+3);
  return 0;
}

I have added 3 to pointer to MemoryInfo, but its value become 3*sizeof(MemoryInfo).