CODEWITHSUNDEEP
cdebugging
Rodolphe Chartier
Rodolphe Chartier

Reputation: 66

Wrong result with sizeof

I have here a really weird issue:

typedef struct        s_mem_chunk
{
  void                *addr;
  unsigned int        size;
  short               is_alloc;
  struct s_mem_chunk  *prev;
}                     t_mem_chunk;

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

int main()
{
  t_mem_chunk *mem_chunk;

  mem_chunk = malloc(sizeof(*mem_chunk));
  mem_chunk->prev = 0;

  printf("%x + %x = %x\n", mem_chunk->prev, sizeof(*mem_chunk), mem_chunk->prev + sizeof(*mem_chunk));
  return 0;
}

So the code here should output: "0 + 18 = 18" And it output instead "0 + 18 = 240"

So I am wondering why, this is may cause by the sizeof ot I dont know... I request your help, thanks in advance for your time and have a nice evening ! :D

Upvotes: 0

Views: 103

Answers (2)

Jean-Baptiste Yun&#232;s
Jean-Baptiste Yun&#232;s

Reputation: 36441

You misinterpreted 0 + 18 = 240 which is the right result!

0 is the value of mem_chunk->prev. 18 is the size of your structure; beware that this is in hexa.

You have pointer arithmetic, so mem_chunk->prev + sizeof(*mem_chunk) is not 0+18 as usual but the address of an hypothetic 19-th element of an array starting at 0. So 0x18*0x18=0x240 in hexa. In pointer arithmetic, adding a number to a pointer calculates a move; the int serves as a distance from the pointer, and units for the distance is the type of objects the pointer points to. If you add 1 to an int pointer, you calculate the memory address one int after...

In your case: mem_chunk->prev+1 is not 1 but 0x18 and mem_chunk->prev+2 is not 2 but 0x30.

Also pay attention to the format and use %p for pointers and %lx (%zx in C99) for sizeof which returns a long int.

Upvotes: 3

ouah
ouah

Reputation: 145899

Your program invokes undefined behavior.

x conversion specifier requires an argument of type unsigned int but mem_chunk->prev is a pointer value. Same for mem_chunk->prev + sizeof(*mem_chunk) which does not perform integer arithmetic but pointer arithmetic and yields an invalid pointer.

Upvotes: 1

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