Wrong result with sizeof

Question

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

a pointer doesn't really know the size of what it is pointing at. the correct method to get the size would be: 'sizeof(t_mem_chunk)' — user3629249
– user3629249, Commented Jan 25, 2015 at 18:08
Pointer math is not the same as integer math. For example, see this answer. — user1233508
– user1233508, Commented Jan 25, 2015 at 18:10
@user3629249: sizeof(*mem_chunk) is a perfectly valid way to obtain the size of the type pointed to by mem_chunk. — Blastfurnace
– Blastfurnace, Commented Jan 25, 2015 at 18:10
Makes perfect sense: sizeof(t_mem_chunk) == 0x240/0x18 == 0x18 == 24. — barak manos
– barak manos, Commented Jan 25, 2015 at 18:26

Jean-Baptiste Yunès · Accepted Answer · 2015-01-25 19:08:18Z

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.

Thanks for your help and your advices, now I know what really happens here :)
sizeof yields a size_t, for which the length modifier (C99, afair) is z (as in %zx).

ouah · Accepted Answer · 2015-01-25 18:09:13Z

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.

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