malloc - C programming, error in getting value using container_of() macro -

i have created following code understand offsetof() , container_of() macros. here printf() show 2 different address instead of same address. doing wrong?

#include <stdio.h> #include <stddef.h>  typedef unsigned char byte;  #define container(ptr, type, member)               \ ({                                                 \    (type *)((byte *)ptr - offsetof(type, member)); \ })  typedef struct {    size_t size;                      void *block;                   }header;  int main() {    void *ptr = malloc(3);    header *phdr = container(ptr, header, block);    printf("%p %p\n", ptr, phdr->block);    return 0; } 

it looks code attempting inspect/demonstrate inner workings of malloc implementation. container macro working, problem header structure definition incorrect:

typedef struct {    size_t size;    void* block; } header; 

this implying malloc keeps size of allocation in memory followed pointer block. not accurate - assuming level of indirection not exist. code ends interpreting contents of data block void*. since haven't initialized memory, see whatever happens there (from values gave, looks may seeing pointer next free block - when malloc(0) you're not allowed write data block implementation may use - see struct malloc_chunk in glibc malloc implementation example).

a more correct structure definition (works me on both clang-503.0.40/llvm 5.1 , vc 2012) be:

typedef struct {    size_t size;    char block[0]; } header; 

you need change printf print address of block:

printf("%p %p\n", ptr, &phdr->block); 

do note, of course, there no guarantee every malloc implementation store block size before block, may not work.