c - Unexpected pointer change after deferenced pointer affectation -

one of pointer being changed after deference , affect value. don't understand why, since code in function runned multiple times work of time. here code :

typedef struct          s_freelist {   int                   ssize;   struct s_freelist     *next;   struct s_freelist     *back;   int                   *esize; }                       t_freelist;  void            *addnode(void *addr, size_t size) {   t_freelist    *freelist;    // working stuff   freelist = (t_freelist *)addr;   freelist->ssize = size * -1;   freelist->next = ((t_freelist *)g_startaddr)->next;   freelist->back = g_startaddr;   ((t_freelist *)g_startaddr)->next->back = freelist;   ((t_freelist *)g_startaddr)->next = freelist;    // not working stuff   printf("addr = %p\nsize = %d\n", addr, (int)size);   freelist->esize = addr + size - 4;   printf("freelist->esize = %p\n", freelist->esize);   *(freelist->esize) = size * -1;   printf("freelist->esize = %p\n\n", freelist->esize);    return (collapsenodes(addr)); } 

here output address :

addr = 0x1647020 size = 32 freelist->esize = 0x164703c freelist->esize = 0xffffffe00164703c 

from printf output, you're running on platform 64-bit pointers. each pointer has size and, importantly, natural alignment of 8 bytes, gives following layout of structure (offsets in decimal):

  00    ssize;   04    <padding>   08    *next;   16    *back;   24    *esize;   32    <total size> 

you're setting freelist->esize = addr + size - 4, which, size = 32, happens overlap higher portion of esize field itself.

if you're going address arithmetic, try use compiler-provided introspection facilities such sizeof , offsetof wherever possible. may reduce incidence of such errors , make code more readable , more portable.

(or, better yet, avoid address arithmetic @ all).