c - struct assignment: segment fault 11 -

#include <stdio.h> #include <string.h> #include <stdlib.h>  union value {     long long i;     unsigned long long u;     double d;     long double ld;     void *p;     void (*g) (); };  struct foo {     struct {         union value max;         union value min;     }limits; };  struct bucket_info {     void *p;                    // free position     void *limit;                // end position     struct bucket_info *next;   // next bucket };  #define nodes 8192  void * my_malloc(size_t size) {     void *p = malloc(size);     if (!p)         exit(1);     memset(p, 0, size);     return p; }  void * alloc_bucket(size_t size) {     struct bucket_info *pb;      pb = my_malloc(sizeof(struct bucket_info) + size);     pb->p = pb + 1;     pb->limit = (char *)pb->p + size;     return pb; }  void * alloc_for_size(struct bucket_info *s, size_t size) {     void *ret;      while (s->next)         s = s->next;      if ((char *)s->p + size > (char *)s->limit) {         struct bucket_info *pb = alloc_bucket(size * nodes);         s->next = pb;         s = pb;     }      ret = s->p;     s->p = (char *)s->p + size;     return ret; }  static void * alloc_node(struct bucket_info **s, size_t size) {     if (!*s)         *s = alloc_bucket(size * nodes);     return alloc_for_size(*s, size); }  static struct bucket_info *foo_info; void * alloc_foo_node() {     void *ret = alloc_node(&foo_info, sizeof(struct foo));     return ret; }  struct foo * new_foo() {     return alloc_foo_node(); }  void test(int t, struct foo *foo1) {     struct foo *foo2 = new_foo();     // crash @ line     *foo2 = *foo1;     // comment switch statement, works. why?     switch (t) {         case 1:             break;          default:             break;     } }  int main(int argc, const char * argv[]) {      struct foo *foo1 = new_foo();     test(10, foo1);      return 0; } 

above complete code. , i've compiled clang, got 'segment fault 11' @ line:

*foo2 = *foo1; 

then, change line to:

memcpy(foo2, foo1, sizeof(struct foo)); 

it works.

then i've tried compiled these 2 cases gcc, there no problem.

the value returned alloc_foo_node may not correctly aligned struct foo.

on system, printing _alignof(struct foo) gives 16, pointers foo1 , foo2 not multiples of 16.

so causes undefined behaviour convert unaligned result of alloc_foo_node have type struct foo *.

to fix have muck around lot more allocation code, make sure ever hands out space on correct boundary struct foo. use max_align_t (it defined _alignof(max_align_t) biggest possible alignment required).