pointers - Accessing points inside structs inside unions - C -

i have following data structures

struct a_str {     union {         struct {             struct c_str *c;         } b_str;                  struct {             int i;         } c_str;     };  }  struct c_str {     struct d_str; }  struct d_str {     int num; } 

i trying access num in struct d_str. reason keep on getting segmentation fault.

struct a_str *a = init_a(); //assume memory allocation , init ok. a->b_str.c->d_str.num = 2; 

what wrong?

probably not allocating memory a->b_str.c in init() function, may reason a->b_str.c pointing garbage location , segmentation fault due accessing memory not allocated - illegal memory operation.
if init() function correct, there should not problem (syntax-wise code correct).

below have suggested inti() function allocated memory nested structure correctly (read comments).

struct a_str *init() {     struct a_str *ret  = malloc(sizeof(struct a_str)); // memory `struct a_str`     struct c_str *cptr = malloc(sizeof(struct c_str)); // memory inner struct     ret->b_str.c = cptr;  //assigning valid memory address ret->b_str.c      return ret; } 

below main() code steps deallocate/free() dynamically allocated memory.

int main(int argv, char **argc) {     struct a_str *ret = init();         ret->b_str.c->d.num = 5;     printf("%d\n", ret->b_str.c->d.num);       //make sure free memory allocated through malloc     free(ret->b_str.c);  // 1 first free in struct      free(ret);           // in reverse order of allocation     return 0; }