OOP implementation on ANSI C

Hi guys!  I've been programming using ANSI C for awhile.  Then one day I thought if I could implement the OOP concept on ANSI C, for interest, and searched for some articles.  The most detailed article I've found is by Axel-Tobias-Schreiner, although I cannot completely understand its contents.  So I tried to mimic the sample codes and use the study-by-code approach.  However, it says "Segmentation fault (core dumped)" each time I try to run my sample program.  It'd be appreciated if anyone can point out where it goes wrong.


#include<stdio.h>

typedef struct {
/* Properties */
      unsigned long      length;
      char *            addr;
/* Methods */
      void      (* setString)(void * self, char * strIn);
      char *      (* getString)(void * self);
} String;

void setString(void * self, char * strIn)
{
      String * str = (String *) self;
      unsigned long str_size = strlen(strIn);/* determine length */
      
      str->addr = (char *) malloc(str_size + 1);
      strcpy(str->addr, strIn);/* copy the actual content */
}

char * getString(void * self)
{
      String * str = (String *) self;
      
      return str->addr;
}

int main(int args, char * argv[])
{
      String strA;
      
      strA.setString(& strA, "Example");
      printf("[%s]\r\n", strA.getString(& strA));
      
      return 0;
}


p.s.  As I've figured out, if I remove the "strA." before the setString() and getString(), which means calling the function directly, not through the "typedef String", the program runs perfectly.  But if I do that, it loses the meaning of implementing the OOP concept, because a method SHOULD belong to a class.  Perhaps I should do something about the two lines in main().