extern void printf();
unsigned int str_len(s)
register char *s;
{
  register char *t;

  t = s;
  for (;;) {
    if (!*t) return t - s; ++t;
    if (!*t) return t - s; ++t;
    if (!*t) return t - s; ++t;
    if (!*t) return t - s; ++t;
  }
}
int main () 
{
  static char *ptr = "makoto%test@harry.ki.nu";
  char i;
      printf("start: %s\n", ptr);
      printf("%0x\n", (int) ptr);
  ptr += str_len(ptr);
      printf("%0x\n",(int) ptr);
  while ( *ptr != '@' ) --ptr; 
      printf("%0x\n",(int) ptr);
      printf("%s\n", ptr);
  i = *ptr;
      printf("%0x\n", i);
  *ptr = 0; 
      printf("done.\n");
  return 0;
}

＠ 出来た。ptr と *p は混ぜてはいけない ?:

extern void   printf();
extern char* strrchr();
static char ptr[] = "makoto%test@harry.ki.nu";

int main () 
{
  char *p;
  p = ptr;
      printf("start: %s\n", ptr);
     p =  strrchr(ptr,'@');
  *p = 0; 
      printf("%s\n", ptr);
  return 0;