Code

 Header...
typedef struct wotsit {
  float val;
  char *str;
  int length;
}Wotsit;

void freeWotsit (Wotsit *w);

 Wotsit* newWotsit(int len){
   ... 

»  Wotsit *w;

»  w=xmalloc(sizeof *w);

»  w->length=len;
»  w->val=0;
»  w->str=xmalloc(w->length);
»  return w;
}

 int main(){
   ... 

»  Wotsit *william;

»  william=newWotsit(20);
»  snprintf(william->str, william->length, 
    "Hello, world\n");
  // Note how snprintf() safely truncated the string
»  snprintf(william->str, william->length, 
    "This buffer is %d bytes long\n", william->length);
  // Note how snprintf() safely truncated the string
»  freeWotsit(william);

»  william=newWotsit(40);
»  snprintf(william->str, william->length, 
    "This buffer is %d bytes long\n", william->length);

»  return 0;
}

 void freeWotsit(Wotsit *w){
   ... 

»  free(w->str);
»  free(w);
}