Code

 Header...
// An extendable string
typedef struct string {
  char *text;
  int len;
  int buflen;
}String;

// Amount to allocate at a time
#define N 8

// Internal function
 String* catstr(String *s,const char *addstr,int len){
   ... 

»  if (s==NULL) {// Create a new String
»    s=xmalloc(sizeof *s);
»    s->buflen=s->len=0;
»    s->text=NULL;
  }
  
»  if (addstr==NULL) // Just create a new empty String
    return s;
  
»  if (len+s->len>=s->buflen) {// Extend text buffer
»    s->buflen=len+s->len;// New string length
»    s->buflen+=N-s->buflen%N;// Make it a multiple of N
»    s->text=realloc(s->text, s->buflen);
  }
»  strcpy(s->text+s->len, addstr);
»  s->len+=len;
  
»  return s;
}

 String* Strcat(String *s,const char *addstr){
   ... 

»  int n=0;
»  if (addstr!=NULL) 
    n=strlen(addstr);
»  return catstr(s, addstr, n);
}

 String* StrCat(String *s,const String *addstr){
   ... 

»  int n=0;
»  if (addstr!=NULL) 
    n=addstr->len;
»  return catstr(s, addstr->text, n);
}

 int main(){
   ... 

»  String *s=NULL;

»  s=Strcat(s, "Hello");
»  Strcat(s, " world");

»  printf("%s\n", s->text);
»  return 1;
}