text: #include <string.h>
  #include <stdlib.h>
  
  // hello return a heap allocated string containing the name appended 
  // to "hello " and followed by "!".
  const char *hello(const char *name) {
  	if (name == NULL)
  		return NULL;
  	const char *txt = "hello ";
      char *buf = malloc(strlen(txt)+strlen(name)+2);
      if (buf == NULL)
      	return NULL;
      buf[0] = '\0';
  	strcat(buf, txt);
  	strcat(buf, name);
  	strcat(buf, "!");
      return buf;
  }
  