I always get error of this question, can someone give me some hints? void free_strings(char **strings); 

The function accepts an array of zero or more strings that has a NULL in the last location. It frees all memory allocated for the array for strings.

Here is an example of some code that would create such an array, using the function copy_string from the page Allocating Strings:

 array = (char **)calloc(6, sizeof(char *)); array[0] = copy_string("dog"); array[1] = copy_string("cat"); array[2] = copy_string("llama"); array[3] = copy_string("mule"); array[4] = copy_string("fish"); 

Here is a complete starter file including the header file you will need to include. stringarray.h does include both string.h and stdio.h so you do not have to include those.

#include "freestrings.h" void free_strings(char **strings) { }