I dont know how to declare this function.
Error: In function \'int main(int, char**).
line 25 colume 16 [Error] \'histo
You will need to define that function before it is used. For larger programs it is usually helpful to have a header file, though not needed for a single function. Just place a
void histogram(char[N][M]);
at the top of your file before MAIN. If you continue to add things, I suggest using a header file though.