Using C language

Write the function

void longestSubstring (char *in, char **out);

which receives the string in and it returns the longest sub-string within in formed only by uppercase letters. The string out must be allocated by the function with the minimum correct length. For example if in = "The longest ENGlISH word is: PSEUDOpseudohypoparaTHYROIDISM" the function has to create and return out = "THYROIDISM".