Where does "tolower()" come from?

Unreal Courses Talk
#1

In running through lecture 43, creating the IsIsogram() function, we leverage a function called tolower() to make a character lower case.

Letter = tolower(Letter);

I don’t understand where this tolower() function is coming from. I did a ctrl+F12, go to declaration, and it was found in something called ctype.h.

I don’t know what ctype.h is, and it’s not included on any of my pages. Is there a standard set of functions that are always available even if they’re not included? If so, how can I know what other functions are already included for me?

#2

Its part of the standard game engine libraries so it comes in under the engine.h (I think)
https://www.tutorialspoint.com/c_standard_library/ctype_h.htm

https://www.tutorialspoint.com/c_standard_library/c_function_islower.htm

int islower(int c)
This function checks whether the passed character is lowercase letter.1

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