If you wrote this code, then you must have some idea of where those functions came from. And if you didn’t write it, and are just copying it from some website or somesuch, you should read the source documentation to find out where it does come from.
So it is,
this is from an example of gl book. i copy it from an article without read the book. search and find it lack of the function in lib.
i shall try and how about other pic format? if i don;t use tga format.