well c is an early version ANSI C is standard adopt by American National Standard Institute which is adopt by ISO which set the standard for C/C++
the compiler are devlop by different companies so the program written in one compiler may not work on other so if it is ANSI C so it is compliance
Specially in microcontroller world, compilers have to have certain extenstions to support certain devices and make programming easy for a particular family of micros. In that case, the C does not comply with ANSI C standards
The early c also known as K&R C (Kernighan and Ritchie) and has some differences .
It is described in their book "The C Programming Language" . Check this link https://www.eskimo.com/~scs/cclass/krnotes/top.html