FILE in C or in other language?Is it an integer or structure or having no particular data type?
It is what is typically termed an opaque data type, meaning it's typically declared as a simple structure, and then internally in the OS libraries the FILE pointer is cast to the actual date-type of the data-structure that the OS will use access data from a file. A lot of these details are system-specific though, so depending on the OS, the definition may differ.