sir i want to know about main(), it is a user defined or library function?

The question is malposed: "user defined" and "library" are not necessarily mutually exclusive, since all function contained in a library (in term of the compiler point of view) are "user defined".

main() is actually called by a function that sits in the default library that is "jumped into" by the OS at program startup.

main() itself is not inside the default library, hence the "user" (in the sense of the "compiler user") must provide it somehow.

He can do that either by writing it in one his own sources, or by linking its project to a some framework library that contains it.