difference between library functions and user defined functions in c language?

Answer 1

Does the phrase "user defined" mean anything to you?
.

Answer 2

A user defined function is something you the programmer has defined to perform a specific action or calculation in your program.

Example, you need to calculate somebody's age to the nearest minute. You create a calculateAge function:

double calculateAge(Date dateOfBirth)
{
}

So it is specific to your needs at this time.

A library function has been written by someone else, and is reusuable by many programmers to solve the same problem over and over.

Example: strcpy in C. If you are using C, you will never need to write functions for string manipulation because somebody else did that a long time ago...