What are the disadvantages of library functions In C?

function does't return more then one value at a time.its needs pointer for such case.

A function must return a result. For it to be useful it must be part of an expression, i.e., not called as a standalone routine. For example, suppose you have a function called 'sum' that adds two...

in user defined function we can make our library according to our need bt in library function , funtions are already defined only we have to call that function

They are very important. The most commonly used functions are the string and file handling ones.