Both Unix and C were created at AT&T's Bell Laboratories in the late 1960s and early 1970s.The C programming language, before it was standardized , did not provide built-in functionalities such as I/O operations . By the beginning of the 1980s compatibility problems between the various C implementations became apparent.
In 1983 the American National Standards Institute (ANSI) formed a committee to establish a standard specification of C known as "ANSI C" . Over time, user communities of C,shared ideas and implementations of what is now called C standard libraries to provide that functionality.
Since the standardisation of the C library , applications written strictly within the bounds mentioned in the standard can vouch to be portable across different platform implementations.
There are lot of online resources which can act as a reference for the standard C libarary :
Books on C :
If you would like to learn and understand the C libarary , you should grab yourself a copy of the The Standard C Library by P. J. Plauger . Dr. Plauger has impeccable qualifications for writing this book - he was secretary of the ANSI C committee.