What's the difference between system calls and library routines?

Question

What are the Linux System Calls which can be classified as I/O calls?

Answer 1

System calls are called in Kernel mode. When a user program invokes a system call, it runs with the privileges of System kernel. Whereas Library routines are functions with similar functionality to system calls with programming language specific parameters, arguments are run in user mode, i.e., with limited privileges. But at underlying layers they make use of System calls.

Answer 2

System calls are provided by the system and are executed in the system kernel. They are entry points into the kernel and are therefore NOT linked into your program. These are not portable calls.

Library calls include the ANSI C standard library and are therefore portable. These functions are linked into your program.