A system call is an entry point to the kernel. system calls are executed in kernel mode. Hence, whenever a system call is made from a user program a mode switching has to be done from "user mode" to "kernel mode". System calls are provided by each OS. They are not portable. example: read.
A library function usually works in a user mode which may or may not require any OS services, for example, strlen() which calculates the length of a particular string.
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