A "system call" is a call to a kernel function. This is needed for functionality managed by the kernel, like accessing devices. For "normal" operation like adding numbers no help from the kernel is needed. Therefore calling a library which is only computing stuff, no call to kernel space is needed, too .

A "system call" is a call to a kernel function. This is needed for functionality managed by the kernel, like accessing devices. For "normal" operation like adding numbers no help from the kernel is needed. Therefore calling a library which is only computing stuff, no call to kernel space is needed, too.

You can use strace to show all the system calls of a given program.