Difference between write() and printf()

Question

Recently I am studying operating system..I just wanna know:

What’s the difference between a system call (like write()) and a standard library function (like printf()

Answer 1

System calls are implemented by the operating system, and run in kernel mode. Library functions are implemented in user mode, just like application code. Library functions might invoke system calls (e.g. printf eventually calls write), but that depends on what the library function is for (math functions usually don't need to use the kernel).