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

I'd like to mention another point that the stdio buffers are maintained in a process’s user-space memory, while system call write transfers data directly to a kernel buffer. It means that if you fork a process after write and printf calls, flushing may bring about to give output three times subject to line-buffering and block-buffering, two of them belong to printf call since stdio buffers are duplicated in the child by fork.