Function syscall4 

pub unsafe fn syscall4(
    n: usize,
    a1: usize,
    a2: usize,
    a3: usize,
    a4: usize,
) -> isize

Make a syscall with 4 arguments.

Safety

This function directly invokes system calls which can have undefined behavior if called with invalid syscall numbers, invalid arguments, or if the kernel state is inconsistent. The caller must ensure the syscall number and arguments are valid for the target system.