Function sysread

#[no_mangle]
pub extern "C" fn sysread(
    fd: Ri,
    offset: isize,
    buf: RequiredPtrMut<u8>,
    len: usize,
    dest_read: OptionalPtrMut<usize>,
) -> SyscallResult

Reads len bytes from the file with the resource id fd at offset offset into buf

if dest_read is not null, it will be set to the number of bytes read