Function open 

pub unsafe extern "system" fn open(
    filename: *const c_char,
    flags: c_int,
    mode: umode_t,
) -> c_long

The open() system call opens the file specified by pathname. If the specified file does not exist, it may optionally (if O_CREAT is specified in flags) be created by open().

RETURN VALUE

On success, the number of bytes read is returned (zero indicates end of file), and the file position is advanced by this number.

ERRORS

EAGAIN(35), EBADF(9), EFAULT(14), EINTR(4), EINVAL(22), EIO(5), EISDIR(21), etc.

Link

Read the docs here