Function read 

pub unsafe extern "system" fn read(
    fd: c_uint,
    buf: *mut c_char,
    count: size_t,
) -> ssize_t

read() attempts to read up to count bytes from file descriptor fd into the buffer starting at buf.

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