IO#sysread

static VALUE
rb_io_sysread(int argc, VALUE *argv, VALUE io)
{
    VALUE len, str;
    rb_io_t *fptr;
    long n, ilen;
    struct io_internal_read_struct iis;
    int shrinkable;

    rb_scan_args(argc, argv, "11", &len, &str);
    ilen = NUM2LONG(len);

    shrinkable = io_setstrbuf(&str, ilen);
    if (ilen == 0) return str;

    GetOpenFile(io, fptr);
    rb_io_check_byte_readable(fptr);

    if (READ_DATA_BUFFERED(fptr)) {
        rb_raise(rb_eIOError, "sysread for buffered IO");
    }

    rb_io_check_closed(fptr);

    io_setstrbuf(&str, ilen);
    iis.th = rb_thread_current();
    iis.fptr = fptr;
    iis.nonblock = 0;
    iis.fd = fptr->fd;
    iis.buf = RSTRING_PTR(str);
    iis.capa = ilen;
    iis.timeout = NULL;
    n = io_read_memory_locktmp(str, &iis);

    if (n < 0) {
        rb_sys_fail_path(fptr->pathv);
    }

    io_set_read_length(str, n, shrinkable);

    if (n == 0 && ilen > 0) {
        rb_eof_error();
    }

    return str;
}