<div>While this question talks abnout XCB, I suppose it is more related to the protocol rather than usage of the XCB library, so I am forwarding this e-mail here.</div><div> </div><div>-------- Beginning of forwarded message --------</div><div>03.02.2021, 23:47, "Junk Mail" <ireallyhatespam@yandex.com>:</div><div> </div><div><div><div>Of course I could simply read what is written here <a href="https://gitlab.freedesktop.org/xorg/lib/libxcb/-/blob/master/src/xcbext.h#L62" rel="noopener noreferrer" target="_blank">https://gitlab.freedesktop.org/xorg/lib/libxcb/-/blob/master/src/xcbext.h#L62</a> but still, I felt the need for examples showing its usage and I found this:</div><div><span style="background-color:#eef2ff;color:#000000;float:none;font-family:'arial' , 'helvetica' , sans-serif;font-size:13.3333px;font-style:normal;font-weight:400;text-decoration-style:initial;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px"><a href="https://github.com/StarchLinux/libxcb/blob/master/xproto.c#L1808" rel="noopener noreferrer">https://github.com/StarchLinux/libxcb/blob/master/xproto.c#L1808</a></div><div><div>xcb_parts[2].iov_base = (char *) &xcb_out;<br style="color:rgb( 0 , 0 , 0 );font-family:monospace;font-size:13.3333px;font-style:normal;font-weight:400;text-decoration-style:initial;text-indent:0px;text-transform:none;white-space:pre;word-spacing:0px" />xcb_parts[2].iov_len = sizeof(xcb_out);<br style="color:rgb( 0 , 0 , 0 );font-family:monospace;font-size:13.3333px;font-style:normal;font-weight:400;text-decoration-style:initial;text-indent:0px;text-transform:none;white-space:pre;word-spacing:0px" /><br style="color:rgb( 0 , 0 , 0 );font-family:monospace;font-size:13.3333px;font-style:normal;font-weight:400;text-decoration-style:initial;text-indent:0px;text-transform:none;white-space:pre;word-spacing:0px" />xcb_parts[3].iov_base = 0;<br style="color:rgb( 0 , 0 , 0 );font-family:monospace;font-size:13.3333px;font-style:normal;font-weight:400;text-decoration-style:initial;text-indent:0px;text-transform:none;white-space:pre;word-spacing:0px" />xcb_parts[3].iov_len = -xcb_parts[2].iov_len & 3; // why?<br style="color:rgb( 0 , 0 , 0 );font-family:monospace;font-size:13.3333px;font-style:normal;font-weight:400;text-decoration-style:initial;text-indent:0px;text-transform:none;white-space:pre;word-spacing:0px" /><br style="color:rgb( 0 , 0 , 0 );font-family:monospace;font-size:13.3333px;font-style:normal;font-weight:400;text-decoration-style:initial;text-indent:0px;text-transform:none;white-space:pre;word-spacing:0px" />xcb_parts[4].iov_base = (char *) value_list;<br style="color:rgb( 0 , 0 , 0 );font-family:monospace;font-size:13.3333px;font-style:normal;font-weight:400;text-decoration-style:initial;text-indent:0px;text-transform:none;white-space:pre;word-spacing:0px" />xcb_parts[4].iov_len = xcb_popcount(value_mask) * sizeof(uint32_t);<br style="color:rgb( 0 , 0 , 0 );font-family:monospace;font-size:13.3333px;font-style:normal;font-weight:400;text-decoration-style:initial;text-indent:0px;text-transform:none;white-space:pre;word-spacing:0px" /><br style="color:rgb( 0 , 0 , 0 );font-family:monospace;font-size:13.3333px;font-style:normal;font-weight:400;text-decoration-style:initial;text-indent:0px;text-transform:none;white-space:pre;word-spacing:0px" />xcb_parts[5].iov_base = 0;<br style="color:rgb( 0 , 0 , 0 );font-family:monospace;font-size:13.3333px;font-style:normal;font-weight:400;text-decoration-style:initial;text-indent:0px;text-transform:none;white-space:pre;word-spacing:0px" />xcb_parts[5].iov_len = -xcb_parts[4].iov_len & 3; // why?</div><div> </div><div>It seems like there are "delimiters" (index 3 and 5 of xcb_parts, which is of type struct iovec), and their lengths are always the negative value of the length of the previous index AND'd with 3, I have found nothing obvious that could shed light here: <a href="https://gitlab.freedesktop.org/xorg/lib/libxcb/-/blob/master/src/xcb_out.c#L221" rel="noopener noreferrer" target="_blank">https://gitlab.freedesktop.org/xorg/lib/libxcb/-/blob/master/src/xcb_out.c#L221</a></div><div>So if I read this code correctly, indexes 3 and 5 are sent directly to the X server, but I failed to find if this is something required by the X Window System protocol spec.</div><div>Could anyone shed light on this?</div></div></div></div><div> </div><div>-------- End of forwarded message --------</div>