<html><body><div style="color:#000; background-color:#fff; font-family:HelveticaNeue, Helvetica Neue, Helvetica, Arial, Lucida Grande, sans-serif;font-size:16px"><div id="yui_3_16_0_1_1443749416749_3577">But if you trace error_object back, you'll see it gets created from an sscanf</div><div id="yui_3_16_0_1_1443749416749_3789" dir="ltr">of a c string. And that string was stored in the tcl/tk "text" widget as state bound</div><div id="yui_3_16_0_1_1443749416749_3902" dir="ltr">to a <ctrl-click> proc (or in post-1980s version of Pd that I work on, a</div><div dir="ltr">"hyperlink").</div><div id="yui_3_16_0_1_1443749416749_4265" dir="ltr"><br></div><div id="yui_3_16_0_1_1443749416749_4264" dir="ltr">That state can persist well past the life of the object it referred to. For example,</div><div id="yui_3_16_0_1_1443749416749_4115" dir="ltr">the error_object could have been deleted by the user.</div><div id="yui_3_16_0_1_1443749416749_4121" dir="ltr"><br></div><div id="yui_3_16_0_1_1443749416749_4126" dir="ltr">That's why I was generating a random long in my contrived example. If we<br></div><div id="yui_3_16_0_1_1443749416749_4197" dir="ltr">cast garbage to void* and put it to the right of the equals sign, isn't libc</div><div id="yui_3_16_0_1_1443749416749_4759" dir="ltr">technically allowed to respond by serving Pd users a listicle of the top 10 C</div><div dir="ltr">programming references available from Amazon with free shipping?<br></div><div id="yui_3_16_0_1_1443749416749_4758" dir="ltr"><br></div><div id="yui_3_16_0_1_1443749416749_4610" dir="ltr">-Jonathan<br></div><div id="yui_3_16_0_1_1443749416749_3448"><span></span></div> <br><div class="qtdSeparateBR"><br><br></div><div style="display: block;" class="yahoo_quoted"> <div style="font-family: HelveticaNeue, Helvetica Neue, Helvetica, Arial, Lucida Grande, sans-serif; font-size: 16px;"> <div style="font-family: HelveticaNeue, Helvetica Neue, Helvetica, Arial, Lucida Grande, sans-serif; font-size: 16px;"> <div dir="ltr"> <font face="Arial" size="2"> On Thursday, October 1, 2015 7:32 PM, Matt Barber <brbrofsvl@gmail.com> wrote:<br> </font> </div> <br><br> <div class="y_msg_container"><div id="yiv8584727487"><div><div dir="ltr"><div class="yiv8584727487gmail_default" style="font-family:verdana, sans-serif;">As I understand it, you can compare void pointers because they just store addresses.</div><div class="yiv8584727487gmail_default" style="font-family:verdana, sans-serif;"><br clear="none"></div><div class="yiv8584727487gmail_default" style="font-family:verdana, sans-serif;">In g_editor.c:</div><div class="yiv8584727487gmail_default" style="font-family:verdana, sans-serif;"><br clear="none"></div><div class="yiv8584727487gmail_default" style=""><div class="yiv8584727487gmail_default" style=""><font face="verdana, sans-serif"><br clear="none"></font></div><div class="yiv8584727487gmail_default" style=""><font face="verdana, sans-serif">static int glist_dofinderror(t_glist *gl, void *error_object)</font></div><div class="yiv8584727487gmail_default" style=""><font face="verdana, sans-serif">{</font></div><div class="yiv8584727487gmail_default" style=""><font face="verdana, sans-serif"> t_gobj *g;</font></div><div class="yiv8584727487gmail_default" style=""><font face="verdana, sans-serif"> for (g = gl->gl_list; g; g = g->g_next)</font></div><div class="yiv8584727487gmail_default" style=""><font face="verdana, sans-serif"> {</font></div><div class="yiv8584727487gmail_default" style=""><font face="verdana, sans-serif"> if ((void *)g == error_object)</font></div><div class="yiv8584727487gmail_default" style=""><font face="verdana, sans-serif"> {</font></div><div class="yiv8584727487gmail_default" style=""><font face="verdana, sans-serif"> /* got it... now show it. */</font></div><div class="yiv8584727487gmail_default" style=""><font face="verdana, sans-serif"> glist_noselect(gl);</font></div><div class="yiv8584727487gmail_default" style=""><font face="verdana, sans-serif"> canvas_vis(glist_getcanvas(gl), 1);</font></div><div class="yiv8584727487gmail_default" style=""><font face="verdana, sans-serif"> canvas_editmode(glist_getcanvas(gl), 1.);</font></div><div class="yiv8584727487gmail_default" style=""><font face="verdana, sans-serif"> glist_select(gl, g);</font></div><div class="yiv8584727487gmail_default" style=""><font face="verdana, sans-serif"> return (1);</font></div><div class="yiv8584727487gmail_default" style=""><font face="verdana, sans-serif"> }</font></div><div class="yiv8584727487gmail_default" style=""><font face="verdana, sans-serif"> else if (g->g_pd == canvas_class)</font></div><div class="yiv8584727487gmail_default" style=""><font face="verdana, sans-serif"> {</font></div><div class="yiv8584727487gmail_default" style=""><font face="verdana, sans-serif"> if (glist_dofinderror((t_canvas *)g, error_object))</font></div><div class="yiv8584727487gmail_default" style=""><font face="verdana, sans-serif"> return (1);</font></div><div class="yiv8584727487gmail_default" style=""><font face="verdana, sans-serif"> }</font></div><div class="yiv8584727487gmail_default" style=""><font face="verdana, sans-serif"> }</font></div><div class="yiv8584727487gmail_default" style=""><font face="verdana, sans-serif"> return (0);</font></div><div class="yiv8584727487gmail_default" style=""><font face="verdana, sans-serif">}</font></div><div class="yiv8584727487gmail_default" style=""><font face="verdana, sans-serif"><br clear="none"></font></div><div class="yiv8584727487gmail_default" style=""><font face="verdana, sans-serif"><br clear="none"></font></div><div class="yiv8584727487gmail_default" style=""><font face="verdana, sans-serif">this function takes a pointer to void (storing the address of an object) as its second argument void *error_object. It can't know what the type of that object is, because it's being called from somewhere else, and it could be any kind of Pd object. That somewhere else knows what kind of object it is and (more importantly) where that object's address is, and just passes that address in. Then t_gobj *g; traverses the canvas, and since the address of each object is known, each of those can be compared to the object address passed in until there's a match. This is kind of a way of getting around the usual strong typing in c; as long as we know from both ends of the transaction that we can get valid addresses of what we're interested in, there's no problem just comparing those addresses to see if we've found the same object.</font></div><div class="yiv8584727487gmail_default" style=""><font face="verdana, sans-serif"><br clear="none"></font></div><div class="yiv8584727487gmail_default" style=""><font face="verdana, sans-serif"><br clear="none"></font></div></div></div><div class="yiv8584727487gmail_extra"><br clear="none"><div class="yiv8584727487gmail_quote">On Thu, Oct 1, 2015 at 4:45 PM, Jonathan Wilkes via Pd-list <span dir="ltr"><<a rel="nofollow" shape="rect" ymailto="mailto:pd-list@lists.iem.at" target="_blank" href="mailto:pd-list@lists.iem.at">pd-list@lists.iem.at</a>></span> wrote:<br clear="none"><blockquote class="yiv8584727487gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;"><div class="yiv8584727487yqt8090524673" id="yiv8584727487yqt49842"><div><div style="color:#000;background-color:#fff;font-family:HelveticaNeue, Helvetica Neue, Helvetica, Arial, Lucida Grande, sans-serif;font-size:16px;"><div>Hi list,</div><div><br clear="none"></div><div>int compare_pointers(t_pd *foo)</div><div>{</div><div dir="ltr"> long bar = generate_random_long();</div><div dir="ltr"> return (((void *)foo) == ((void *)bar));</div><div dir="ltr">}</div><div dir="ltr"><br clear="none"></div><div dir="ltr">(I probably have unnecessary parens there...)</div><div dir="ltr"><br clear="none"></div><div dir="ltr">Is the check for equality a case of undefined behavior?</div><div dir="ltr"><br clear="none"></div><div dir="ltr">If so, doesn't glob_findinstance of s_print.c also lead to the same</div><div dir="ltr">undefined behavior?</div><span class="yiv8584727487HOEnZb"><font color="#888888"></font></span><div dir="ltr"><br clear="none"></div><div dir="ltr">-Jonathan</div><div dir="ltr"> </div></div></div></div><br clear="none">_______________________________________________<br clear="none">
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