{"id":25,"date":"2009-07-31T11:58:00","date_gmt":"2009-07-31T09:58:00","guid":{"rendered":"\/post\/2009\/07\/31\/The-Nested-Procedure-Menace-in-Oberon.aspx"},"modified":"2014-02-22T21:06:52","modified_gmt":"2014-02-22T20:06:52","slug":"the-nested-procedure-menace-in-oberon","status":"publish","type":"post","link":"https:\/\/cymbeline.ch\/2009\/07\/31\/the-nested-procedure-menace-in-oberon\/","title":{"rendered":"The Nested Procedure Menace in Oberon"},"content":{"rendered":"<p>Just stumbled across this <a title=\"Niklaus Wirth: Geek of the Week\" href=\"http:\/\/www.simple-talk.com\/opinion\/geek-of-the-week\/niklaus-wirth-geek-of-the-week\/\">article and interview with Niklaus Wirth<\/a>, from July 2 this year. There are quite a few interesting things in there, and also historical facts which are good to know. I&#8217;ve run into Wirth a couple of times when I was doing my master thesis at ETH. In fact, I&#8217;ve done my thesis in the group which was originally headed by Wirth and now is headed by Gutknecht (also a well-known name from the article).<\/p>\n<p>That&#8217;s also when I detected the following flaw in Oberon. Imagine the following module:<\/p>\n<pre class=\"brush: plain; title: NestedProcTest.mod; notranslate\" title=\"NestedProcTest.mod\">\r\nMODULE NestedProcTest;\r\n\r\nVAR SomeProc: PROCEDURE(val: LONGINT);\r\n\r\n    PROCEDURE ProcA();\r\n    VAR someLocalVar: LONGINT;\r\n\r\n        PROCEDURE NestedProc(val: LONGINT);\r\n        BEGIN\r\n            someLocalVar := val\r\n        END NestedProc;\r\n\r\n    BEGIN\r\n        someLocalVar := 1234H;\r\n        SomeProc := NestedProc;\r\n        NestedProc(5678H);\r\n        SomeProc(0DEADH)\r\n    END ProcA;\r\n\r\nEND NestedProcTest.\r\n<\/pre>\n<p>On line 15 we assign the nested procedure (&#8216;NestedProc&#8217;) to the procedure variable &#8216;SomeProc&#8217;. This is valid because the signatures match, right? <strong>Wrong.<\/strong> The signature for &#8216;NestedProc&#8217; is different: Because nested procedures have access to the parent procedures&#8217; variables, their signature is added a hidden parameter, the so called <em>Static Link<\/em>. If this parameter is not passed to the procedure when it&#8217;s called, the procedure is going to read either its local variables or its parents&#8217; variables from a wrong place (depending on the actual implementation of the calling convention). This of course is wrong and can even make this code crash in case we&#8217;d read from an unallocated or protected region.<\/p>\n<p>That&#8217;s why I recommended disallowing the assignment of nested procedures to procedure variables. Back in 2006 I have also proposed a change for the Oberon compiler which comes with <a title=\"BlueBottle Operating System\" href=\"http:\/\/www.bluebottle.ethz.ch\">BlueBottle<\/a>, but I cannot tell at this point whether it has made it in to the release or not.<\/p>\n<p>This and other findings can be found in the report for my master thesis (dated March 2006):\u00c2\u00a0<a href=\"http:\/\/www.cymbeline.ch\/wp-content\/uploads\/2014\/02\/Improved-Stackmanagement-in-the-Active-Object-Kernel-Master-Thesis-Roger-Keller.pdf\">Improved Stackmanagement in the Active Object Kernel &#8211; Master Thesis, Roger Keller<\/a>  (742.85 kb)<\/p>\n<p><\/p>","protected":false},"excerpt":{"rendered":"<p>Just stumbled across this article and interview with Niklaus Wirth, from July 2 this year. There are quite a few interesting things in there, and also historical facts which are good to know. I&#8217;ve run into Wirth a couple of times when I was doing my master thesis at ETH. In fact, I&#8217;ve done my &hellip; <a href=\"https:\/\/cymbeline.ch\/2009\/07\/31\/the-nested-procedure-menace-in-oberon\/\" class=\"more-link\">Continue reading<span class=\"screen-reader-text\"> &#8220;The Nested Procedure Menace in Oberon&#8221;<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":[],"categories":[61],"tags":[46,12,18,45],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v19.13 - https:\/\/yoast.com\/wordpress\/plugins\/seo\/ -->\n<title>The Nested Procedure Menace in Oberon - Tales of a Code Monkey<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/cymbeline.ch\/2009\/07\/31\/the-nested-procedure-menace-in-oberon\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"The Nested Procedure Menace in Oberon - Tales of a Code Monkey\" \/>\n<meta property=\"og:description\" content=\"Just stumbled across this article and interview with Niklaus Wirth, from July 2 this year. 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