* Question about circular elaboration order error (GNAT). @ 2008-04-13 18:12 Peter C. Chapin 2008-04-13 19:43 ` Samuel Tardieu ` (2 more replies) 0 siblings, 3 replies; 12+ messages in thread From: Peter C. Chapin @ 2008-04-13 18:12 UTC (permalink / raw) I'm using GNAT GPL 2007. Consider the following packages. The specifications and bodies are each in their own files, as usual. ----> parent.ads <---- package Parent is -- Needed so this package requires/allows a body. procedure Dummy; end Parent; ----> parent-child.ads <---- package Parent.Child is procedure Print_Stuff; end Parent.Child; ----> parent.adb <---- with Parent.Child; package body Parent is procedure Dummy is begin null; end; begin Parent.Child.Print_Stuff; -- Note: invoking child here. end Parent; ----> parent-child.adb <---- with Ada.Text_IO; package body Parent.Child is procedure Print_Stuff is begin Ada.Text_IO.Put_Line("Printing stuff in package Parent.Child"); end Print_Stuff; end Parent.Child; My main procedure just does a "with Parent" but otherwise does nothing (it contains only a null statement). Note that the elaboration code in package Parent is calling a subprogram in package Parent.Child. With the -gnatwl option, GNAT tells me warning: implicit pragma Elaborate_All for "Parent.Child" generated when compiling parent.adb. This does not surprise me and seems fine. However, when trying to build the executable I get: error: elaboration circularity detected info: "parent (body)" must be elaborated before "parent (body)" info: reason: implicit Elaborate_All in unit "parent (body)" info: recompile "parent (body)" with -gnatwl for full details info: "parent (body)" info: must be elaborated along with its spec: info: "parent (spec)" info: which is withed by: info: "parent.child (spec)" info: which is withed by: info: "parent (body)" gnatmake: *** bind failed. I don't understand where the circularity is coming from. Isn't the following elaborate order acceptable: parent (spec) parent.child (spec) parent.child (body) parent (body) How is it the case that parent (body) must be elaborated before parent (body)? I've tried tinkering around with some explicit elaboration related pragmas to control the order of elaboration, but GNAT insists there is circularity regardless. I'm left with the impression that all my attempts to control the elaboration order are fruitless. Note my actual program involves a task in the parent that is trying to use subprograms in the child. However, the difficulties I'm having appear to be unrelated to tasking. Thanks in advance for any thoughts you might have. Peter ^ permalink raw reply [flat|nested] 12+ messages in thread
* Re: Question about circular elaboration order error (GNAT). 2008-04-13 18:12 Question about circular elaboration order error (GNAT) Peter C. Chapin @ 2008-04-13 19:43 ` Samuel Tardieu 2008-04-13 20:20 ` Robert A Duff 2008-04-13 19:46 ` Robert A Duff 2008-04-14 17:33 ` Jeffrey R. Carter 2 siblings, 1 reply; 12+ messages in thread From: Samuel Tardieu @ 2008-04-13 19:43 UTC (permalink / raw) >>>>> "Peter" == Peter C Chapin <pchapin@sover.net> writes: Peter> I don't understand where the circularity is coming from. Isn't Peter> the following elaborate order acceptable: No. pragma Elaborate_All is transitive, and forces the elaboration of Parent body (since Parent.Child has an implicit dependency on Parent) before... Parent body elaboration. Hence the circular elaboration order. But you can fix the situation by providing GNAT with the necessary elaboration information: - put a "pragma Elaborate (Parent.Child)" in Parent body to indicate that Parent elaboration requires Parent.Child to be elaborated; this pragma isn't transitive, so you have to ensure that subprograms of Parent.Child called during the elaboration of Parent do not themselves require that other packages be elaborated first, or add the required pragma; - put a "pragma Elaborate_Body" in Parent.Child spec if you know that this package is likely to be called during the elaboration of other packages; this requires that the body be elaborated just after the spec. Using only the "pragma Elaborate_Body" will not be enough for GNAT, so the "pragma Elaborate" is required. That is because GNAT is overcautious and adds implicit "pragma Elaborate_All" unless you provide explicit "pragma Elaborate". Note that having GNAT be overcautious by default is actually a good thing: I spent three hours two weeks ago debugging a tricky elaboration problem in a software written for an embedded system. The original author incorrectly used "pragma Elaborate". Had he done nothing, GNAT would have warned him about the dangerous situation. I hope this clarifies things. Sam -- Samuel Tardieu -- sam@rfc1149.net -- http://www.rfc1149.net/ ^ permalink raw reply [flat|nested] 12+ messages in thread
* Re: Question about circular elaboration order error (GNAT). 2008-04-13 19:43 ` Samuel Tardieu @ 2008-04-13 20:20 ` Robert A Duff 2008-04-13 21:20 ` Samuel Tardieu 0 siblings, 1 reply; 12+ messages in thread From: Robert A Duff @ 2008-04-13 20:20 UTC (permalink / raw) Samuel Tardieu <sam@rfc1149.net> writes: > No. pragma Elaborate_All is transitive, and forces the elaboration of > Parent body (since Parent.Child has an implicit dependency on Parent) Yes, it has a dependence. But why do you call it "implicit"? - Bob ^ permalink raw reply [flat|nested] 12+ messages in thread
* Re: Question about circular elaboration order error (GNAT). 2008-04-13 20:20 ` Robert A Duff @ 2008-04-13 21:20 ` Samuel Tardieu 2008-04-14 20:21 ` Robert A Duff 0 siblings, 1 reply; 12+ messages in thread From: Samuel Tardieu @ 2008-04-13 21:20 UTC (permalink / raw) >>>>> "Robert" == Robert A Duff <bobduff@shell01.TheWorld.com> writes: Robert> Samuel Tardieu <sam@rfc1149.net> writes: >> No. pragma Elaborate_All is transitive, and forces the elaboration >> of Parent body (since Parent.Child has an implicit dependency on >> Parent) Robert> Yes, it has a dependence. But why do you call it "implicit"? Because it is not spelt explicitely using a with clause or an Elaborate/Elaborate_All pragma. And if it's not explicit, it must be implicit. One could say that when you write "package Parent.Child", you explicitely spell "Parent" so this is an explicit dependency declaration. However, for me it is a declaration of the "Parent.Child" package, which implicitely creates a semantic dependency on Parent. Now, I think we are both nitpicking here, as I don't think the RM says anything about "explicit" vs. "implicit" dependencies, it only consider "static dependences" and "elaboration dependences". Btw, I never noticed the use of "dependence" vs. "dependency" before. Is there a difference in English? Sam -- Samuel Tardieu -- sam@rfc1149.net -- http://www.rfc1149.net/ ^ permalink raw reply [flat|nested] 12+ messages in thread
* Re: Question about circular elaboration order error (GNAT). 2008-04-13 21:20 ` Samuel Tardieu @ 2008-04-14 20:21 ` Robert A Duff 2008-04-14 23:36 ` Adam Beneschan 0 siblings, 1 reply; 12+ messages in thread From: Robert A Duff @ 2008-04-14 20:21 UTC (permalink / raw) Samuel Tardieu <sam@rfc1149.net> writes: >>>>>> "Robert" == Robert A Duff <bobduff@shell01.TheWorld.com> writes: > > Robert> Samuel Tardieu <sam@rfc1149.net> writes: >>> No. pragma Elaborate_All is transitive, and forces the elaboration >>> of Parent body (since Parent.Child has an implicit dependency on >>> Parent) > > Robert> Yes, it has a dependence. But why do you call it "implicit"? > > Because it is not spelt explicitely using a with clause or an > Elaborate/Elaborate_All pragma. And if it's not explicit, it must be > implicit. OK. To me, the primary purpose of a with clause, and the primary purpose of a parent name, is their effect on visibility. Both appear explicitly in the code. Secondary issues are semantic dependences, compilation dependences, and elaboration-order dependences. So I don't see one as more "implicit" than the other. The reason I asked is that "implicit" is often a matter of opinion, and I'm curious about what it means to people in cases like this. There was some argument at AdaCore recently about what pragma Restrictions (No_Implicit_Heap_Allocations); means in a certain obscure case. > One could say that when you write "package Parent.Child", you > explicitely spell "Parent" so this is an explicit dependency > declaration. Yes, one could, and I do. ;-) >...However, for me it is a declaration of the "Parent.Child" > package, which implicitely creates a semantic dependency on Parent. > > Now, I think we are both nitpicking here,... Nitpicking? Yes, I plead guilty as charged. ;-) >... as I don't think the RM says > anything about "explicit" vs. "implicit" dependencies, it only > consider "static dependences" and "elaboration dependences". > > Btw, I never noticed the use of "dependence" vs. "dependency" > before. Is there a difference in English? I don't know. Most people say "dependency". But the Ada 83 RM used "dependence", and we didn't change it for Ada 95 (or 2005), and I've gotten used to typing that. Note that the primary author of Ada 83 was a Frenchman. ;-) - Bob ^ permalink raw reply [flat|nested] 12+ messages in thread
* Re: Question about circular elaboration order error (GNAT). 2008-04-14 20:21 ` Robert A Duff @ 2008-04-14 23:36 ` Adam Beneschan 2008-04-15 7:13 ` Georg Bauhaus 0 siblings, 1 reply; 12+ messages in thread From: Adam Beneschan @ 2008-04-14 23:36 UTC (permalink / raw) On Apr 14, 1:21 pm, Robert A Duff <bobd...@shell01.TheWorld.com> wrote: > > Btw, I never noticed the use of "dependence" vs. "dependency" > > before. Is there a difference in English? > > I don't know. Most people say "dependency". But the Ada 83 RM used > "dependence", and we didn't change it for Ada 95 (or 2005), > and I've gotten used to typing that. I checked Merriam-Webster Online (www.m-w.com). The first meaning of "dependency" is simply a reference to the first meaning of "dependence". (The other two meanings of "dependency" aren't close to being relevant.) The first meaning of "dependence" is "the quality or state of being dependent; especially: the quality or state of being influenced or determined by or subject to another". I think this is meaning of "dependence" that applies here (I'm pretty sure the "drug addiction" meaning does not). So I'd say that for our purpose, the two words are interchangeable, at least in American English usage, which is what Merriam-Webster is a dictionary of. -- Adam ^ permalink raw reply [flat|nested] 12+ messages in thread
* Re: Question about circular elaboration order error (GNAT). 2008-04-14 23:36 ` Adam Beneschan @ 2008-04-15 7:13 ` Georg Bauhaus 0 siblings, 0 replies; 12+ messages in thread From: Georg Bauhaus @ 2008-04-15 7:13 UTC (permalink / raw) Adam Beneschan wrote: > On Apr 14, 1:21 pm, Robert A Duff <bobd...@shell01.TheWorld.com> > wrote: > >>> Btw, I never noticed the use of "dependence" vs. "dependency" >>> before. Is there a difference in English? >> I don't know. Most people say "dependency". But the Ada 83 RM used >> "dependence", and we didn't change it for Ada 95 (or 2005), >> and I've gotten used to typing that. > > I checked Merriam-Webster Online (www.m-w.com). [...] > > So I'd say that for our purpose, the two words are interchangeable, at > least in American English usage, which is what Merriam-Webster is a > dictionary of. Uhm, there is---or was---a distinction which is of some historical interest, if not helpful in rediscovering the distinguished meaning of either word. Not sure whether everyone will like Independency Day 8-) My old 1974/82 (but not my new, 2005) Oxford Advanced Learner's Dictionary has only this to say about "dependency": "country governed or controlled by another." The newer edition additionally explains that "dependency" now also refers to a state of unneccessary reliance on the part of the dependents. Dependency is exemplified in "dependency culture". The definition of "dependence" includes the word "need". E.g. A needs the help of X in order to survive. Both "dependence" and "dependency" are used in place of "addiction", according to OALD. Otherwise the dictionary seems to agree with M-W online. I found that "concurrence" and "occurrency" ... had a funny taste when I tried them ;) Can the widespread use of "dependency" for "dependence" be attributed to learning words in globalized technical contexts and not through a study of English? -- I hadn't been aware of this before to be sure. Thanks for the question. ^ permalink raw reply [flat|nested] 12+ messages in thread
* Re: Question about circular elaboration order error (GNAT). 2008-04-13 18:12 Question about circular elaboration order error (GNAT) Peter C. Chapin 2008-04-13 19:43 ` Samuel Tardieu @ 2008-04-13 19:46 ` Robert A Duff 2008-04-13 22:49 ` Peter C. Chapin 2008-04-14 17:33 ` Jeffrey R. Carter 2 siblings, 1 reply; 12+ messages in thread From: Robert A Duff @ 2008-04-13 19:46 UTC (permalink / raw) "Peter C. Chapin" <pchapin@sover.net> writes: > I'm using GNAT GPL 2007. Consider the following packages. The > specifications and bodies are each in their own files, as usual. > > ----> parent.ads <---- > > package Parent is > -- Needed so this package requires/allows a body. > procedure Dummy; > end Parent; > > ----> parent-child.ads <---- > > package Parent.Child is > procedure Print_Stuff; > end Parent.Child; > > ----> parent.adb <---- > > with Parent.Child; > package body Parent is > procedure Dummy is > begin > null; > end; > begin > Parent.Child.Print_Stuff; -- Note: invoking child here. > end Parent; > > ----> parent-child.adb <---- > > with Ada.Text_IO; > package body Parent.Child is > procedure Print_Stuff is > begin > Ada.Text_IO.Put_Line("Printing stuff in package Parent.Child"); > end Print_Stuff; > end Parent.Child; > > My main procedure just does a "with Parent" but otherwise does nothing > (it contains only a null statement). Note that the elaboration code in > package Parent is calling a subprogram in package Parent.Child. With the > -gnatwl option, GNAT tells me > > warning: implicit pragma Elaborate_All for "Parent.Child" generated This pragma (on Parent body) means Parent.Child, and it's body, and everything they depend on, and their bodies, and so on, must all be elaborated before Parent body. One of those is Parent body -- hence the cycle. If you write pragma Elaborate, I think GNAT will not generate the implicit pragma Elab_All. Pragma Elaborate(Parent.Child) on Parent body means elaborate the body of Parent.Child before Parent body, but it's not transitive. Note that the default GNAT rules are stricter than standard Ada. To get the standard Ada rules, use -gnatE. But it's not a good idea -- the stricter rules are beneficial. For your program, if you use the standard rules, it is implementation dependent whether or not you get Program_Error. That's bad language design! The stricter rules are conservative, and modular -- when compiling Parent, it sees that you're calling Parent.Child, and assumes the worst WITHOUT looking at Parent.Child body. For example, it assumes that Parent.Child might call Dummy, causing a real cycle. Pragma Elaborate is somewhat evil, since it breaks this modularity (it requires one package body to "know" what's in another package body). I suggest you read the section in the GNAT docs about elaboration. It explains all this stuff in great detail. >... I'm left with the impression that all > my attempts to control the elaboration order are fruitless. Well, they're not fruitless, but elab cycles are indeed frustrating. I find the error messages (from all compilers I've tried, not just GNAT) to be confusing. And every time you add or delete one of those pragmas you have to recompile a whole bunch of stuff. > Note my actual program involves a task in the parent that is trying to > use subprograms in the child. However, the difficulties I'm having > appear to be unrelated to tasking. Don't be too sure. Tasks get activated "early", and can easily cause elab cycles. Look at the docs for details. - Bob ^ permalink raw reply [flat|nested] 12+ messages in thread
* Re: Question about circular elaboration order error (GNAT). 2008-04-13 19:46 ` Robert A Duff @ 2008-04-13 22:49 ` Peter C. Chapin 2008-04-14 13:56 ` Robert A Duff 0 siblings, 1 reply; 12+ messages in thread From: Peter C. Chapin @ 2008-04-13 22:49 UTC (permalink / raw) Robert A Duff wrote: > The stricter rules are conservative, and modular -- when compiling > Parent, it sees that you're calling Parent.Child, and assumes the worst > WITHOUT looking at Parent.Child body. For example, it assumes that > Parent.Child might call Dummy, causing a real cycle. Thanks to you and Samuel Tardieu for your replies. They do help to clarify things for me somewhat. It is easy for me to understand how Parent.Child's body would have an elaboration dependency on Parent's spec, but I did not expect Parent.Child's body to necessarily have an elaboration dependency on Parent's body. Are you saying that it doesn't (necessarily) have such a dependency in standard Ada, but that I'm experiencing an example of GNAT's stricter rules? In any event I got my example to work with an appropriate application of pragma Elaborate. I guess I didn't stumble into the right combination in my earlier tinkering. It helps to have a better idea of what is happening. :-) > I suggest you read the section in the GNAT docs about elaboration. > It explains all this stuff in great detail. Yes, actually I read that stuff, but I didn't see anything in particular about elaboration dependencies between parent and child packages. >> Note my actual program involves a task in the parent that is trying to >> use subprograms in the child. However, the difficulties I'm having >> appear to be unrelated to tasking. > > Don't be too sure. Tasks get activated "early", and can easily cause > elab cycles. Look at the docs for details. I'm not saying that the errors in the tasking example (my real program) are wrong. Indeed, I assumed there was some non-trivial stuff going on during elaboration that was stimulating the error. I was just glad when I could (apparently) reproduce the problem with a simpler example. That makes it easier to study, of course. Peter ^ permalink raw reply [flat|nested] 12+ messages in thread
* Re: Question about circular elaboration order error (GNAT). 2008-04-13 22:49 ` Peter C. Chapin @ 2008-04-14 13:56 ` Robert A Duff 0 siblings, 0 replies; 12+ messages in thread From: Robert A Duff @ 2008-04-14 13:56 UTC (permalink / raw) "Peter C. Chapin" <pchapin@sover.net> writes: > Thanks to you and Samuel Tardieu for your replies. You're welcome. >...They do help to > clarify things for me somewhat. It is easy for me to understand how > Parent.Child's body would have an elaboration dependency on Parent's > spec, but I did not expect Parent.Child's body to necessarily have an > elaboration dependency on Parent's body. Are you saying that it doesn't > (necessarily) have such a dependency in standard Ada, but that I'm > experiencing an example of GNAT's stricter rules? Yes. Your original example (with no pragmas added) is legal, and a valid Ada compiler (such as GNAT with -gnatE) will compile and link it just fine. However, it might fail at run time. Worst case: it works fine, but 2 years later, you port it to a new compiler (new machine, or even a new version of the same compiler), and you get a mysterious Program_Error. If you get an elab cycle when you modify your program (e.g., you add a with_clause), it's easy to deal with -- your change is small, and you know it caused the cycle. But if you get a Program_Error when porting a giant program to a new compiler, it can easily take several hours to figure out the problem. - Bob ^ permalink raw reply [flat|nested] 12+ messages in thread
* Re: Question about circular elaboration order error (GNAT). 2008-04-13 18:12 Question about circular elaboration order error (GNAT) Peter C. Chapin 2008-04-13 19:43 ` Samuel Tardieu 2008-04-13 19:46 ` Robert A Duff @ 2008-04-14 17:33 ` Jeffrey R. Carter 2008-04-14 17:52 ` Robert A Duff 2 siblings, 1 reply; 12+ messages in thread From: Jeffrey R. Carter @ 2008-04-14 17:33 UTC (permalink / raw) Peter C. Chapin wrote: > > package Parent is > -- Needed so this package requires/allows a body. > procedure Dummy; > end Parent; Not related to your question, but perhaps of interest to you: Dummy is not needed so the pkg allows a body. Pragma Elaborate_Body is a better way to achieve that: package Parent is pragma Elaborate_Body; end Parent; -- Jeff Carter "My name is Jim, but most people call me ... Jim." Blazing Saddles 39 ^ permalink raw reply [flat|nested] 12+ messages in thread
* Re: Question about circular elaboration order error (GNAT). 2008-04-14 17:33 ` Jeffrey R. Carter @ 2008-04-14 17:52 ` Robert A Duff 0 siblings, 0 replies; 12+ messages in thread From: Robert A Duff @ 2008-04-14 17:52 UTC (permalink / raw) "Jeffrey R. Carter" <spam.jrcarter.not@spam.acm.org> writes: > Peter C. Chapin wrote: >> package Parent is >> -- Needed so this package requires/allows a body. >> procedure Dummy; >> end Parent; > > Not related to your question, but perhaps of interest to you: Dummy is > not needed so the pkg allows a body. Pragma Elaborate_Body is a better > way to achieve that: It's better in many cases, but in this particular case, it would make the program illegal. ;-) > package Parent is > pragma Elaborate_Body; > end Parent; - Bob ^ permalink raw reply [flat|nested] 12+ messages in thread
end of thread, other threads:[~2008-04-15 7:13 UTC | newest] Thread overview: 12+ messages (download: mbox.gz / follow: Atom feed) -- links below jump to the message on this page -- 2008-04-13 18:12 Question about circular elaboration order error (GNAT) Peter C. Chapin 2008-04-13 19:43 ` Samuel Tardieu 2008-04-13 20:20 ` Robert A Duff 2008-04-13 21:20 ` Samuel Tardieu 2008-04-14 20:21 ` Robert A Duff 2008-04-14 23:36 ` Adam Beneschan 2008-04-15 7:13 ` Georg Bauhaus 2008-04-13 19:46 ` Robert A Duff 2008-04-13 22:49 ` Peter C. Chapin 2008-04-14 13:56 ` Robert A Duff 2008-04-14 17:33 ` Jeffrey R. Carter 2008-04-14 17:52 ` Robert A Duff
This is a public inbox, see mirroring instructions for how to clone and mirror all data and code used for this inbox