Variable on stack or heap?
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Variable on stack or heap?
by Gavin Rehkemper
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Hi everyone,
We are inspecting the AST in CDT. Is there any way to figure out if a particular variable was declared on the stack or on the heap? Any help on this topic would be appreciated. Thanks, Gavin _______________________________________________ cdt-dev mailing list cdt-dev@... https://dev.eclipse.org/mailman/listinfo/cdt-dev |
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Re: Variable on stack or heap?
by Alena Laskavaia
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Never herd of syntax to declare variable in C in the heap. What do you mean?
On Thu, Nov 5, 2009 at 7:18 PM, Gavin Rehkemper <gavreh@...> wrote: > Hi everyone, > > We are inspecting the AST in CDT. Is there any way to figure out if a > particular variable was declared on the stack or on the heap? Any help on > this topic would be appreciated. > > Thanks, > Gavin > > _______________________________________________ > cdt-dev mailing list > cdt-dev@... > https://dev.eclipse.org/mailman/listinfo/cdt-dev > > cdt-dev mailing list cdt-dev@... https://dev.eclipse.org/mailman/listinfo/cdt-dev |
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Re: Variable on stack or heap?
by Pete Vidler
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Gavin Rehkemper wrote:
> We have a variable, and we know it is a pointer. How do we determine > whether it points to something allocated on the heap (malloc'd, for > example), or something allocated on the stack (an array of finite length). In the general case this is not possible from an AST, as it must be determined at run-time (pointers can be changed to point at other things, for example). If the specific scenario is *very* simple, you could look for all the assignments in the AST and *may* be able to determine from that exactly what it is pointing to after the assignment completes. Probably not very likely or useful though. At run-time you might be able to do this by checking the address that it points to, assuming you know where the stack and heap are in memory. What are you actually trying to achieve, if you don't mind me asking? Pete -- Peter J. Vidler Senior Systems Developer, TTE Systems Ltd p.vidler@... +44 (0)116 223 1684 Registered address: 106 New Walk, Leicester, LE1 7EA Company number: 05058157 VAT registration: 913 4859 12 www.tte-systems.com _______________________________________________ cdt-dev mailing list cdt-dev@... https://dev.eclipse.org/mailman/listinfo/cdt-dev |
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Re: Variable on stack or heap?
by Tobias Hahn-3
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You cannot.
int x = 0; int* py = new int; int* pInt = &x; pInt = py; Am 06.11.2009 um 16:42 schrieb Gavin Rehkemper: > We have a variable, and we know it is a pointer. How do we determine > whether it points to something allocated on the heap (malloc'd, for > example), or something allocated on the stack (an array of finite > length). > > Thanks for the help! > Gavin > On Fri, Nov 6, 2009 at 6:51 AM, <cdt-dev-request@...> wrote: > Date: Thu, 5 Nov 2009 20:09:28 -0500 > From: Alena Laskavaia <elaskavaia.cdt@...> > Subject: Re: [cdt-dev] Variable on stack or heap? > To: "CDT General developers list." <cdt-dev@...> > Message-ID: > <5498d4500911051709k290c4e5eocaf19a8f50c155ce@...> > Content-Type: text/plain; charset=ISO-8859-1 > Never herd of syntax to declare variable in C in the heap. What do > you mean? > On Thu, Nov 5, 2009 at 7:18 PM, Gavin Rehkemper <gavreh@...> > wrote: > > Hi everyone, > > > > We are inspecting the AST in CDT. Is there any way to figure out > if a > > particular variable was declared on the stack or on the heap? Any > help on > > this topic would be appreciated. > > > > Thanks, > > Gavin > _______________________________________________ > cdt-dev mailing list > cdt-dev@... > https://dev.eclipse.org/mailman/listinfo/cdt-dev Ableton AG, Sitz Berlin, Amtsgericht Berlin-Charlottenburg, HRB 72838 Vorstand: Gerhard Behles, Jan Bohl, Bernd Roggendorf Vorsitzender des Aufsichtsrats: Uwe Struck _______________________________________________ cdt-dev mailing list cdt-dev@... https://dev.eclipse.org/mailman/listinfo/cdt-dev |
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Re: Variable on stack or heap?
by Pete Vidler
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On 7 Nov 2009, at 17:09, Gavin Rehkemper wrote:
> So you're saying that because of this particular case we can never > know if pInt is on the stack or heap? Is there a way to check the > last assignment? How would you know which comes last without running it? An AST is for one translation unit only remember - a global (for example) could also be changed elsewhere. Statically tracing the execution path of a non- trivial program is virtually impossible. > Pete, we are writing an eclipse plugin that is doing some > intelligent code refactoring. To take an example, say we're trying > to get the size of an array. We could either use sizeof() or > malloc_usable_size() - and we want to figure out which one to use > given a particular variable. You can tell the difference between a pointer and an array statically, but not if it's been passed to a function or cast to something else. After all, a function taking a pointer could be called with both malloc'ed and array-based memory in the same program... Pete -- Peter J. Vidler Senior Systems Developer, TTE Systems Ltd p.vidler@... +44 (0)116 223 1684 Registered address: 106 New Walk, Leicester, LE1 7EA Company number: 05058157 VAT registration: 913 4859 12 www.tte-systems.com _______________________________________________ cdt-dev mailing list cdt-dev@... https://dev.eclipse.org/mailman/listinfo/cdt-dev |
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Re: Variable on stack or heap?
by Alena Laskavaia
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This is classic static analysis problem and can be partially solved by
data-flow analysis, but never can be solved completely, i.e. but doing data-flow you can figure out if variable at given line contains pointer to heap, pointer to non-heap, either or unknown. Depends on depth and complexity you reduce "unknown" factor. You cannot do it from AST you need to build control-flow graph, data-flow graph and perform data-flow evaluation for particular know "seeds" i.e. call to malloc or new is known to return value in heap, then you propagate this value through the graph. On Sat, Nov 7, 2009 at 12:09 PM, Gavin Rehkemper <gavreh@...> wrote: > Thanks for all the help everyone. > > Tobias, we're a bit confused by your example. x is on the stack, and *py is > on the heap. In the first case, pint points to something on the stack, and > the second, it points to something on the heap. So you're saying that > because of this particular case we can never know if pInt is on the stack or > heap? Is there a way to check the last assignment? > > Pete, we are writing an eclipse plugin that is doing some intelligent code > refactoring. To take an example, say we're trying to get the size of an > array. We could either use sizeof() or malloc_usable_size() - and we want to > figure out which one to use given a particular variable. > > We were hoping there would be some aspect of the AST that would indicate > this without having to run the code. So it sounds like you're saying there's > no easy way to do this before runtime? > > Thanks, > Gavin > > On Fri, Nov 6, 2009 at 9:50 AM, <cdt-dev-request@...> wrote: > Gavin Rehkemper wrote: >> We have a variable, and we know it is a pointer. How do we determine >> whether it points to something allocated on the heap (malloc'd, for >> example), or something allocated on the stack (an array of finite length). > > In the general case this is not possible from an AST, as it must be > determined at run-time (pointers can be changed to point at other > things, for example). > > If the specific scenario is *very* simple, you could look for all the > assignments in the AST and *may* be able to determine from that exactly > what it is pointing to after the assignment completes. Probably not > very likely or useful though. > > At run-time you might be able to do this by checking the address that it > points to, assuming you know where the stack and heap are in memory. > > What are you actually trying to achieve, if you don't mind me asking? > > Pete > > -- > Peter J. Vidler > Senior Systems Developer, TTE Systems Ltd > > p.vidler@... > +44 (0)116 223 1684 > > Registered address: 106 New Walk, Leicester, LE1 7EA > Company number: 05058157 > VAT registration: 913 4859 12 > > www.tte-systems.com > > _______________________________________________ > cdt-dev mailing list > cdt-dev@... > https://dev.eclipse.org/mailman/listinfo/cdt-dev > > cdt-dev mailing list cdt-dev@... https://dev.eclipse.org/mailman/listinfo/cdt-dev |
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Re: Variable on stack or heap?
by Tobias Hahn-3
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Thanks Alena, I couldn't have phrased it better :)
Am 08.11.2009 um 02:06 schrieb Alena Laskavaia: > This is classic static analysis problem and can be partially solved by > data-flow analysis, but never can be solved completely, > i.e. but doing data-flow you can figure out if variable at given line > contains pointer to heap, pointer to non-heap, either or unknown. > Depends on depth and complexity > you reduce "unknown" factor. You cannot do it from AST you need to > build control-flow graph, data-flow graph and perform data-flow > evaluation for particular know "seeds" > i.e. call to malloc or new is known to return value in heap, then you > propagate this value through the graph. > > On Sat, Nov 7, 2009 at 12:09 PM, Gavin Rehkemper <gavreh@...> > wrote: >> Thanks for all the help everyone. >> >> Tobias, we're a bit confused by your example. x is on the stack, >> and *py is >> on the heap. In the first case, pint points to something on the >> stack, and >> the second, it points to something on the heap. So you're saying that >> because of this particular case we can never know if pInt is on the >> stack or >> heap? Is there a way to check the last assignment? >> >> Pete, we are writing an eclipse plugin that is doing some >> intelligent code >> refactoring. To take an example, say we're trying to get the size >> of an >> array. We could either use sizeof() or malloc_usable_size() - and >> we want to >> figure out which one to use given a particular variable. >> >> We were hoping there would be some aspect of the AST that would >> indicate >> this without having to run the code. So it sounds like you're >> saying there's >> no easy way to do this before runtime? >> >> Thanks, >> Gavin >> >> On Fri, Nov 6, 2009 at 9:50 AM, <cdt-dev-request@...> wrote: >> Gavin Rehkemper wrote: >>> We have a variable, and we know it is a pointer. How do we determine >>> whether it points to something allocated on the heap (malloc'd, for >>> example), or something allocated on the stack (an array of finite >>> length). >> >> In the general case this is not possible from an AST, as it must be >> determined at run-time (pointers can be changed to point at other >> things, for example). >> >> If the specific scenario is *very* simple, you could look for all the >> assignments in the AST and *may* be able to determine from that >> exactly >> what it is pointing to after the assignment completes. Probably not >> very likely or useful though. >> >> At run-time you might be able to do this by checking the address >> that it >> points to, assuming you know where the stack and heap are in memory. >> >> What are you actually trying to achieve, if you don't mind me asking? >> >> Pete >> >> -- >> Peter J. Vidler >> Senior Systems Developer, TTE Systems Ltd >> >> p.vidler@... >> +44 (0)116 223 1684 >> >> Registered address: 106 New Walk, Leicester, LE1 7EA >> Company number: 05058157 >> VAT registration: 913 4859 12 >> >> www.tte-systems.com >> >> _______________________________________________ >> cdt-dev mailing list >> cdt-dev@... >> https://dev.eclipse.org/mailman/listinfo/cdt-dev >> >> > _______________________________________________ > cdt-dev mailing list > cdt-dev@... > https://dev.eclipse.org/mailman/listinfo/cdt-dev _______________________________________________ cdt-dev mailing list cdt-dev@... https://dev.eclipse.org/mailman/listinfo/cdt-dev |
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