Generic packages, unconstrained types and access types

Why OSVVM™? Forums VHDL Generic packages, unconstrained types and access types


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  • #1026

    I’m implementing a kind of dictionary type in VHDL-08 for using in simulation. I use ModelSim DE 10.4c.

    I’m using a protected type which contains various procedures & functions that form the methods to interact with the dictionary type. The dictionary is implemented as (double) linked list of records entries, a record consists of key, value & access types to last/next entry:

       type t_key;

        type t_key_ptr is access t_key;

        type t_key is record

          key        : string;

          data       : G_DATA_TYPE;

          last_entry : t_key_ptr;

          next_entry : t_key_ptr;

        end record t_key;

    Until now, the key type was fixed to (unconstrained) string, which works without problems. The type of the value field can be given by a generic list to the package. Now, when I try to make the key field generic also with a second generic type (replace the string type by G_KEY_TYPE in the record above), the simulation fails with a runtime error:

        # ** Fatal: (SIGSEGV) Bad handle or reference.

        #    Time: 0 ps  Iteration: 0  Process: /simulationt/DictP File: ../../DictP.vhd

        # Fatal error in Subprogram t_dict.set at ../../DictP.vhd line 52

    For me this looks like a memory access error, which could occur when using access types in a wrong way. This error occurs in ModelSim DE 10.4c. The same code works with Riviera Pro 2015.06 on EDA playground.

    My question before making a bug report to Mentor is: Is my design valid VHDL-08 code? Or are there restrictions when using generic packages and records with unconstrained types which I didn’t considered?

    You can find the test case on EDA playground:


    With older Versions of Riviera Pro on EDA Playground, I get a segmentation fault:

    ./ line 7: 23 Segmentation fault (core dumped) vcom ‘-2008’ design.vhd DictP.vhd testbench.vhd

    Jim Lewis

    Hi Torsten,
    Can you send me the code at jim at synthworks dot com.   I can take a look at it to see what I can.

    I have a scoreboard that does all of the above.  It works well in 10.3.  You may wish to run your test in one of the 10.3 versions.  The socoreboard model is a release candidate for OSVVM, so if you send me an email, I will send it to you.

    Best Regards,

    Jim Lewis

    Hi Torsten,
    I just remembered a work around for a problem that may apply to your situation.

    In VHDL, we can do allocation of this sort of structure as either:
         TailPointer := new ListType'(Item1, Item2, Item3, NULL);
         TailPointer := new ListType ;
         TailPointer.Item1  := Item1;
         TailPointer.Item2  := Item2;
         TailPointer.Item3  := Item3;
         TailPointer.NextPtr   := NULL ;

    I now use the second one as it seems to work around problems in some simulators.



    Hi Jim,

    I tried your advice, now Modelsim prints a error message, which says more about the real problem it has with the code:

    #** Fatal: (vsim-3420): Array lengths do not match. Left is (UNDEFINED) (UNCONSTRAINED ARRAY). Right is 0 (1 to 1)

    So it seems. that Modelsim cannot handle assignments of constrained strings to a  object of unconstrained string type. With unconstrained std_logic_vectors for the data record item, it works.


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