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Here in the above image there are three variables,Where pr1 and pr2 are pointers and num is an integer variable.The double pointer comes into picture when a pointer is pointing to another pointer, here pr1 is pointing to pr2Variable num has address: XX771230 Address of Pointer pr1 is: XX661111 Address of Pointer pr2 is: 66X123X1 num ==*pr2==**pr1 &num==pr2==*pr1 &pr2==pr1 pr1 will have the memory address of pr2Therefore *pr1=pr2and**pr1 =num**pr1 => double referencing of pointer pr1 which will give you the value of num

A pointer which points to another pointer is known as double pointer.example:-As per the figure,pr2 is a pointer for num (as pr2 is having address of variable num),similarly pr1 is a pointer for another pointer pr1 (as pr1 is having the address of pointer pr2).In this example pr1 is a double pointer.

After some good study and research I'm writing this answer.The vacuous type of results occur in assertion based verification.In concurrent assertions, the usage of operators such as |->, |=>, if..else, -> (implications and if..else) may result in vacuous results.First Vacuous Success, it could be explained with an example,Property pr1;@(posedge clk)x |-> yendpropertyassert property(pr1)$info("Pass");else$error("Fail");Here, x and y may be some expressions.At some scenarios, if x fails to trigger or starts but never ends throughout the simulation and assertion gives a pass info which is known as a Vacuous success or Vacuous pass.Similarly, if the assertion gives a failure info without starting or ending of the sequence then it is a Vacuous Failure.Vacuous Success and Failure should be eliminated and not to be considered for the proper Verification measure.How to eliminate Vacuous success?Usage of cover keywordproperty pr1;@(posedge clk)x |-> yendpropertyassert property(pr1)else$error("Warning");cover property(pr1)$info("Pass");Here, the true action block of assertion is removed; it has only the false block.The true action block is added using cover keyword for same assertion which eliminates the vacuous success to be passed.2. Usage of Default statementOther scenario where vacuous occurs is, when we use a case in assertion property.property encrypt (logic [1:0] pass_Type);case (pass_Type)2’b00: $fell(frame_) ##1(cmd==READ);2’b01: $fell(frame_) ##1(cmd==WRITE);2’b10: $fell(frame_) ##1(cmd==TABORT);2’b11: $fell(frame_) ##1(cmd==MABORT);default: $fell(frame_) ##1 (cmd==ILLEGAL); //default is optionalendcaseendpropertyNote that if the default statement is not given and all of the comparisons fail,then none of the case item property statements are evaluated.If there is no default and no case branch match, we get a vacuous pass.3. By using assert system tasks,$assertpassoff: This system task turns off the action block associated with PASS of an assertion. This includes PASS indication because of both the vacuous and non-vacuous success.To re-enable the PASS action block, use $assertpasson. It will turn on the PASS action of both the vacuous and non-vacuous successes.If you want to turn on only the non-vacuous PASS then use $assertnonvacuouson system task.Note that these system tasks do not affect an assertion that is already executing.$assertfailoff: This system task turns off the action block associated with the FAIL of an assertion.In order to turn it on, use $assertfailon. Here also, these system tasks do not affect an assertion that is already executing.$assertvacuousoff system task turns off the PASS indication based on a vacuous success. An assertion that is already executing is not affected. By default, we get a PASS indication on vacuous pass.Non-vacuous Success and Non-Vacuous Failure:When assertion satisfies the proper expressions and succeeds as expected then it will be the Non-Vacuous success.When assertion not satisfies the proper expressions and fails as expected then it will be the Non-Vacuous Failure.For further reference,Please referSystemVerilog Assertionsand Functional CoverageBy Ashok B Mehta