Accessing unallocated memory

Many times a user experiences a null pointer dereference (or null object access) error during the simulation. What does it mean?

By default, the class handle points to no memory location which is commonly called null. When the user tries to access a memory that does not exist, a null pointer dereference is expected.

Note: null is not a string. So, a comparison with “null” will give a compilation error.

Accessing unallocated memory Example

Let’s understand this with the below example:

tr1 and tr2 are two handles of the same class. Out of them, memory is allocated for tr1 (i.e. object tr1 is created).

tr1.data = 5;   // successfully value is assigned
tr2.data = 10; // since memory was not allocated, simulation terminated with an error

class transaction;
  bit [31:0] data;
  int id;
endclass

module class_example;
  transaction tr1, tr2;
  initial begin
    tr1 = new();
    tr1.data = 5;
    tr1.id = 1;
    $display("Object pkt1.data = %0h, id = %0h", tr1.data, tr1.id);
    
    tr2.data = 10;
    tr2.id = 2;
    $display("Object pkt2.data = %0h, id = %0h", tr2.data, tr2.id);
  end
endmodule

Output:

Object pkt1.data = 5, id = 1
*E,TRNULLID: NULL pointer dereference.
          File: ./testbench.sv, line = 16, pos = 6
         Scope: class_example
          Time: 0 FS + 0

./testbench.sv:16     tr2.data = 10;

Check for null memory access

Premature termination of code can also be avoided by checking whether the handle is null or not.

Example:

class transaction;
  bit [31:0] data;
  int id;
endclass

module class_example;
  transaction tr1, tr2;
  initial begin
    tr1 = new();
    if(tr1 != null) begin  // string "null" is not expected
      tr1.data = 5;
      tr1.id = 1;
      $display("Object tr1.data = %0h, tr1.id = %0h", tr1.data, tr1.id);
    end
    else $display("tr1 object is not created");
    
    if(tr2 != null) begin
      tr2.data = 10;
      tr2.id = 2;
      $display("Object tr2.data = %0h, tr2.id = %0h", tr2.data, tr2.id);
    end
    else $display("tr2 object is not created");   
  end
endmodule

Output:

Object tr1.data = 5, tr1.id = 1
tr2 object is not created

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