WWW.TESTBENCH.IN - Vera Constructs

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Specman E
Interview Questions

SHADOW VARIABLES
Shadow Variables

By default, all child processes have access to the parentÂ´s variables.However, if multiple processes independently use the same variable,races can occur. To avoid races within fork/join blocks, shadow variables should be used.

EXAMPLE : without shadow variable

program main {
call();
delay(40);
printf(" END OF SIMUALTION \n");
}

task call(){
integer i;
delay(10);
for(i = 0; i < 3; i++) {
    fork
    {
    delay(10);
     printf(" time = %0d: i is %0d \n",get_time(LO),i);
    }
   join none
}
}

RESULTS

time = 20: i is 3
time = 20: i is 3
time = 20: i is 3
END OF SIMUALTION

Look at the solution, i is 3 in all the threads.

EXAMPLE :with shadow variable

program main {
call();
delay(40);
printf(" END OF SIMUALTION \n");
}

task call(){
shadow integer i;  // using shadow variable
delay(10);
for(i = 0; i < 3; i++) {
    fork
    {
    delay(10);
     printf(" time = %0d: i is %0d \n",get_time(LO),i);
    }
   join none
}
}

RESULTS

time = 20: i is 0
time = 20: i is 1
time = 20: i is 2
END OF SIMUALTION

The solution shows that ,Using the shadow keyword forces the Vera compiler to create a copy of the variable i local to each child process, which eliminates race conditions. Any descendants of the child processes will also have a copy of the variable local to that descendant.

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