Fork Join tricky example

Lets write some code

module fork_test;
initial begin
for (int j=0; j<3; j++)
begin

fork

$display(j);

join_none

end
end
endmodule

 

Output 

3
3
3

It outputs 3 because there is only one variable j shared by all the threads, and its value is 3 before any of the threads start executing. None of the threads spawned by the fork/join_none start executing until the thread spawned by the initial block finishes.

module fork_test1;
initial begin
for (int j=0; j<3; j++)
begin
#1;
fork

$display(j);

join_none

end
end
endmodule:fork_test1

if you put delay , thread will see different values of j ..but will miss first value .. 0

Output 

1
2
3

Using AutoMatic 

module fork_test;
initial begin
for (int j=0; j<3; j++)
begin

fork
automatic int k=j;
$display(k);

join_none

end
end
endmodule:fork_test

Output

0
1
2

This works because k, as an automatic variable, is created for each iteration of the fork block and initialized with the current value if j. Creation and initialization of automatic variables occurs as each block is activated and does not wait for execution of the block