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What does the return boolean "Good" do in VHDL textio read function?

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matrixofdynamism

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The read function has multiple overloaded versions in the textio package. Among them I have noticed that there is a set of read functions that have 3 parameters instead of 2. The third parameter is always GOOD: out BOOLEAN

How is this GOOD used?
 

good can be used to determine if the attempted read was valid or not.
consider the following line from a text file:

1 2 3 4 5 6 7 8 9 A B C

if you used:

Code VHDL - [expand]
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variable l : line;
variable i : integer;
 
....
read(l,i);


would cause a fatal error when you got to the character A.
but if you use the good parameter, it wont fail:

Code VHDL - [expand]
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variable l : line;
variable i : integer;
variable good : boolean;
 
....
read(l,i,good);
 
if not good then 
  report "You tried to read an illegal value. Im going to set i to 100 for you" severity warning;
  i := 100;
end if;


It allows you to act on bad reads.
 
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