Aggressive GC? Was Re: Untimely garbage collection

Bjorn Gustavsson <>
Sun Jul 14 17:32:38 CEST 2002


Answer to the part about the GC:

Scott Lystig Fritchie <> writes:

> While hacking drivers, I've come across a GC question, similar to
> Shawn's, that I cannot answer.  
> 
> Consider this code snippet:
> 
> 	iolist2binary(B) when binary(B) ->
> 	    {B, size(B)};
> 	iolist2binary(L) when List(L) ->
> 	    B = list_to_binary(L),
> 	    {B, size(B)}.
> 	
> 	foofoo(Port, IOList) when port(Port) ->
> 	    {IOListBinary, IOListBinaryLen} = iolist2binary(IOList),
> 	    C = [ <<?S1_FOOFOO, IOListBinaryLen:32/integer>>, IOListBinary],
> 	    erlang:port_command(Port, C),
> 	    %% Vulnerability window begins here?

Yes. If your driver haven't incremented the reference counter, you are
in trouble.

The compiler knows that the variables C and IOListBinary will not be
used anymore; therefore, at this point no reference to them will
be kept, and the next garbage collection the reference counts
for the binaries will be decremented.

> 	    get_port_reply(Port).
> 	
> 	get_port_reply(Port) ->
> 	    receive
> 	        {Port, ok} -> ok;
> 	        [...]
> 

/Bjorn
-- 
Björn Gustavsson            Ericsson Utvecklings AB
      ÄT2/UAB/F/P
			    BOX 1505
+46 8 727 56 87 	    125 25 Älvsjö



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