GNU bug report logs - #20087
'gensym' is not guaranteed to return a fresh symbol

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: ludo <at> gnu.org (Ludovic Courtès)
To: bug-guile <at> gnu.org
Subject: 'gensym' is not guaranteed to return a fresh symbol
Date: Wed, 11 Mar 2015 18:15:02 +0100

‘gensym’ returns interned symbols, but the algorithm to determine the
new symbol is simplistic and predictable.

Thus, one can arrange to produce a symbol before ‘gensym’ does, leading
‘gensym’ to return a symbol that’s not fresh (in terms of ‘eq?’), as is
the case with the second call to ‘gensym’ here:

--8<---------------cut here---------------start------------->8---
scheme@(guile-user)> (gensym "x")
$1 = x379
scheme@(guile-user)> 'x405
$2 = x405
scheme@(guile-user)> (gensym "x")
$3 = x405
--8<---------------cut here---------------end--------------->8---

Should we worry about it?  I think it may have hard to anticipate
security implications.

Ludo’.

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