Re: Maxima and thread safety

Le 09/06/2026 à 17:33, Stavros Macrakis a écrit :
> Binding it locally just saves the outside value and sets the value 
> cell to the new value.

I don't think this is what happens with sbcl with threading support -at 
the lisp level. As David has shown by looking at the value of 
sb-vm::*free-tls-index*, when binding a special variable in a form like 
let, prog, etc. what people call dynamic binding, the value is not 
stored in the value cell of the  special variable, but in thread local 
storage. Unfortunately this does not happen for special variables 
defined in the global scope like those defined by defvar or defparameter 
at the top level, which can be seen and modified everywhere, and in all 
threads, with the "exception" that the value in a child thread doesn't 
inherit from the value in the parent thread as modified in a let form. 
The example i have provided shows that. This is the cause of problems 
with maxima which has a lot of global special variables.

By the way, for the ignorant like me, the binding of the special 
variable *x* is called dynamic because it subsists for all duration of 
the form which establishes it, for example (let ((*x* 3)...)....) 
including eventual function calls in the let form, and disappears at the 
end of the form. There are examples of this in the simptimes program, 
which puzzled me a lot. Finally, in view of that, one can understand the 
following sentence of CLTL:

"It is convenient to define /dynamic scope/ to mean /indefinite scope 
and dynamic extent/. Thus we speak of ``special'' variables as having 
dynamic scope, or being dynamically scoped, because they have indefinite 
scope and dynamic extent: a special variable can be referred to anywhere 
as long as its binding is currently in effect"

Conversely lexical variables have lexical scope, can be referred to only 
inside the form where they are defined, but may have indefinite extent, 
that is the binding can subsist outside of the form creating it in closures.

-- 
Michel Talon

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