[m-users.] Global variables that are local to the thread

[Julien Fischer]

On Sun, 5 Nov 2023, Volker Wysk wrote:

> I'm considering to build a new version of the ODBC library, that's thread-
> safe. There's a point which I don't understand yet.
>
> The ODBC library has a number of global variables. For instance, the ODBC
> environment handle. This makes it thread-unsafe. 
>
> Can these variables be made local to the thread, while still being global,
> so all foreign procedures in the same thread can access it? Or do I need to
> introduce a new argument of some new type odbc_data, which is to be passed
> to all predicates of the library? This type would hold all the formerly
> global data, but it would be burdensome to pass it around all the time.

Either approach could work. Personally, I would pass the extras
infromation to all the predicates and avoid global state at all.
>
> Maybe use a thread-local mutable variable?

You could use a thread-local mutable; I note that the reference manual
doesn't currently talk about how to access the value of a thread-local
mutable from C code.  (It's done using the
MR_{get,set}_thread_local_mutable macros in runtime/mercury_thread.h.)

Julien.