>   for (i=start; i<=stop; i++)  { /* i is private by default */
> 
>     scm_init_guile();
>     scm_call_1( func , scm_from_int(i) );

IIUC, you are calling scm_init_guile once per index, whereas calling it 
once per thread would suffice.  For better performance, I propose doing 
it once per thread.

On 13-01-2023 12:10, Damien Mattei wrote:
> there should be difference in implementation of Guile between Mac OS and 
> Linux but i do not know the inner mechanism and algorithm used to run 
> Guile in a C environment,what  scm_init_guile() is doing?

Guile is free software, you can download the source code to read what 
scm_init_guile is doing.

> why must it be placed under the // region on Linux (with slower result)

 From the manual:

6.4  Each thread that wants to use functions from the Guile API needs to 
put itself into guile mode with either ‘scm_with_guile’ or 
‘scm_init_guile’. The global state of Guile is initialized automatically 
when the first thread enters guile mode.

OpenMP does multi-threading, so you need to call scm_init_guile or 
scm_with_guile.

> and anywhere under MacOS ? (speed up code)

You need to do it on non-Linux too -- the extract from the manual does 
not make a 'except on MacOS' exception.  It might work without 
scm_init_guile in some circumstances, but this is not at all guaranteed.

If you want to know the difference between MacOS and Linux in the 
implementation of scm_init_guile, you can read the source code of 
scm_init_guile as mentioned before.

Greetings,
Maxime.