Annotation--annotation

LogicalCaptain said (2021-02-26T16:37:45):

So what's with the return value?

Consider a process whih exits with the value that was passed to it, in other words:

#include<stdio.h>
#include<stdlib.h>

int main(int argc,char *argv[]) {
  if (argc==1) {
     exit(0);
  }
  else {
     int x = atoi(argv[1]);
     exit(x);
  }
}

Then:

?- process_create('/home/paquette/returnit',[0],[]).
true.

?- process_create('/home/paquette/returnit',[-1],[]).
ERROR: Process "/home/paquette/returnit": exit status: 255

?- process_create('/home/calvin/returnit',[12],[]).
ERROR: Process "/home/paquette/returnit": exit status: 12

Okay, but maybe too radical.

What about returning the exit status in an option instead? We have Prolog, we don't necessarily want to get an exception, just fail if the an unification fails

Some sleuthing

process_create/3' source code is available by clicking on the top righ ':-' orange marker in this page.

That Prolog predicate calls process_create(Term, Options3) which is a foreign code function in packages/clib/process.c

That function calls static int do_create_process(p_options *info) which comes in a Windows and POSIX version.

It is not foreseen to get the child process exit code out of there with the current code....

Here is a little temporary replacement for process_create/3 which does the same thing but:

deterministically succeeds
unifies the Term and Options used in the call to the 4th and 5th argument
if an exception is thrown by process_create/2 it is caught, unified with the 6th argument, and then ignored

:- module(extra,[ my_process_create/6 ]).

:- use_module(library(process)).

my_process_create(Exe, Args, Options, TermOnCall, OptionsOnCall, ExceptionTerm) :-
   must_be(var,TermOnCall),
   must_be(var,OptionsOnCall),
   must_be(var,ExceptionTerm),
   process:exe_options(ExeOptions),
   absolute_file_name(Exe, PlProg, ExeOptions),
   must_be(list, Args),
   maplist(process:map_arg, Args, Av),
   prolog_to_os_filename(PlProg, Prog),
   TermOnCall =.. [Prog|Av],
   process:expand_cwd_option(Options, Options1),
   process:expand_env_option(env, Options1, Options2),
   process:expand_env_option(environment, Options2, OptionsOnCall),
   catch(
      process:process_create(TermOnCall, OptionsOnCall),
      ExceptionTerm,
      true).

So you can have:

?- my_process_create(returnit, [12], [], Term, Options, ExeTerm).
Term = '/home/paquette/returnit'(12),
Options = [],
ExeTerm = error(process_error('/home/paquette/returnit',exit(12)),_116).

See also:

So what's with the return value?

Some sleuthing