Return variable through start command - windows

Question

I am looking for a way to get a value return from a start-command launched batch script. Let me explain:
I need to take advantage of multiprocessing by launching multiple sub-batch scripts simultaneously from a main batch script, then retrieve every sub batch file return value when they're done.
I've been using return variables with the call-command as very well explained by dbenham.
That solution does not allow multithreading, since sub-batch scripts are run one after the other.
Using the start-command allows multiple running batch scripts, but values are not returned to my main script because apparently the start-command creates a whole new variable context.
Does anybody have a solution/workaround to return values from the sub-scripts to the main script ?
Below is a model of what I need:
mainScript.bat
#echo off
setlocal
set "retval1=0"
set "retval2=0"
REM run two scripts in parallel:
start "" subscript1.bat arg1 retval1
start "" subscript2.bat arg1 retval2
REM wait for returned value
:waiting
call :sleep 1
set /a "DONE=1"
if %retval1% equ 0 set "DONE=0"
if %retval2% equ 0 set "DONE=0"
if %DONE% equ 0 goto :waiting
echo returned values are %retval1% %retval2%
exit /b
subscript1.bat
#echo off
setlocal
set "arg1=%~1"
set "retval1=%~1"
REM do some stuff...
REM return value
(endlocal
set "%retval1%=%foo%"
)
exit /b

Answer 1

Can't see any alternative to writing your return values to files, so
main
#ECHO OFF
SETLOCAL
for %%a in (1 2) do (
del "%temp%\retval%%a" 2>nul
)
start /min "" q225220791.bat arg1 retval1
choice /t 1 /d y >nul
start /min "" q225220791.bat arg1 retval2
:waiting
choice /t 1 /d y >nul
ECHO wait...%retval1%...%retval2%
if not exist "%temp%\retval1" goto waiting
if not exist "%temp%\retval2" goto waiting
for %%a in (1 2) do (
for /f "usebackqdelims=" %%i in ("%temp%\retval%%a") do set "retval%%a=%%i"
)
for %%a in (1 2) do (
del "%temp%\retval%%a" 2>nul
)
echo returned values are %retval1% %retval2%
GOTO :EOF
q225220791.bat
#ECHO OFF
SETLOCAL
:: wait a random time 2..10 sec.
SET /a timeout=%RANDOM% %% 8 + 2
choice /t %timeout% /d y >nul
:: return a random result 12..20
SET /a foo=%RANDOM% %% 8 + 12
>"%temp%\%2" echo %foo%
ENDLOCAL
exit
Relying on the value of the second parameter to the sub-process to set the tempfile name. I've changed the names of the batches to suit my system.

Answer 2

Not sure if this is even practical, but just testing to avoid the temporary file. So looking for a place in child process that can be readed from parent process i decided to use the window title.
task.cmd
#echo off
setlocal
rem Retrieve task information and set window title
set "taskID=%~1"
title [task];%taskID%;working;
rem Retrieve the rest of parameters. For this sample, a random value
set /a "timeToWait=%~2 %% 30"
rem Simulate command processing
timeout /t %timeToWait%
rem Calculate a return value for this task
for /f "tokens=1-10 delims=,.:/ " %%a in ("%date%%time%_%~2") do set "returnValue=%%a%%b%%c%%d%%e%%f%%g%%h%%i%%j"
rem Signal the end of the task
title [task];%taskID%;ended; my return value is %returnValue% ;
rem Wait for master to end this tasks
cls
echo Waiting for master....
waitfor %taskID%
rem Cleanup
endlocal
master.cmd
#echo off
setlocal enableextensions enabledelayedexpansion
rem Configure tasks
set "taskPrefix=myTSK"
set "numTasks=5"
rem Start tasks
for /l %%a in (1 1 %numTasks%) do (
set "return[%taskPrefix%%%a]=unknown"
start "[task];%taskPrefix%%%a;working;" cmd /c "task.cmd %taskPrefix%%%a !random!"
)
rem Wait for tasks to start
timeout /t 2 > nul
rem Wait for tasks to end. Get the list of cmd.exe windows with window title
rem to see the state of the task
rem Tasks in working state indicate master needs to keep working
rem Tasks in ended state have the return value in the window title and are
rem waiting for the master to retrieve the value and end them
:wait
set "keepWaiting="
for /f "tokens=9 delims=," %%a in ('tasklist /fi "imagename eq cmd.exe" /fo csv /v ^| findstr /l /c:"[task];%taskPrefix%"'
) do for /f "tokens=2-4 delims=;" %%b in ("%%a") do (
if "%%c"=="working" (
set "keepWaiting=1"
) else if "%%c"=="ended" (
set "return[%%b]=%%d"
start "" /min waitfor.exe /si %%b
)
)
rem If any task has been found in working state, keep waiting
if defined keepWaiting (
echo %time% : waiting ...
timeout /t 5 > nul
goto wait
)
rem All tasks have ended. Show return values
for /l %%a in (1 1 %numTasks%) do (
echo task #%%a ended with exit value :[!return[%taskPrefix%%%a]!]
)
endlocal
In this sample, the task waits for master using waitfor command. In XP this is not available, and can be replaced with just a pause, and from master.cmd the waiting loop must be modified to include the processID token in the tasklist processing, so the waiting task can be closed with taskkill.