gist.nim

type ServiceMain = proc(gSvcStatus: SERVICE_STATUS)
proc wrapServiceMain(mainProc: ServiceMain): LPSERVICE_MAIN_FUNCTION {.closure.} = 
    ## wraps a nim proc in a LPSERVICE_MAIN_FUNCTION
    return proc(dwArgc: DWORD, lpszArgv: LPTSTR) {.stdcall.} =
        gSvcStatusHandle = RegisterServiceCtrlHandler(
            SERVICE_NAME,
            svcCtrlHandler
        )
        gSvcStatus.dwServiceType = SERVICE_WIN32_OWN_PROCESS
        gSvcStatus.dwServiceSpecificExitCode = 0
        reportSvcStatus(SERVICE_RUNNING, NO_ERROR, 0)   
        mainProc(gSvcStatus) # call the wrapped proc ## <------------ This failes with `illegalCapture`
        reportSvcStatus(SERVICE_STOPPED, NO_ERROR, 0) # we have to report back when we stopped!

when isMainModule:
    import times, os
    proc serviceMain(gSvcStatus: SERVICE_STATUS) =
        ## a service main
        ## use gScvStatus to check if we should stop periodically!
        var fh = open("C:/servicelog.txt", fmAppend)
        fh.write("SERVICE STARTED\n")
        fh.flushFile()
        while gSvcStatus.dwCurrentState == SERVICE_RUNNING:
            reportSvcStatus(SERVICE_RUNNING, NO_ERROR, 0)
            fh.write($epochTime() & "\n")
            fh.flushFile()
            sleep(1_000)   
        fh.write("SERVICE CLOSED BY MANAGER!\n")
        fh.flushFile()
        reportSvcStatus(SERVICE_STOPPED, NO_ERROR, 0) # we have to report back when we stopped!

    var wrapped = wrapServiceMain(serviceMain)
    
    
Solution:
[...]
template wrapServiceMain(mainProc: ServiceMain): LPSERVICE_MAIN_FUNCTION = 
    ## wraps a nim proc in a LPSERVICE_MAIN_FUNCTION
    proc foo(dwArgc: DWORD, lpszArgv: LPTSTR) {.stdcall.} =
        gSvcStatusHandle = RegisterServiceCtrlHandler(
            SERVICE_NAME,
            svcCtrlHandler
        )
        gSvcStatus.dwServiceType = SERVICE_WIN32_OWN_PROCESS
        gSvcStatus.dwServiceSpecificExitCode = 0
        reportSvcStatus(SERVICE_RUNNING, NO_ERROR, 0)   
        mainProc(gSvcStatus) # call the wrapped proc
        reportSvcStatus(SERVICE_STOPPED, NO_ERROR, 0) # we have to report back when we stopped!
    foo
[...]