Question

I want to start a potentially long running background process from Delphi. I want to leave the process running independently, but first I want to check that the process started OK.

If anything went wrong on startup, I want to capture any output written to standardErr and log it. If the background process starts OK, my program needs to be able to exit and leave the spawned process running.

The psuedo code would be something like this:

process:=RunProgramInBackground('someCommand.exe');
sleep(1000); // Wait a bit to see if the program started OK
if process.Finished and process.ExitCode=FAIL then 
    Raise Exception.Create(process.ErrorStream);
process.Dispose; // Close any connection we may still have to the running process
Program.Exit; // Background process keeps running

I’ve looked at a few things (WinExec, CreateProcess, ShellExecute, JclMiscel) but can’t find any examples for what I’m trying to do. What is the best way to do this?

I’m using Delphi 2010

The background process is a 3rd party program I don’t have the source to.

Answer 1

I ended up using the example No’Am linked to, and adding code to check that the process started OK. This function only checks the exit code of the background process, it doesn’t read the StdErr output.

Here is what I did:

/// <summary> Runs a new process in the background. Waits for a short period, then checks that the process started succesfully.
/// If the process has already finished, checks the exit status. Otherwise, leaves it to run. </summary>
/// <param> ProgramName The executable name, including any parameters.</param>
/// <param> TimeOut Milliseconds to wait before checking the process has executed correctly</param>
/// <param> Directory The full path of the working directory</param>
/// <exception> Exception If the process was not started correctly or if the process was started but returned
/// an error before the timeout.</exception>
procedure ExecBackgroundProcess(ProgramName : String; TimeOut: Integer; Directory:string);
var
    StartInfo : TStartupInfo;
    ProcInfo : TProcessInformation;
    CreateOK : Boolean;
    status: Cardinal;
    theExitCode: Cardinal;
begin
    FillChar(StartInfo,SizeOf(TStartupInfo),#0);
    FillChar(ProcInfo,SizeOf(TProcessInformation),#0);
    StartInfo.cb := SizeOf(TStartupInfo);
    UniqueString(ProgramName); // Required if a const string is passed in. Otherwise the program crashes.
    CreateOK := CreateProcess(nil, PChar(ProgramName), nil, nil,False,
              CREATE_NEW_PROCESS_GROUP+NORMAL_PRIORITY_CLASS+CREATE_NO_WINDOW,
              nil, PChar(Directory), StartInfo, ProcInfo);

    if CreateOK then
    begin
        status:=WaitForSingleObject(ProcInfo.hProcess, TimeOut);
        if status<> WAIT_TIMEOUT then
        begin
            // Program has exited. Get exit code.
            GetExitCodeProcess(ProcInfo.hProcess, theExitCode);
            if theExitCode<>0 then raise Exception.Create('Program '''+ProgramName+''' failed with exit code '+IntToStr(theExitCode));
        end
    end
    else
      Raise Exception.Create('Unable to run '+ProgramName+' in directory '+Directory);

    CloseHandle(ProcInfo.hProcess);
    CloseHandle(ProcInfo.hThread);
end;