Code to use NSTask with ps to verify a process is running

Question

I am having endless problems checking to see if the screen saver is running. If I use an NSTask with ps, it crashes or hangs on a lot of users. If I use notifications it seems to be spotty for others.

Any ideas as to why this NSTask is flakey? (Yes, I know it's messy for now as I debug)

  -(BOOL)checkScreenSaverRunning
{
    MYLog(@"Building task to check screen saver running");
    BOOL foundSaver=FALSE;

    NSTask *task;
    int i;
    task = [[NSTask alloc] init];
    [task setLaunchPath: @"/bin/ps"];

    NSArray *arguments;
    arguments = [NSArray arrayWithObjects:  @"-ax", nil];
    [task setArguments: arguments];

    NSPipe *stdpipe;
    stdpipe = [NSPipe pipe];
    [task setStandardOutput: stdpipe];
    NSFileHandle *stdfile;
    stdfile = [stdpipe fileHandleForReading];
    MYLog(@"Launching task to check screen saver running");
    [task launch];

    while ([task isRunning]){
        NSData *stddata;
        stddata = [stdfile readDataToEndOfFile];
        if([stddata length]>0){
            NSString *stdstring = [[NSString alloc] initWithData:stddata
                                            encoding:NSUTF8StringEncoding];
            NSArray *stdReturnValues=[stdstring componentsSeparatedByString:@"\n"];

            for(i=0;i<[stdReturnValues count];i++){

                if([[stdReturnValues objectAtIndex:i]
                     rangeOfString:@"ScreenSaverEngine"].location != NSNotFound){
                    foundSaver=TRUE;
                    MYLog(@"Found screensaver in running processes");
                }
            }
            [stdstring release];
            stdstring=nil;
        }
    }
    MYLog(@"Task ended");
    [task release];
    if(foundSaver)screenSaverIsActive=TRUE;
    else screenSaverIsActive=FALSE;
    return(foundSaver);
}

Why are you trying to check for a screensaver process? Wouldn't you be better checking if the screen is locked? http://stackoverflow.com/questions/11505255/osx-check-if-the-screen-is-locked — Anya Shenanigans, Jan 28 '13 at 23:38
Using system wide notifications such as com.apple.screensaverstarted. Petesh, I will certainly take a look at the other api — JeremyLaurenson, Jan 29 '13 at 01:18

score 1 · Answer 1 · answered Jan 28 '13 at 23:43

What is your higher-level purpose for wanting to know if the screen saver is running? There may be a better way to accomplish that.

If you're trying to diagnose a crash or a hang, show the crash or hang report.

Anyway, if you're going to spawn a subprocess for this, you should probably use killall -0 ScreenSaverEngine instead of ps. killall will find a process by name for you. Using the signal 0 (-0) means "just test for process existence, don't actually signal it". Do [task setStandardError:[NSFileHandle fileHandleWithNullDevice]] to make sure its output goes nowhere. You determine if the process existed by examining the success or failure status of the task after it terminates.

Code to use NSTask with ps to verify a process is running

1 Answers1