NSThread setThreadPriority: is not working

Question

I am using + (BOOL)setThreadPriority:(double)p; to change priority of NSThread but threadPriority is always 0.5. Return value of setThreadPriority: is TURE.

_thread =  [[NSThread alloc] initWithTarget:self selector:@selector(runThread) object:nil];

-(void)runThread {
    @autoreleasepool {
        [[NSThread currentThread] setName:@"3DF7EF974A80"];
        NSRunLoop *runLoop = [NSRunLoop currentRunLoop];
        [runLoop addPort:[NSMachPort port] forMode:NSDefaultRunLoopMode];
        [_condition lock];
        [_condition signal];
        [_condition unlock];
        [NSThread setThreadPriority:1.0];
        CFRunLoopRun();

        [runLoop removePort:[NSMachPort port] forMode:NSDefaultRunLoopMode];
        [_condition lock];
        [_condition signal];
        [_condition unlock];
    }
}

I am using Xcode 7.0.1 and OS X 10.10.5.

Answer 1

I cannot reproduce the behavior you describe. When I do the following on either iOS or Mac OS X, it correctly sets the thread priority:

@interface ViewController ()
{
    NSThread *_thread;
    NSCondition *_condition;
}
@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];

    _condition = [[NSCondition alloc] init];
    _thread = [[NSThread alloc] initWithTarget:self selector:@selector(runThread) object:nil];
    [_thread start];
    [self performSelector:@selector(checkThreadStatus) onThread:_thread withObject:nil waitUntilDone:false];
}

- (void)checkThreadStatus {
    NSLog(@"%.2f", [[NSThread currentThread] threadPriority]);
}

- (void)runThread {
    @autoreleasepool {
        [[NSThread currentThread] setName:@"3DF7EF974A80"];
        NSRunLoop *runLoop = [NSRunLoop currentRunLoop];
        [runLoop addPort:[NSMachPort port] forMode:NSDefaultRunLoopMode];
        [_condition lock];
        [_condition signal];
        [_condition unlock];
        [NSThread setThreadPriority:1.0];
        CFRunLoopRun();

        [runLoop removePort:[NSMachPort port] forMode:NSDefaultRunLoopMode];
        [_condition lock];
        [_condition signal];
        [_condition unlock];
    }
}

@end

We need minimal, yet reproducible, example of the problem in order to help you diagnose this further.