EXC_BAD_ACCESS error in Objective-C - objective-c

Question

Every now and then I get one of the EXC_BAD_ACCESS errors that seem to plague new Objective-C programmers.
In my program I'm trying to get a get the time in seconds, convert that to a string and then convert that to a NSData object for writing to a file. Here is the code I'm using, but it crashes with an EXC_BAD_ACCESS every time I run it. What am I doing wrong?
-(void) startTheClock{
NSTimeInterval cloqInTime = [NSDate timeIntervalSinceReferenceDate];
NSString * dateStr = [self stringFromTimeInterval:cloqInTime];
NSData * data = [[dateStr stringByAppendingString:#", "] dataUsingEncoding:NSUTF8StringEncoding];
NSLog([#"Data:" stringByAppendingString:[data description]]);
// [data retain]; // <-- Uncommenting this and the [data release] line doesn't prevent the error
[self writeData:data]; // <-- EXC_BAD_ACCESS happens here!
// [data release];
}
When I run this method I get the following output:
timeString: |2011-11-04 16:17:12|
Data:<32303131 2d31312d 30342031 363a3137 3a31322c 20>
As requested here is my stringFromTimeInterval method:
-(NSString *) stringFromTimeInterval:(NSTimeInterval)t{
NSDate * date = [NSDate dateWithTimeIntervalSinceReferenceDate:t];
NSDateFormatter *dateFormat = [[NSDateFormatter alloc] init];
[dateFormat setDateFormat:#"yyyy-MM-dd HH:mm:ss"];
NSString *timeString = [dateFormat stringFromDate:date];
NSLog(#"timeString: %#", timeString);
return timeString;
}
Here is the stacktrace:
#0 0x00007fff84407e90 in objc_msgSend ()
#1 0x00000001054543f0 in 0x00000001054543f0 ()
#2 0x00000001000012ee in -[WorqAppDelegate startTheClock] at /Users/slayton/Documents/Xcode/Worq/Worq/WorqAppDelegate.m:61
#3 0x0000000100001164 in -[WorqAppDelegate cloqInAction:] at /Users/slayton/Documents/Xcode/Worq/Worq/WorqAppDelegate.m:37
#4 0x00007fff8cbb9a1d in -[NSObject performSelector:withObject:] ()
#5 0x00007fff86f69710 in -[NSApplication sendAction:to:from:] ()
#6 0x00007fff86f69642 in -[NSControl sendAction:to:] ()
#7 0x00007fff86f6956d in -[NSCell _sendActionFrom:] ()
#8 0x00007fff86f68a30 in -[NSCell trackMouse:inRect:ofView:untilMouseUp:] ()
#9 0x00007fff86fe88e0 in -[NSButtonCell trackMouse:inRect:ofView:untilMouseUp:] ()
#10 0x00007fff86f6763a in -[NSControl mouseDown:] ()
#11 0x00007fff86f320e0 in -[NSWindow sendEvent:] ()
#12 0x00007fff86eca68f in -[NSApplication sendEvent:] ()
#13 0x00007fff86e60682 in -[NSApplication run] ()
#14 0x00007fff870df80c in NSApplicationMain ()
#15 0x0000000100001092 in main ()
Here is the writeData method, recordFile is an instance of NSFileHandle:
-(BOOL) writeData:(NSData *)data{
if (recordFile != NULL)
[recordFile writeData:data];
else
NSLog(#"Record file is null! No data written");
}

Answer 1

Keep an eye on your recordFile object. I'm wondering if this object has been over-released (and thus deallocated) but is not nil. This would cause the [recordFile writeData:data] method to throw an EXC_BAD_ACCESS. What is the lifespan of recordFile?

Answer 2

Going out on a limb, but I'd say that data is probably nil. Have you checked the value of data?

Answer 3

the debugger and in particular the stack trace is your friend, but yeah, most likely data is nil.
in the debugger window at the prompt, type: po data
or better, just look at the local variables in the left pane and see what data's value is.