In our multi - user application we are continuously interacting with database. We have a common class through which we are sending POST queries to database and obtaining xml files in return. We are using delegates of NSXMLParser to parse the obtained file. The problem with us is we are facing many crashes in it generally when application is idle and changed data in database is being fetched in background through timer which is invoked after every few seconds. We have also dealt with error handling through try and catch but it proves to be of no use in this case and mostly application crashes with following error : Exception Type: EXC_BAD_ACCESS (SIGBUS) Exception Codes: KERN_PROTECTION_FAILURE at 0x0000000000000020 Strange thing is that many times the fetching of updated data at background works very fine, same methods being successfully executed under similar conditions but suddenly it crashes on one of them. The codes we are using is as follows:

// we are using timer in this way:
chkOnlineUser=[NSTimer scheduledTimerWithTimeInterval:15 target:mmObject selector:@selector(threadOnlineUser) userInfo:NULL repeats:YES];


// this method being called in timer
-(void)threadOnlineUser{//HeartBeat in Thread
    [NSThread detachNewThreadSelector:@selector(onlineUserRefresh) toTarget:self withObject:nil];
}

// this performs actual updation
-(void)onlineUserRefresh{
    NSAutoreleasePool *pool =[[NSAutoreleasePool alloc]init];
    @try{
    	if(chkTimer==1){
    		return;
    	}
    	chkTimer=1;

    	if([allUserArray count]==0){
    		[user parseXMLFileUser:@"all" andFlag:3];
    		[allUserArray removeAllObjects];
    		[allUserArray addObjectsFromArray:[user users]];
    	}
    	[objHeartBeat parseXMLFile:[loginID intValue] timeOut:10];
    	NSMutableDictionary *tDictOL=[[NSMutableDictionary alloc] init];
    	tDictOL=[objHeartBeat onLineList];

    	NSArray *tArray=[[NSArray alloc] init];
    	tArray=[[tDictOL objectForKey:@"onlineuser"] componentsSeparatedByString:@","];
    	[loginUserArray removeAllObjects];
    	for(int l=0;l less than [tArray count] ;l++){
    		int t;//=[[tArray objectAtIndex:l] intValue];
    		if([[allUserArray valueForKey:@"Id"] containsObject:[tArray objectAtIndex:l]]){
    			t = [[allUserArray valueForKey:@"Id"] indexOfObject:[tArray objectAtIndex:l]];
    			[loginUserArray addObject:[allUserArray objectAtIndex:t]];
    		}
    	}
    	[onlineTable reloadData];

    	[logInUserPopUp removeAllItems];
    	if([loginUserArray count]==1){
    		[labelLoginUser setStringValue:@"Only you are online"];
    		[logInUserPopUp setEnabled:YES];
    	}else{
    		[labelLoginUser setStringValue:[NSString stringWithFormat:@"  %d users online",[loginUserArray count]]];
    		[logInUserPopUp setEnabled:YES];
    	}

    	NSMenu *menu = [[NSMenu alloc] initWithTitle:@"menu"];
    	NSMenuItem *itemOne = [[NSMenuItem alloc] initWithTitle:@"" action:NULL keyEquivalent:@""];
    	[menu addItem:itemOne];
    	for(int l=0;l less than [loginUserArray count];l++){
    		NSString *tempStr= [NSString stringWithFormat:@"%@ %@",[[[loginUserArray objectAtIndex:l] objectForKey:@"user_fname"] stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceAndNewlineCharacterSet]],[[[loginUserArray objectAtIndex:l] objectForKey:@"user_lname"] stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceAndNewlineCharacterSet]]];
    		if(![tempStr isEqualToString:@""]){
    			NSMenuItem *itemOne = [[NSMenuItem alloc] initWithTitle:tempStr action:NULL keyEquivalent:@""];
    			[menu addItem:itemOne];
    		}else if(l==0){
    			NSMenuItem *itemOne = [[NSMenuItem alloc] initWithTitle:tempStr action:NULL keyEquivalent:@""];
    			[menu addItem:itemOne];
    		}
    	}
    	[logInUserPopUp setMenu:menu];

    	if([lastUpdateTime isEqualToString:@""]){
    	}else {
    		[self fetchUpdatedInfo:lastUpdateTime];


    		[self fetchUpdatedGroup:lastUpdateTime];// function same as fetchUpdatedInfo
    		[avObject fetchUpdatedInfo:lastUpdateTime];// function same as fetchUpdatedInfo
    		[esTVObject fetchUpdatedInfo:lastUpdateTime];// function same as fetchUpdatedInfo
    	}

    	lastUpdateTime=[[tDictOL objectForKey:@"lastServerTime"] copy];
    }
    @catch (NSException * e) {
    	[queryByPost insertException:@"MainModule" inFun:@"onlineUserRefresh" excp:[e description] userId:[loginID intValue]];
    	NSRunAlertPanel(@"Error Panel", @"Main Module- onlineUserRefresh....%@", @"OK", nil, nil,e);
    }
    @finally {
    	NSLog(@"Internal Update Before Bye");
    	chkTimer=0;
    	NSLog(@"Internal Update Bye");// Some time application crashes after this log

    	// Some time application crahses after "Internal Update Bye" log
    }
}

// The method which we are using to obtain updated data is of following form:
-(void)fetchUpdatedInfo:(NSString *)UpdTime{
    @try {
    	if(initAfterLoginComplete==0){
    		return;
    	}

    	[user parseXMLFileUser:UpdTime andFlag:[loginID intValue]];
    	[tempUserUpdatedArray removeAllObjects];
    	[tempUserUpdatedArray addObjectsFromArray:[user users]];
    	if([tempUserUpdatedArray count]>0){
    		if([contactsView isHidden]){
    			[topContactImg setImage:[NSImage imageNamed:@"btn_contacts_off_red.png"]];
    		}else {
    			[topContactImg setImage:[NSImage imageNamed:@"btn_contacts_red.png"]];
    		}
    	}else {
    		return;
    	}
    	int chkprof=0;
    	for(int l=0;l less than [tempUserUpdatedArray count];l++){
    		NSArray *tempArr1 = [allUserArray valueForKey:@"Id"];
    		int s;
    		if([[[tempUserUpdatedArray objectAtIndex:l] objectForKey:@"Id"] intValue]==profile_Id){
    			chkprof=1;
    		}
    		if([tempArr1 containsObject:[[tempUserUpdatedArray objectAtIndex:l] objectForKey:@"Id"]]){
    			s = [tempArr1 indexOfObject:[[tempUserUpdatedArray objectAtIndex:l] objectForKey:@"Id"]];
    			[allUserArray replaceObjectAtIndex:s withObject:[tempUserUpdatedArray objectAtIndex:l]];
    		}else {
    			[allUserArray addObject:[tempUserUpdatedArray objectAtIndex:l]];
    		}

    		NSArray *tempArr2 = [tempUser valueForKey:@"Id"];
    		if([tempArr2 containsObject:[[tempUserUpdatedArray objectAtIndex:l] objectForKey:@"Id"]]){
    			s = [tempArr2 indexOfObject:[[tempUserUpdatedArray objectAtIndex:l] objectForKey:@"Id"]];
    			[tempUser replaceObjectAtIndex:s withObject:[tempUserUpdatedArray objectAtIndex:l]];
    		}else {
    			[tempUser addObject:[tempUserUpdatedArray objectAtIndex:l]];
    		}
    	}

    	NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"user_fname" ascending:YES];
    	[tempUser sortUsingDescriptors:[NSMutableArray arrayWithObject:sortDescriptor]];
    	[userListTableView reloadData];

    	[groupsArray removeAllObjects];
    	for(int z=0;z less than [tempGroups count];z++){
    		NSMutableArray *tempMArr=[[NSMutableArray alloc] init];
    		for(int l=0;l less than [allUserArray count];l++){
    			if([[[allUserArray objectAtIndex:l] objectForKey:@"GroupId"] intValue]==[[[tempGroups objectAtIndex:z] objectForKey:@"group_id"] intValue]){
    				[tempMArr addObject:[allUserArray objectAtIndex:l]];
    			}
    		}
    		[groupsArray insertObject:tempMArr atIndex:z];
    		[tempMArr release];
    		tempMArr= nil;
    	}

    	for(int n=0;n less than [tempGroups count];n++){
    		[[groupsArray objectAtIndex:n] addObject:[tempGroups objectAtIndex:n]];
    	}
    	[groupsListOV reloadData];
    	if(chkprof==1){
    		[self profileShow:profile_Id];
    	}else {
    	}
    	[self selectUserInTable:0];
    }@catch (NSException * e) {
    	NSRunAlertPanel(@"Error Panel", @"%@", @"OK", nil, nil,e);
    }
}

// The method which we are using to frame select query and parse obtained data is:
-(void)parseXMLForUser:(int)UId stringVar:(NSString*)stringVar{
    @try{

    	if(queryByPost)
    		[queryByPost release];

    	queryByPost=[QueryByPost new];  // common class used to invoke method to send request via POST method

    	//obtaining data for xml parsing
    	NSString *query=[NSString stringWithFormat:@"Select * from userinfo update_time >= '%@' AND NOT owner_id ='%d' ",stringVar,UId];

    	NSData *obtainedData=[queryByPost executeQuery:query WithAction:@"query"];  // method invoked to perform post query

    	if(obtainedData==nil){
    		// data not obtained so return
    		return;
    	}

    	// initializing dictionary to be obtained after parsing
    	if(obtainedDictionary)
    		[obtainedDictionary release];

    	obtainedDictionary=[NSMutableDictionary new];

    	// xml parsing
    	if (updatedDataParser) // airportsListParser is an NSXMLParser instance variable 
    		[updatedDataParser release]; 

    	updatedDataParser = [[NSXMLParser alloc] initWithData:obtainedData];
    	[updatedDataParser setDelegate:self]; 
    	[updatedDataParser setShouldResolveExternalEntities:YES]; 

    	BOOL success = [updatedDataParser parse];  

    }
    @catch (NSException *e) {
    	NSLog(@"wtihin parseXMLForUser- parseXMLForUser:stringVar: - %@",[e description]);
    }

}
//The method which will attempt to interact 4 times with server if interaction with it is found to be unsuccessful , is of following form:
-(NSData*)executeQuery:(NSString*)query WithAction:(NSString*)doAction{
    NSLog(@"within ExecuteQuery:WithAction: Query is: %@ and Action is: %@",query,doAction);
    NSString *returnResult;
    @try {

    	NSString *returnResult;
    	NSMutableURLRequest *postRequest;
    	NSError *error;
    	NSData *searchData;
    	NSHTTPURLResponse *response;
    	postRequest=[self directMySQLQuery:query WithAction:doAction];  // this method sends actual POST request

    	NSLog(@"after directMYSQL in QueryByPost- performQuery... ErrorLogMsg");
    	searchData = [NSURLConnection sendSynchronousRequest:postRequest returningResponse:&response error:&error];
    	returnResult = [[NSString alloc] initWithData:searchData encoding:NSASCIIStringEncoding];

    	NSString *resultToBeCompared=[returnResult stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceAndNewlineCharacterSet]];
    	NSLog(@"result obtained - %@/ resultToBeCompared - %@",returnResult,resultToBeCompared);


    	if(![resultToBeCompared isEqualToString:@""]){
    	}else {
    		sleep(10);
    		postRequest=[self directMySQLQuery:query WithAction:doAction];
    		searchData = [NSURLConnection sendSynchronousRequest:postRequest returningResponse:&response error:&error];
    		if(![resultToBeCompared isEqualToString:@""]){
    		}else {
    			sleep(10);
    			postRequest=[self directMySQLQuery:query WithAction:doAction];
    			searchData = [NSURLConnection sendSynchronousRequest:postRequest returningResponse:&response error:&error];
    			if(![resultToBeCompared isEqualToString:@""]){
    			}else {
    				sleep(10);
    				postRequest=[self directMySQLQuery:query WithAction:doAction];
    				searchData = [NSURLConnection sendSynchronousRequest:postRequest returningResponse:&response error:&error];
    				if(![resultToBeCompared isEqualToString:@""]){
    				}else {
    					sleep(10);
    					postRequest=[self directMySQLQuery:query WithAction:doAction];
    					searchData = [NSURLConnection sendSynchronousRequest:postRequest returningResponse:&response error:&error];
    					if(![resultToBeCompared isEqualToString:@""]){
    					}else {
    						return nil;	
    					}
    				}
    			}
    		}
    	}




    	returnResult = [[NSString alloc] initWithData:searchData encoding:NSASCIIStringEncoding];

    	return searchData;
    }
    @catch (NSException * e) {
    	NSLog(@"within QueryByPost , execurteQuery:WithAction - %@",[e description]);
    	return nil;		
    }
}

// The method which sends POST request to server , is of following form:
-(NSMutableURLRequest *)directMySQLQuery:(NSString*)query WithAction:(NSString*)doAction{   
    @try{
    	NSLog(@"Query is: %@ and Action is: %@",query,doAction);

    	// some pre initialization
    	NSString *stringBoundary,*contentType;
    	NSURL *cgiUrl ;
    	NSMutableURLRequest *postRequest;
    	NSMutableData *postBody;
    	NSString *ans=@"434";

    	cgiUrl = [NSURL URLWithString:@"http://keysoftwareservices.com/API.php"];
    	postRequest = [NSMutableURLRequest requestWithURL:cgiUrl];
    	[postRequest setHTTPMethod:@"POST"];

    	stringBoundary = [NSString stringWithString:@"0000ABCQueryxxxxxx"];
    	contentType = [NSString stringWithFormat:@"multipart/form-data; boundary=%@", stringBoundary];
    	[postRequest addValue:contentType forHTTPHeaderField: @"Content-Type"];

    	//setting up the body:
    	postBody = [NSMutableData data];
    	[postBody appendData:[[NSString stringWithFormat:@"\r\n\r\n--%@\r\n",stringBoundary] dataUsingEncoding:NSUTF8StringEncoding]];
    	[postBody appendData:[[NSString stringWithString:@"Content-Disposition: form-data; name=\"code\"\r\n\r\n"] dataUsingEncoding:NSUTF8StringEncoding]];
    	[postBody appendData:[[NSString stringWithString:ans] dataUsingEncoding:NSUTF8StringEncoding]];

    	[postBody appendData:[[NSString stringWithFormat:@"\r\n--%@\r\n",stringBoundary] dataUsingEncoding:NSUTF8StringEncoding]];
    	[postBody appendData:[[NSString stringWithString:@"Content-Disposition: form-data; name=\"action\"\r\n\r\n"] dataUsingEncoding:NSUTF8StringEncoding]];
    	[postBody appendData:[[NSString stringWithString:doAction] dataUsingEncoding:NSUTF8StringEncoding]];

    	[postBody appendData:[[NSString stringWithFormat:@"\r\n--%@\r\n",stringBoundary] dataUsingEncoding:NSUTF8StringEncoding]];
    	[postBody appendData:[[NSString stringWithString:@"Content-Disposition: form-data; name=\"devmode\"\r\n\r\n"] dataUsingEncoding:NSUTF8StringEncoding]];
    	[postBody appendData:[[NSString stringWithString:[[[NSBundle mainBundle] infoDictionary] objectForKey:@"devmode"]] dataUsingEncoding:NSUTF8StringEncoding]];

    	[postBody appendData:[[NSString stringWithFormat:@"\r\n--%@\r\n",stringBoundary] dataUsingEncoding:NSUTF8StringEncoding]];
    	[postBody appendData:[[NSString stringWithString:@"Content-Disposition: form-data; name=\"q\"\r\n\r\n"] dataUsingEncoding:NSUTF8StringEncoding]];
    	[postBody appendData:[[NSString stringWithString:query] dataUsingEncoding:NSUTF8StringEncoding]];

    	[postBody appendData:[[NSString stringWithFormat:@"\r\n--%@--\r\n",stringBoundary] dataUsingEncoding:NSUTF8StringEncoding]];
    	[postRequest setHTTPBody:postBody];

    	NSLog(@"Direct My SQL ok");// Some time application crashes afte this log

    	//Some time application crashes after "Direct My SQL ok" log
    	return [postRequest mutableCopy];

    }@catch (NSException * e) {
    	NSLog(@"NSException %@",e);
    	NSRunAlertPanel(@"Error Panel", @"Within QueryByPost- directMySQLQuery...%@", @"OK", nil, nil,e);
    	return nil;
    } 
}