I have a threadMethod which shows in console robotMotorsStatus every 0.5 sec. But when I try to change robotMotorsStatus in changeRobotStatus method I receive an exception. Where I need to put locks in that program.

#import "AppController.h"

@implementation AppController
extern char *robotMotorsStatus;

- (IBAction)runThread:(id)sender
{
 [self performSelectorInBackground:@selector(threadMethod) withObject:nil];
}

- (void)threadMethod
{
 char string_to_send[]="QFF001100\r"; //String prepared to the port sending (first inintialization)
 string_to_send[7] = robotMotorsStatus[0];
 string_to_send[8] = robotMotorsStatus[1];
 while(1){
  [theLock lock];
  usleep(500000);
  NSLog (@"Robot status %s", robotMotorsStatus);
  [theLock unlock];
 }

}

- (IBAction)changeRobotStatus:(id)sender
{
 robotMotorsStatus[0]='1';
}