Prismatic Joint causes a crash-Box2D(cocos2d)

Question

I tried to set up a prismatic joint, but I get this error upon run time: Assertion failed: (IsLocked() == false), function CreateBody, file /Users/Aether/Developer/JFRecode/JFRecode/libs/Box2D/Dynamics/b2World.cpp, line 109.

Here is the code for the screen boundaries since I am using it as bodyB.

CGSize s = [CCDirector sharedDirector].winSize;


    //define screen boundaries
     // Create edges around the entire screen
     b2BodyDef groundBodyDef;
     groundBodyDef.position.Set(0,0);
     _groundBody = _world->CreateBody(&groundBodyDef);

     b2EdgeShape groundBox;
     b2FixtureDef groundBoxDef;
     groundBoxDef.shape = &groundBox;

     groundBox.Set(b2Vec2(0,0), b2Vec2(s.width/PTM_RATIO, 0));
     _bottomFixture = _groundBody->CreateFixture(&groundBoxDef);

     groundBox.Set(b2Vec2(0,0), b2Vec2(0, s.height/PTM_RATIO));
     _groundBody->CreateFixture(&groundBoxDef);

     groundBox.Set(b2Vec2(0, s.height/PTM_RATIO), b2Vec2(s.width/PTM_RATIO,
     s.height/PTM_RATIO));
     _groundBody->CreateFixture(&groundBoxDef);

     groundBox.Set(b2Vec2(s.width/PTM_RATIO, s.height/PTM_RATIO),
     b2Vec2(s.width/PTM_RATIO, 0));
     _groundBody->CreateFixture(&groundBoxDef);

Here is the code for the prismatic joint itself

    b2PrismaticJointDef jointDef;
    b2Vec2 worldAxis(1.0f,0.0f);
    jointDef.collideConnected = true;
    jointDef.Initialize(_shipBody,_groundBody, 
                        _shipBody->GetWorldCenter(), worldAxis);
    _world->CreateJoint(&jointDef);

Answer 1

isLocked indicates that the physics world is being updated. That means you're running this code during the world step method, likely in or as a result of a contact callback method. Delay executing this code until after the world step.