Fixed bug #1056 (Frequent crashes in Touch events by simply touching the screen)

 Joseba García Echebarria      2010-12-15 01:55:22 PST

I believe the crash is caused by a check not being performed on wether an
SDL_Touch element is NULL before using it in the SDL_SendTouchMotion function
in src/events/SDL_touch.c around line 400.
Judging from the rest of the code, there's a missing

    if (!touch) {
        return 0;
    }

before using "touch" as SDL_GetFinger(), SDL_GetFingerIndexId() use
touch->num_fingers without checking.

I can attach a patch if you like. It seems pretty straightforward, though.

I have yet to discover why touch is being returned as NULL as this error is
only triggered when an actual gesture has been performed, maybe something
related to SDL_AddTouch()?

src/events/SDL_touch.c

@@ -397,8 +397,8 @@ SDL_SendTouchMotion(SDL_TouchID id, SDL_FingerID fingerid, int relative,
                     float xin, float yin, float pressurein)
 {
     int index = SDL_GetTouchIndexId(id);
-    SDL_Touch *touch = SDL_GetTouch(id);
-    SDL_Finger *finger = SDL_GetFinger(touch,fingerid);
+    SDL_Touch *touch;
+    SDL_Finger *finger;
     int posted;
     Sint16 xrel, yrel;
     float x_max = 0, y_max = 0;
@@ -406,6 +406,7 @@ SDL_SendTouchMotion(SDL_TouchID id, SDL_FingerID fingerid, int relative,
     Uint16 y;
     Uint16 pressure;
     
+    touch = SDL_GetTouch(id);
     if (!touch) {
       return SDL_TouchNotFoundError(id);
     }
@@ -418,6 +419,7 @@ SDL_SendTouchMotion(SDL_TouchID id, SDL_FingerID fingerid, int relative,
         return 0;
     }
     
+    finger = SDL_GetFinger(touch,fingerid);
     if(finger == NULL || !finger->down) {
         return SDL_SendFingerDown(id,fingerid,SDL_TRUE,xin,yin,pressurein);        
     } else {
@@ -496,14 +498,16 @@ SDL_SendTouchMotion(SDL_TouchID id, SDL_FingerID fingerid, int relative,
         return posted;
     }
 }
+
 int
 SDL_SendTouchButton(SDL_TouchID id, Uint8 state, Uint8 button)
 {
-    SDL_Touch *touch = SDL_GetTouch(id);
+    SDL_Touch *touch;
     int posted;
     Uint32 type;
 
     
+    touch = SDL_GetTouch(id);
     if (!touch) {
       return SDL_TouchNotFoundError(id);
     }