Use of undeclared identifier error in my case

My code invokes a C library function:

@implementation Store
  ...
  -(void) doWork {
    // this is a C function from a library
    int data = getData(); 
    ...
  }
end

I am unit testing the above function, I want to mock the C function getData() in my test, here is my test case:

@interface StoreTests : XCTestCase {
    int mData;
    Store *store;
}
@end

@implementation StoreTests

-(void) setUp {
  [super setUp];
   mData = 0;
   store = [[Store alloc] init];
}

-(void) testDoWork {
  // this call will use the mocked getData(), no problem here.
  [store doWork];
}

// mocked getData()
int getData() {
   mData = 10; // Use of undeclared identifier 'mData', why?
   return mData;
}

...
@end

Why I get complier error: Use of undeclared identifier 'mData' inside mocked getData() function?