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PWR014: Out-of-dimension-bounds matrix access

Issue​

An index outside the dimensions of a matrix is used.

Actions​

Change the array access to explicitly access the desired dimension.

Relevance​

C/C++ array access syntax allows the use of indices outside the ranges of array elements. Using indices corresponding to positions outside the array is an out- of-memory-bounds error. For matrices, however, using an index outside the dimension bounds doesn't necessarily cause an out-of-memory-bound error, since this access can result in accessing a valid array element in a different dimension. However, it produces obscure code which is difficult to understand and reason about. Moreover, parallelization of such code can lead to the introduction of data races.

Code example​

The following code iterates over part of a two-dimensional array:

void example() {
  int A[100][100];
  for (int i = 1; i < 100; ++i) {
    for (int j = 0; j < 100; ++j) {
      A[i][j - 1] = 1;
    }
  }
}

Notice that the second dimension is accessed by subtracting 1 from j. Since the first value of j will be 0, the first iteration will access the second dimension using an index of -1. This is valid syntax in C/C++: given that the two-dimensional array is stored in contiguous memory, it results in accessing the last element of the previous dimension. For instance, for the first iteration of the loop, A[1][-1] will be accessed, which results in the same element as accessing A[0][99]. Notice that if i range started in 0 instead of 1, this would be an out-of-memory-bounds access defect; and the same would happen if the two-dimensional array was dynamically allocated.

Although it may not be an error, this practice is discouraged since it makes it harder to reason about the code. For example, the programmer might have wanted to iterate over j up to 98, in which case the fixed code would be:

void example() {
  int A[100][100];
  for (int i = 1; i < 100; ++i) {
    for (int j = 0; j < 99; ++j) {
      A[i][j] = 1;
    }
  }
}

References​