PWR014: Out-of-dimension-bounds array access

Issue

An index outside the dimension size of an array is being used to access it.

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. Using an index outside the dimension bounds that results in accessing a valid array element in a different dimension is technically possible and correct in C/C++; however, it produces obscure code which is difficult to understand and reason about. Moreover, doing this in parallel code can lead to the introduction of data races.

Actions

Change the array access to explicitly accessing the desired dimension.

Code example

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

void foo() {
   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 bi-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 bi-dimensional array was dynamically allocated not using contiguous memory.

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

Related resources

• PWR014 examples at GitHub 
• PWD004: Out-of-memory-bounds array access

References

Index checking – Bounds checking [last checked October 2020]

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