|
| 1 | +# algorithmOutOfBounds |
| 2 | + |
| 3 | +**Message**: The algorithm 'std::copy' accesses 5 elements through the iterator 'v1.begin()' but only 3 elements are available.<br/> |
| 4 | +**Category**: Correctness<br/> |
| 5 | +**Severity**: Error<br/> |
| 6 | +**Language**: C++ |
| 7 | + |
| 8 | +## Description |
| 9 | + |
| 10 | +Many STL algorithms take an iterator that denotes the beginning of a second range (typically an output range) and |
| 11 | +assume that this range is large enough. If it is not, the algorithm writes or reads past the end of the container, |
| 12 | +which is undefined behavior. |
| 13 | + |
| 14 | +This checker uses the ValueFlow analysis to compare the number of elements an algorithm accesses with the number of |
| 15 | +elements that are actually available through the iterator, and warns when the access is out of bounds. Three groups |
| 16 | +of algorithms are checked: |
| 17 | + |
| 18 | +- Algorithms that access exactly `last1 - first1` elements through the other iterator: `std::copy`, `std::move`, |
| 19 | + `std::swap_ranges`, `std::transform`, `std::replace_copy`, `std::replace_copy_if`, `std::reverse_copy`, |
| 20 | + `std::equal`, `std::mismatch`, `std::is_permutation`, `std::partial_sum`, `std::adjacent_difference` and |
| 21 | + `std::inner_product`. |
| 22 | +- Algorithms that access at most `last1 - first1` elements, depending on the values in the input range: |
| 23 | + `std::copy_if`, `std::remove_copy`, `std::remove_copy_if` and `std::unique_copy`. Since the actual number of |
| 24 | + accessed elements is not known, these are only reported as inconclusive warnings (with `--inconclusive`). |
| 25 | +- Count-based algorithms that access as many elements as the count argument says: `std::copy_n`, `std::fill_n` and |
| 26 | + `std::generate_n`. |
| 27 | + |
| 28 | +The severity is `error` when the out of bounds access always happens. When the analysis depends on an earlier |
| 29 | +condition in the code, the severity is `warning` and the message has the form "Either the condition 'v.size()==3' |
| 30 | +is redundant or the algorithm ... ". |
| 31 | + |
| 32 | +The checker does not warn when: |
| 33 | + |
| 34 | +- The second range is given with both a begin and an end iterator (for example the two-range overloads of |
| 35 | + `std::equal`, `std::mismatch` and `std::is_permutation`), since such overloads do not access the second range out |
| 36 | + of bounds. |
| 37 | +- An iterator adaptor such as `std::back_inserter` or `std::inserter` is used, since those grow the container as |
| 38 | + needed. |
| 39 | +- The proof would rely on "possible" (non-known) values on both the accessed and the available side. |
| 40 | + |
| 41 | +## How to fix |
| 42 | + |
| 43 | +Make sure the destination range is large enough before calling the algorithm, or use an iterator adaptor such as |
| 44 | +`std::back_inserter` that grows the container as needed. |
| 45 | + |
| 46 | +Before: |
| 47 | +```cpp |
| 48 | +void f(const std::vector<int>& v0) { |
| 49 | + std::vector<int> v1(3); |
| 50 | + // If v0 has more than 3 elements, this writes past the end of v1 |
| 51 | + std::copy(v0.begin(), v0.end(), v1.begin()); |
| 52 | +} |
| 53 | +``` |
| 54 | +
|
| 55 | +After: |
| 56 | +```cpp |
| 57 | +void f(const std::vector<int>& v0) { |
| 58 | + std::vector<int> v1(v0.size()); |
| 59 | + std::copy(v0.begin(), v0.end(), v1.begin()); |
| 60 | +} |
| 61 | +``` |
| 62 | + |
| 63 | +Or let the container grow: |
| 64 | +```cpp |
| 65 | +void f(const std::vector<int>& v0) { |
| 66 | + std::vector<int> v1; |
| 67 | + std::copy(v0.begin(), v0.end(), std::back_inserter(v1)); |
| 68 | +} |
| 69 | +``` |
0 commit comments