OOP56-CPP. Honor replacement handler requirements

The handler functions new_handler, terminate_handler, and unexpected_handler can be globally replaced by custom implementations, as specified by [handler.functions], paragraph 2 of the C++ Standard [ISO/IEC 14882-2014]. For instance, an application could set a custom termination handler by calling std::set_terminate(), and the custom termination handler may log the termination for later auditing. However, the C++ Standard, [res.on.functions], paragraph 1, states:

In certain cases (replacement functions, handler functions, operations on types used to instantiate standard library template components), the C++ standard library depends on components supplied by a C++ program. If these components do not meet their requirements, the Standard places no requirements on the implementation.

Paragraph 2 goes on to state, in part:

In particular, the effects are undefined in the following cases:
— for handler functions, if the installed handler function does not implement the semantics of the applicable Required behavior: paragraph

A replacement for any of the handler functions must meet the semantic requirements specified by the appropriate Required behavior: clause of the replaced function.

Noncompliant Code Example

In this noncompliant code example, a replacement new_handler is written to attempt to release salvageable resources when the dynamic memory manager runs out of memory. However, this example does not take into account the situation where all salvageable resources have been recovered, and there is still insufficient memory to satisfy the allocation request. Instead of terminating the replacement handler with an exception of type std::bad_alloc, or terminating the execution of the program without returning to the caller, the replacement handler returns as normal. Under low memory conditions, this will result in an infinite loop with the default implementation of ::operator new(). Since such conditions are rare in practice, it is likely for this bug to go undiscovered under typical testing scenarios.

#include <new>
 
void custom_new_handler() {
  extern std::size_t reclaim_resources();
  reclaim_resources();
}
 
int main() {
  std::set_new_handler(custom_new_handler);
 
  // ...
}

Compliant Solution

In this compliant solution, custom_new_handler() does not disregard the return value from reclaim_resources(), which is returning the number of bytes freed by the reclamation. If it returns 0, then an exception of type std::bad_alloc is thrown, meeting the requirements for the replacement handler.

#include <new>

void custom_new_handler() noexcept(false) {
  extern std::size_t reclaim_resources();
  if (0 == reclaim_resources()) {
    throw std::bad_alloc();
  }
}
 
int main() {
  std::set_new_handler(custom_new_handler);
 
  // ...
}

Risk Assessment

Failing to meet the required behavior for a replacement handler results in undefined behavior.

Automated Detection

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Related Guidelines

Bibliography