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According to the The C++ Standard, [stmt.return], paragraph 2 [ISO/IEC 14882-2014], states the following: 

Flowing off the end of a function is equivalent to a return with no value; this results in undefined behavior in a value-returning function.

A value-returning function must return a value from all code paths; otherwise, it will result in undefined behavior. This includes returning through less-common code paths, such as from a function-try-block, as explained in the C++ Standard, [except.handle], paragraph 15:

Flowing off the end of a function-try-block is equivalent to a return with no value; this results in undefined behavior in a value-returning function (6.6.3).

Noncompliant Code Example

In this noncompliant code example, the programmer forgot to return the input value for positive input, and so not all code paths return a value:.

int absolute_value(int a) {
  if (a < 0) {
    return -a;
  }
}

Compliant Solution

In this compliant solution, all code paths now return a value:.

int absolute_value(int a) {
  if (a < 0) {
    return -a;
  }
  return a;
}

Noncompliant Code Example

In this noncompliant code example, the function-try-block handler does not return a value, resulting in undefined behavior when an exception is thrown:.

#include <vector>
 
std::size_t f(std::vector<int> &v, std::size_t s) try {
  v.resize(s);
  return s;
} catch (...) {
}

Compliant Solution

In this compliant solution, the exception handler of the function-try-block also returns a value:.

#include <vector>
 
std::size_t f(std::vector<int> &v, std::size_t s) try {
  v.resize(s);
  return s;
} catch (...) {
  return 0;
}

Exceptions

MSC54-CPP-EX1: Flowing off the end of the main() function is equivalent to a return 0; statement, according to the C++ Standard, [basic.start.main], paragraph 5 [ISO/IEC 14882-2014].  Thus, flowing off the end of the main() function does not result in undefined behavior.

MSC54-CPP-EX2: It is permissible for a control path to not return a value if that code path is never expected to be taken and a function marked [[noreturn]] is called as part of that code path or if an exception is thrown. For example:, as is illustrated in the following code example.

#include <cstdlib>
#include <iostream>
[[noreturn]] void unreachable(const char *msg) {
  std::cout << "Unreachable code reached: " << msg << std::endl;
  std::exit(1);
}

enum E {
  One,
  Two,
  Three
};

int f(E e) {
  switch (e) {
  case One: return 1;
  case Two: return 2;
  case Three: return 3;
  }
  unreachable("Can never get here");
}

Risk Assessment

Failing to return a value from a code path in a value-returning function results in undefined behavior that might be exploited to cause data integrity violations.

Automated Detection

Related Vulnerabilities

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

Bibliography

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