According to the C++ Standard, [stmt.return], paragraph 2 [ISO/IEC 14882-2014]:
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, or will otherwise 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> size_t f(std::vector<int> &v, size_t s) try { v.resize(s); return s; } catch (...) { return 0; }
Exceptions
MSC36-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. It does not result in undefined behavior when flowing off the end of the main()
function.
MSC36-EX2: It is permissible for a control path to not return a value if that code path is never taken and a function marked [[noreturn]]
is called as part of that code path, or an exception is thrown. For example:
#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.
Rule | Severity | Likelihood | Remediation Cost | Priority | Level |
---|---|---|---|---|---|
MSC54-CPP | High | Probable | Medium | P6 | L2 |
Automated Detection
Tool | Version | Checker | Description |
---|---|---|---|
Clang | 3.9 | -Wreturn-type | Does not catch all instances of this rule, such as function-try-blocks. |
Related Vulnerabilities
Search for vulnerabilities resulting from the violation of this rule on the CERT website.
Related Guidelines
Bibliography
[ISO/IEC 14882-2014] | 6.6.3, "The |