What Happens When a C Function Lacks a Return Value?

Function with Missing Return Value: Runtime Behavior Exploration

The absence of a return statement in a non-void function raises concerns about the behavior of the program at runtime. VisualStudio 2008 aptly warns about the missing return value in the following code:

int doSomethingWith(int value)
{
    int returnValue = 3;
    bool condition = false;

    if(condition)
        // returnValue += value; // DOH

    return returnValue;
}

int main(int argc, char* argv[])
{
    int foo = 10;
    int result = doSomethingWith(foo);
    return 0;
}

Despite the warning, the program compiles and runs. However, the return value of the 'doSomethingWith' function is not what one might expect.

Is it Undefined Behavior?

As the ISO C standard (section 6.6.3) states:

"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."

This means that the behavior of the program is unpredictable. In this specific instance, the return value is set to 0.

How is the Result Value Computed?

When a function does not explicitly return a value, the compiler tries to determine the return type based on the context. In this case, since the function is declared to return an 'int', the compiler initializes the return value to 0.

What Happens with Non-POD Datatypes?

The behavior of functions with missing return values for non-POD datatypes is even more unpredictable. Without explicit initialization, the compiler cannot assume a default value and the return value will likely be garbage data.

Conclusion

While the program in the provided example may run without noticeable issues, relying on undefined behavior is never advisable. It can lead to subtle and hard-to-debug problems. Always ensure that every non-void function has an explicit return statement that sets an appropriate return value.

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