What Happens When a C Function with a Non-Void Return Type Doesn't Return a Value?

Compiler Handling of Functions Returning Garbage in C

In C , a function with a non-void return type is expected to return a value. However, if the function does not explicitly return anything, the compiler's behavior in dealing with this situation is somewhat controversial.

According to the C standard, such code has undefined behavior. This means that the compiler is allowed to generate any value, including garbage, as the return value. Consequently, relying on the behavior observed in specific implementations is not portable.

However, most modern compilers will issue a warning for such code, indicating that it's best practice to explicitly return a value or use void as the return type. This is because leaving a function without a return value can lead to unexpected behavior.

The reason why the compiler does not always report an error for this case lies in the difficulty of determining whether the function actually "flows off the end" or exits through another mechanism, such as an exception.

For example, consider the following code:

int func3() {
    func4();
}

If func4() throws an exception, then func3() will return via that exception. In this case, there is no issue with not returning a value explicitly.

Moreover, the compiler may not have access to the definition of func4(), making it difficult to determine whether it will return or not. Additionally, it would require a complete program analysis to prove that func3() is always called, which is generally impractical.

Therefore, while it is considered a bad practice, the C compiler is allowed to return garbage in cases where a non-void function does not explicitly return a value. It is the programmer's responsibility to ensure that proper return values are always provided to maintain program correctness and avoid undefined behavior.

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