Why Does a C Function Without a Return Statement Still Run?

Function with Missing Return Value: Understanding Runtime Behavior

In C , a function that returns a value must always provide a return statement, or the compiler will issue a warning. However, as observed in the example code presented, the program may still run without any apparent issues, even when a return statement is missing. This behavior can be puzzling.

Undefined Behavior

The absence of a return statement in a value-returning function is expressly defined as undefined behavior in the ISO C standard (section 6.6.3). This means that the compiler is free to generate any value as the result of such a function.

Default Value for Uninitialized Variables

In the given example, the function doSomethingWith doesn't initialize its local variables. As a result, they are initialized with default values. In this case, returnValue is initialized to 0, which is the default value for int variables.

What Happens at Runtime

When the function executes without a return statement, it effectively "falls off the end" of its body. In such cases, the compiler typically generates code that returns the default value of the return type. For primitive data types like int, this value is 0.

Behavior with Non-POD Datatypes

For non-POD (Plain Old Data) datatypes, such as objects or structs, the behavior becomes more complex. Without a specific return statement, the compiler may attempt to return a partially constructed or uninitialized object, which can lead to memory corruption and undefined behavior.

Conclusion

While a function with a missing return value may appear to run correctly in some cases, it is essential to understand that such behavior is undefined. Ignoring the compiler warning can lead to unpredictable and potentially dangerous consequences. Always ensure that your value-returning functions have explicit return statements to guarantee well-defined behavior.

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