Can a C Function Safely Omit a `return` Statement if it's Not `void`?

Is Omitting 'Return' in a Non-void Function Acceptable?

When a function returns no value but has a non-void return type, it's expected that a compiler would issue an error. However, in certain cases, compilers can allow such code to pass without raising an error.

Example

Consider the following code:

int func1() {
    return; // ERROR: no return value
}

int func2() {
    // Does not return anything
}

While func1 generates an error due to the missing return statement, func2 doesn't. This may seem inconsistent.

Underlying Reason

In C , allowing non-returning functions with a non-void return type has undefined behavior. However, compilers often issue warnings instead of errors to maintain compatibility with legacy code.

The C standard deliberately avoids mandating a compile-time error in this situation because determining whether a function genuinely terminates without a return value is challenging.

Consider this example:

int func3() {
    // ...
    if (condition) {
        // Does not return
    }

    return 0; // Reached only if condition is false
}

In this scenario, the compiler cannot know if the code will run off the end of the function or exit through an exception. If an error was enforced for all cases without a return value, the program might be rejected even though it's valid.

Conclusion

Allowing non-returning functions with non-void return types is a potential source of undefined behavior. While compilers may issue warnings, they often opt for permissive behavior to ensure compatibility. However, it's strongly recommended to always include an appropriate return statement to maintain code clarity and avoid potential issues.

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