What Happens When a C Function Doesn't Return a Value?

Compiler Behavior When Function Returns Garbage for Non-Void Return Type

In C , a function that does not explicitly return a value for its defined return type is considered to have undefined behavior. However, some compilers do not issue any errors in such cases, leaving the returned value as garbage.

Reason for Compiler's Tolerance

This behavior stems from the fact that it can be challenging for compilers to determine definitively whether a function will reach its end and return garbage or if it will exit through an exception or another non-standard return mechanism.

Example

Consider the following code:

int func1() {
    return; // Error
}

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

The first function, func1(), raises an error during compilation because it does not explicitly return a value for its specified return type (int). On the other hand, func2() does not produce an error despite not explicitly returning a value.

Implication for Variable Initialization

The undefined behavior in func2() is not analogous to uninitialized variables in C . The C standard differentiates between variables that are explicitly left uninitialized and variables that are designated as "undefined behavior" due to incomplete execution or other exceptional conditions.

Compiler Warning and Standard Definition

Even though most compilers will not raise an error for func2(), they may issue a warning to indicate the potential for undefined behavior. The C standard states that exiting a function without a return value for a non-void return type leads to undefined behavior:

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.

Conclusion

Although some compilers do not raise errors for functions that do not return values, this behavior is considered undefined by the C standard. Compilers may provide warnings to flag such cases, but it is ultimately the developer's responsibility to ensure that all functions have explicit return statements or handle exceptional return conditions appropriately.

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