Why Does My C g Compiler Compile a Function Without a Return Statement, Even Though It Returns a Structure?

Omitting Return Statement in C

Problem:

A developer encountered unexpected behavior when using a version of g for Windows obtained through Strawberry Perl. The g compiler allowed them to omit a return statement in a non-void function, despite the function returning a structure.

In-Depth Analysis:

The code snippet provided:

struct boundTag Box::getBound(int side) {
    struct boundTag retBoundTag;
    retBoundTag.box = this;
    switch (side)
    {
        // set retBoundTag.bound based on value of "side"
    }
}

In a non-void function like this, omitting the return statement leads to undefined behavior. The ISO C -98 standard specifies that:

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.

Why Did It Compile Without Warnings?

While omitting the return statement is undefined behavior, some compilers may not issue warnings by default. To enable more thorough warnings, it is recommended to use the -Wall option when compiling.

Consequences of Omitting the Return Statement

Omitting the return statement in a non-void function can have unpredictable consequences. The function may return uninitialized values or cause the program to crash. Additionally, using the returned value in subsequent code may lead to unexpected results.

Conclusion

Although the g compiler allowed the omission of a return statement in this case, it is crucial to adhere to the ISO C standard and always include a return statement in non-void functions. Omitting the return statement can lead to undefined behavior and unpredictable consequences.

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